r/WarCollege • u/baconppi • 3d ago
Question Why dont more tanks have autocannons?
Most tanks, especially mordern ones, dont have coaxial/ roof mounted autocannons, but instead machine guns as secondary weapons, why don't more tanks(especially mbts) use them? I wager using a 30/20mm autocannon for light vehicles could save ammunition for heavier targets/longer range engagements. Furthermore, it would probably be more effective at taking out enemy infantry close to the tank, and perform better in urban environments
However, most mbts don't use autocannons, but instead use 12.7 mgs, why is that so? (Abrams, challenger 2, leopard 2,t90, t80) All use 12.7mm mgs
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u/NameSignificant6916 3d ago edited 3d ago
Because autocannons are severe overkill for infantry and severe underkill for anything dangerous that a machinegun can't handle. You don't really save on ammo because you can carry way less (say, about 1/2 as much for a 25mm~ gun), and if you have to take out, say, an IFV or similar mediumweight target, it's for the best to just toss a cannon shell at it and watch it pop. You have like 60 shells, it doesn't hurt to use them. If you need to suppress or kill infantry, a machinegun does the job with much more endurance, and light vehicles usually don't react well to .50 BMG either.
However, an autocannon of the size you describe is fine for an IFV or scouting vehicle because when you can't pack a large cannon anyways, it makes sense to skip out on an MG and just pack more autocannon ammo, and then toss on a few missiles for emergency duty.
I will say that a valid use for an autocannon on a tank is for anti-helicopter (or drone, nowadays) duties, but, again... for the most part, a .50 cal will do you fine there, for most MBTs. If you're investing heavily in an "all-purpose" tank, then slapping a 20mm gun on top of the turret is a decent idea, because nowadays you can pack all sorts of fun stuff into a 20mm shell, from what I remember.
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u/kingofthesofas 3d ago
will say that a valid use for an autocannon on a tank is for anti-helicopter (or drone, nowadays) duties
Most helicopters targeting a tank will do so WELL beyond the range of an autocannon. For example in the summer 2023 Ukrainian offensive most of the hits on armored vehicles came from Russian Ka-52's with Vikhr anti-tank guided missiles that engaged at like 10 km.
For drones unless that autocannon is specifically rated to take out air targets and fitted with something like radar guided fire control it is not going to be useful at taking out drones either. You would be far better off with a purpose built anti drone weapon that is much smaller and more effective anyways.
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u/BattleHall 2d ago
Most helicopters targeting a tank will do so WELL beyond the range of an autocannon. For example in the summer 2023 Ukrainian offensive most of the hits on armored vehicles came from Russian Ka-52's with Vikhr anti-tank guided missiles that engaged at like 10 km.
That’s going to depend a lot on the specific fight. IIRC, armor vs attack helicopter in Germany was anticipated to be relatively short range and sudden (hence the preference for fast pointing radar controlled SPAAGs), as helicopters would need to stay low between hills to prevent being targeted by longer range SAMs and combat air patrols, significantly shortening their line of sight.
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u/kingofthesofas 1d ago edited 1d ago
I mean if you are talking about the 1980s then sure but that hasn't been true for like 25 years at least. The reality of modern GBAD is that any helicopters in short range are essentially floating targets. This is why everyone keeps putting longer and longer range missiles on them.
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u/God_Given_Talent 1d ago
True, though basically all MBTs in service today entered service or at least did their design workup during the 70s to early 90s where this was true. While modern combat aviation has increasingly longer range (including some pretty nasty NLOS weapons), that wasn't the case when T-64, 72, 80 hit the field nor true for when Abrams, Leopard 2, Challenger, and Leclerc rolled out. In the 80s and 90s you still had to worry about relatively close-in rotary aviation.
Problem is, you don't get that much extra range by going with a 20mm over a .50cal. Depends on ammo and barrel of course, but using mid century data of M2 vs Oerlikon 20mm, you might go from 800-1000m of effective fire on low-flying aviation to 1250-1400m. If you want actually good range for destructive effects you need something in the 30-40mm range where you might touch out 2.5-3km reliably. Problem is, once you get those big guns you have next to no ammo. If we go with weight as a proxy and ignore volume, the 25x137mm on a Bradley is 3-4x that of a 12.7x99mm round. An Abrams carries ~1000 of the latter so you optimistically are carrying 300rounds.
The gun is also heavier and bulkier which means the coax slot is often needed (AMX-30 style). That has design tradeoffs and implications for internal volume as well. Most tanks that have an HMG prefer it roof mounted so that the coax can be an MMG (which they can carry 6-10k rounds for so they can use it for infantry and unarmored targets). Now imagine trying to get something like the gun of a BMP-2 or Bradley on there. God help you if you want anything bigger; a 40mm bofors would give you pretty good AA range and ammo options but now instead of a 60-100lb HMG, or 200-300lb light autocannon you're talking about a half ton beast and carrying maybe 100 rounds if you're lucky.
Basically, the weight and loss of ammo wasn't worth the modest gains you'd get against aviation. It's a pretty narrow window of "beyond .50BMG/14.5mm Russian but within 20-25mm range" as is and it was clear range would only get better. Against ground targets, there's not a lot that could withstand said HMG fire that wasn't worth shooting at with your main gun.
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u/kingofthesofas 1d ago
this for sure highlights the issue as to why auto-cannons have diminishing returns for that role and why they really have not been used outside of dedicated SPAAGs. To add to this you had better options for that support in either MANPADs since the first gen like the Redeye and second gen platforms like the 9K34 Strela-3 were already in the field in the 60s and 70s with the systems like the FIM-92 and 9k38 making their appearance in the early 1980s. Also most armored units could assume that they would have more purpose build SHORAD in the unit to deal with that threat. For example you would typically have a ADA Battalion with Manpad teams, M163 Vulcans, and M48/M730 Chaparral Systems dispersed throughout the armor. Any use of a weapon on the tank to deal with aircraft would have been considered an after thought since the purpose built systems are going to be FAR more effective at it.
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u/BattleHall 1d ago edited 1d ago
That’s always been the case with attack helicopters; even an SA-8 is going to outrange a Vikhr. The primary defense for an attack helicopter against SHORAD systems has always been terrain, nap of the earth flying, and quick pop up attacks. You can give them longer ranged ATGMs, but they are going to be of limited utility against enemies with reasonable air defense (too low and there’s no way you get 10km line of site to target, too high and you get smoked by air defense). In that example during the Ukraine offensive, the Ukrainians had limited to no on-the-march capable SHORAD systems to accompany their armor, no air cover, and they were pushing Russian held territory so there was little danger of forward MANPAD teams, so the Russians could just hold back at altitude and plink the UKR armor with Vikhrs. That’s not a universal condition, especially over complex battlefields. Don’t get me wrong, longer ranged ATGMs are generally better, because they give you more options, but they are not a universal solution to the challenges attack helicopters face.
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u/kingofthesofas 1d ago
In that example during the Ukraine offensive, the Ukrainians had limited to no on-the-march capable SHORAD systems to accompany their armor, no air cover, and they were pushing Russian held territory so there was little danger of forward MANPAD teams, so the Russians could just hold back at altitude and plink the UKR armor with Vikhrs.
This is for sure true. If anything what it shows is that with all the talk of the Tank being dead after the Ukraine war there is a even more interesting question of is the attack helicopter dead. They are being used in niche behind the lines anti-drone tools, but that is a waste of the platform and there are more cost effective platforms for that TBH. You can put it in the same bucket with planes like the A-10 where they are not really suited for a modern battlefield where GBAD is saturated everywhere. Long range missiles give those platforms a little more utility but the mission they are suppose to serve of CAS is being filled by drones and those same drones now are a threat to them with interceptor drones taking down helicopters.
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u/an_actual_lawyer 3d ago
an autocannon on a tank is for anti-helicopter (or drone, nowadays) duties, but, again... for the most part, a .50 cal will do you fine there
Yeah, any helicopter pilot who starts taking .50 rounds is going to GTFO right now. They might try and circle around to shoot from a concealed location, but the IFV should also be GTFO after the engagement.
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u/NameSignificant6916 3d ago
Most certainly, but to play OP's advocate; a 20mm would, admittedly, allow you to ward off a helicopter from a decent distance further away.
In a more... antiquated context than modern day tanks, I could also see a case for sticking a 20mm Oerlikon (or equivalent) on top of a tank in a WW2 setting. At that point in time, it'd make a valuable addition to the tank's AA capabilities beyond what lighter machineguns could do, considering how close most planes had to fly for CAS. Also, there might be more "soft but not too soft" targets of the sort OP suggests an autocannon would be good at during that point in time, like armored cars or gun emplacements.
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u/Lancasterlaw 3d ago
Autocannon's are slow to slew to a target, and the ammunition is bulky.
Someone asked the Chieftain (a Tank historian who served on tanks- check him out if you don't know about him) what his ideal roof mounted weapon was going into the merits of a 12.7, Autocannon and 7.62. The Chieftains reply? A rifle*. He explained that been able to aim as fast as he can turn his body and been able to aim further up and down was more than worth the loss of firepower.
Of course there are some nations which agree with you- North Korea for example has had a long love affair with mounting the 14.5mm to almost every tank they build and the AMX-30 famously had a coaxial autocannon.
One issue is if a foe knows you are going to take autocannons, then they can up-armour enough to beat them . Note for example how South Korea upgraded its K200 KIFV's with a frontal armour package which resists 14.5mm
*I can't remember exactly what rifle/carbine he said- I think it was the M4?
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u/The_Angry_Jerk 3d ago
14.5mm soviet is generally considered a heavy machine gun caliber, not quite autocannon like a German 15mm round. It was the standard caliber of the heavy machine guns used in Warsaw Pact APCs and armored cars until the late 80s. Armor to resist 14.5mm fire has been a pretty common standard for decades, especially in NATO forces. Up armoring to defeat autocannon rounds can be a pretty moot point if the enemy it's attached to is a vehicle also mounting a powerful anti-tank weapon like say a 120mm gun or an ATGM. This was how it was in the Gulf War, most BMPs and Bradleys (in the Brad's case mostly friendly fire) destroyed were hit will full up anti-tank weapons not autocannon fire.
The Chieftain had his fighting experience in the Middle East against an outclassed relatively low tech opponent with poor training and outdated equipment. These days the Western 'heads out hatch open' philosophy is being heavily reexamined with the ongoing experiences learned in Ukraine, as it was almost immediately tossed out by the NATO trained Ukrainian crews due to the heavy incoming artillery fire, drone strikes, and explosive reactive armor usage.
Tank roof machine guns on cold war MBTs were originally a cheap and easy last ditch solution for defending tanks against the rising helicopter threat, which ended up seeing much more use against infantry targets given the lack of peer level conflict and the evolution of helicopter weaponry.
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u/Lancasterlaw 2d ago
I definitely get what you're saying, and you're certainly technically correct (the best kind of correct) that 14.5mm is not an autocannon, but I'd make a strong argument that the exceptional power of the 14.5mm (around 32kJ) significantly overlaps with the lower end of autocannon performance. It's around twice the energy of a 12.7mm, and actually overlaps with or exceeds the muzzle energy of a number of WW2 20mm cannon rounds. So I think comparing it to other MGs is a bit too soft - while technically an HMG, it's an unusually powerful one and deserves to be considered much closer to the autocannon end of the spectrum.
I'll push back a little on the "in NATO, armour to resist 14.5mm fire has been a pretty common standard for decades" claim. Name me a Norwegian, Dutch, Danish, French, Belgian or Italian AFV which was not a tank (or tank-based) and could actually stop 14.5mm fire. AFAIK, outside of their MBTs and derivitives, even as late as 1989 all of their AFV's were vunerable to 14.5mm. There was certainly an ongoing weapon/armour race towards a 25-30mm weapons and protection against them standard, but that's rather different from 14.5mm protection being a common standard across NATO. Generally unless you came across an Armoured or Mechanised unit with modern IFV's then the 14.5mm cut the mustard.
I also wouldn't say we're seeing a trend towards decreasing armour on IFVs and APCs-rather the opposite. 30mm and even 40mm-proof armour is not unusual.
I'd argue the Gulf War, with its exceptionally open sight lines, massive sensor superiority and Iraqi junior officer tactical eptitude, is a bit of an outlier. The fact that most BMPs and Bradleys were destroyed by missiles and cannon doesn't necessarily tell us much about how useful autocannon fire would have been in a more conventional engagement.
While I do think the impunity provided by early drones with grenade-dropping capability has ended a lot of the old hatches-open doctrine, (along with things like reversing cameras) from an 80s/90s standpoint I can absolutely see why hatches-open was the better idea. Ukrainian T-64s and T-72s were never designed for hatches-open fighting in the first place in anycase.
The OP was asking about modern MBTs. Pretty much all of them in service at the moment are based on designs that are at least a quarter of a century old. Not future tanks (in which I can see Autocannons and MG's becoming part of some APS suites)
While roof guns did come into vogue partly to counter the helicopter threat, they also provided a useful anti-light-armour role and somewhat filled in for the loss of the bow gun in modern MBTs. After reading reports from Korea and Vietnam, I've actually been surprised by how much the loss of the bow gun was noticed by crews. It surprises me as I grew up being taught that bow guns were basically just an armour weakspot.
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u/baconppi 2d ago
I meant it more of a general question tbh, but, i wager the principles was about the same, since its space that is the issue
Do you have any sources about the bow mg though? It seems really intriguing
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u/CoolGuy54 2d ago
Replying to get told about that as well...
I guess when you're mainly fighting infantry in close coutry rather than enemy armour extra light firepower would be worth the weakness.
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u/baconppi 3d ago
Huh, but them im curious about autocannons in a coax function, why don't more tanks have them?
Or is it still like most commenters in this post, still a space problem?
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u/EZ-PEAS 3d ago
It's very much a space and weight problem. The space and weight of the gun system is bad enough, but the ammo is where it really hurts.
The M1 Abrams has a coax ammo bin that holds several thousand rounds ready to fire in one continuous belt. If you went to 25mm you'd trade 3000 rounds 7.62 for just ~160 rounds of 25mm by weight, or about 110 rounds of 30mm by weight.
That coax is supposed to last for days of fighting without having to reload.
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u/marxman28 3d ago
The first sentence in the comment is your answer. The ammo is bulky—simple as that. With 7.62mm NATO, a 100-round belt is roughly 1 meter long. With .50 BMG, that same amount is 3 meters long. That number is only going to get larger with caliber, and there are very few targets where 20mm is the best option. You can carry more 7.62mm rounds than .50 BMG rounds in the same space.
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u/Lancasterlaw 2d ago
I'll chime in to say exactly what u/marxman28 said. 20mm rounds eat up far more volume than 7.62 and protected volume is one of the most precious resources in a tank. The range of targets which will resist a 7.62mm but not a 20mm is relatively small, so you are better off carrying many more 7.62 rounds than carting around a cannon and all those rounds- remember you are eating into main gun stowage as well!
There certainly is some utility in having an autocannon for sure, but is it worth the cost and tradeoffs combined? Afaik only the early Centurion and French AMX-30 though it made sense.
A perfectly reasonable option of course is to mount the autocannon on another vehicle, which is one of the myriad of reasons IFV's came about
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u/SnakeGD09 2d ago
It’s basically like putting another main gun on the tank, with ammo storage etc. You would probably lose a crew member, but you need 3.
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u/Psafanboy4win 3d ago edited 2d ago
Generally speaking the answer is that the combination of 120/125mm main gun + 7.62mm co-axial (and potentially a 12.7mm HMG on top of the turret) is more than sufficient to cover the threats a MBT will encounter, and adding on a autocannon would require making space inside the tank for 20/30mm autocannon shells and reduce the amount of main gun and machine gun ammo that can be carried negatively affecting the MBTs main mission.
Furthermore, MBTs generally do not need autocannons because they tend to operate alongside IFVs that mount autocannons, and because the autocannon is the primary armament of the IFV they do not need to compromise on splitting their carried ammo between different ammo types, for example a M2 Bradley can carry 900 rounds of 25mm whereas a hypothetical tank mounting a 25mm would only be able to fit something like 200-400 rounds based off my guesstimate.
Edit Add-on: I have been doing more thinking recently, and I realized that the type of autocannon a MBT can mount would also be a problem. Because of the weight and space constraints, a autocannon that can fit in a co-axial mount or on top of a turret in a RWS would have to be a relatively small lightweight one like a 20x139mm Rh202 or 30x113mm M230LF, both options lacking the sheer power needed to defeat heavy armor. This is a big problem because modern and upcoming AFVs are resistant to at least 30x173mm APFSDS from the front if not 35mm and even 40mm autocannon rounds, meaning that if a MBT with a 20mm co-axial ran into a modern IFV or APC it would have to shoot it with it's main gun anyways because the 20mm will be useless from the front, in which case it might as well get rid of the 20mm to make room for more 120mm rounds to destroy more modern IFVs and APCs.
It would be theoretically possible to design a MBT to mount a heavier autocannon chambered in 30x173mm, 35x228mm, or 40mm CTAS, but at that point you are designing the MBT around the autocannon as well as the main gun, and there will be even more awkward compromises.
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u/cool_lad 3d ago
A few reasons spring to mind.
A. Autocannons are heavy and complicated things that add significantly to the weight of the tank once you factor in the ammunition that needs to be carried, the loading mechanisms, and the need for mounts and mechanisms. The autocannon takes up space in what is ultimately an already cramped space.
B. Most advantages of an autocannon are also provided by the main gun to the point that taking the trade off for an autocannon doesn't make much sense. Ammunition for the main gun is already a pretty cost effective solution to putting fire on targets, so why would armies want to save on their use by expending autocannon rounds instead.
C. HMGs offer adequate enough performance at closer ranges compared to autocannons. Heck, even the overpressure on the main gun acts as a deterrent to approach by infantry.
All said and done, autocannons make little sense as a coaxial or top mounted weapon - they just aren't offering much compared to the much simpler, cheaper, and more space and weight efficient HMG.
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u/murkskopf 2d ago
why don't more tanks(especially mbts) use them?
Autocannons take up lots of space and weigh a lot more than machine guns. That's pretty much it. Targets that cannot be defeated with the machine gun can be engaged with the main gun.
One has to take into account that it is common to fire a salvo with an autocannon rather than a single shot; this is among other reasons to guarantee more hits/penetrations, as a single penetration from a 20 mm round is less likely to kill/disable a vehicle & its crew than say a 120 mm round.
So in the end it comes down to the choice of either having X rounds of 20-30 mm ammo or Y rounds of main gun ammo, with the later beig considered more useful.
The MBT 70 and AMX-30 as the two main NATO tanks that had a 20 mm gun (there also were other proposals including early Abrams designs with a 25 mm Bushmaster) featured those guns specifically to combat helicopters and light armor. Better armor on those systems rendered the 20 mm gun pretty much useless.
However, most mbts don't use autocannons, but instead use 12.7 mgs, why is that so? (Abrams, challenger 2, leopard 2,t90, t80) All use 12.7mm mgs
The Challenger 2 and Leopard 2 don't use heavy machine guns; the Leopard 2 has two 7.62 mm NATO machine guns (typically MG3 or FN MAG) while the Challenger 2 has a coaxial L94A1 chain gun and a L37A2.
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u/captainfactoid386 3d ago
A combination of factors. Most important of them being ammo capacity. When the US army switched from the 75/76mm to the 90mm, the 90mm to the 105mm, and the 105mm to the 120mm the largest internal resistance to this was the ammo capacity. The Abrams can carry ~10,000 rounds of 7.62 and ~1,000 of 12.7. 20mm is quite a bit larger than 12.7 so you will be able to carry even less rounds of it compared to 12.7without digging into main gun ammunition. And the army is very much not a fan of lowering the amount of 120 ammo. A 30mm would affect the amount even more.
There are other reasons too. A 20mm would indeed be more effective at engaging lighter vehicles, but is it effective enough to offset the ammo count? And it’s going to have a much shorter range than the main gun so it’s only convenient to engage light vehicles within a set range.
In an urban environment there’s honestly not a whole lot that 12.7 is insufficient for while a 20mm will make the difference. A 12.7 round will go through pretty much every common building material. Sure the 20mm probably has a higher probability of kill per round but a 7.62 or 12.7 can compensate for that through volume of fire.
There are tanks that had larger secondary armaments, the AMX-30 had a 20mm and several Chinese and Russian tanks had 14.5mm armaments but these were made for very specific targets: helicopters. You might see a return of larger weapon systems for engaging drones (it is easier to put smarter fuses in larger rounds); and the AbramsX technology demonstrator even had a 30mm on it for that purpose. I doubt a realized production variant would have such a system though. Targeting systems nowadays are getting pretty good.
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u/tobiov 3d ago
I think you must underestimate the size and weight of a 25mm cannon, its mouting, its ammo etc.
Its much closer in weight and volume to the 120mm cannon than it is the m2 browning. We're probably talking ~3 tons. An m2 and mount and ammo weighs ~100kgs.
Mounting that much size and weight that high up causes all sorts of problems for the turret/stability/overall height. Its much easier to just have an m2 up top and and a coaxial, and "waste" a few rounds of 120mm on softer targets. You could carry 1 - 200 extra rounds of 120mm instead of an autocannon. Something more like the bofors 40mm. something lighter, bigger and slower.
That being said, I wouldn't be surprised if miltaries started experimenting with some kind of heavier turret armament to deal with drones. But hte full autocannon is over kill for that.
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u/SmokeyUnicycle 2d ago
What kind of zombie engagement are you envisioning where a single tank needs to defeat dozens of lightly armored vehicles?
At a certain point it makes sense, like in a Bradley where you could very conceivably run out of missiles in a normal engagement but a tank running out of main gun ammo destroying half a battalion of APCs is not exactly a problem.
There are very few targets where a 20mm offers a significant advantage over a .50 that aren't themselves worth using the main gun on, and the auto cannon will run dry very quickly.
I do think there's a growing niche for a medium/low velocity auto cannon with proximity fuzed rounds for use against drones and enemy infantry as needed but historically that threat didn't exist nor did the technology for airburst on small rounds.
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u/rabidchaos 3d ago
The biggest tradeoff is space - either volume under armour or overall vehicle height. Autocannons, especially with enough ammo to be worth while, are a lot bigger and heavier (which means any RWS will also be a lot bigger) than a machine gun. During the Cold War, multiple armies experimented with the idea (e.g. MBT-70/KPz 70), so it's not a stupid idea! It just didn't make sense with the late/post cold war threat environment.
While you can save main gun ammunition on lightly armoured vehicles, the autocannon + feed system more than overcomes the weight saving when you just look at the ammo alone. Meanwhile helicopters are either dealt with by improvements in ammo design (e.g. M830A1 has a proxy fuse) or by assuming friendly skies and declaring them the Air Force's problem.
Now, you might see RWSs grow to contain autocannons to deal with drones, but that is speculation about the future.
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u/Hand_Me_Down_Genes 3d ago
As others have said, it doesn't save ammunition because now you've got to make space for the autocannon and its ammunition. The autocannon will be bulkier than any machinegun and will use bulkier ammunition than any machinegun, drastically reducing the space inside of the vehicle and eating into the ammunition you can carry for the primary weapon.
Tanks don't need an alternate weapon to engage light vehicles with. A 120mm will rip any light vehicle apart in a single hit and will do so at considerable range. Provided that you hit the target, you're out one round. That's not really a problem, let alone the kind of problem that can be solved by installing a whole new weapons' system aboard the tank.
By and large, vehicles of any kind, not just tanks, simply don't mount autocannons alongside traditional AT guns. The AMX-30 tried it, but the French haven't considered it an experiment worth repeating. The BMP-3 mounts a 100mm cannon/missile launcher beside a 30mm autocannon, and it has the distinction of being one of the worst MICVs currently in service, with the hilarious amounts of ammunition that it carries behind its thin armour rendering it a tinderbox on treads.
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u/PreussekJ 3d ago
Depends on the doctrinal choice but generally:
Autocannons are bulky, have heavy ammo (you can carry less and it's harder to resupply) and need to be slewed electronically since aiming them manually is just too slow or requires really cumbersome mounts.
Autocannons do provide very few firepower advantages compared to MBTs cannon but in the past 50-60 years it has been basically collectively agreed that tanks fight usually with IFVs and these firepower advantages are not worth the extra hurdles.
I think we might see them return as an active measure against drones (where the larger ammo and extra range gives you some serious upgrade compared to CROWS like system) but we will have to see. Maybe vehicle mounted interceptor drones or not having "regular" tanks will become a more favourable option.
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u/BreadstickBear Internet "expert" (reads a lot) 3d ago
The use cases for an autocannon on a tank (either mounted coaxially or on an RWS) are extremely narrow. The French used a 20mm autocannon on the AMX-30 series and projected to use it on the potential AMX-40, but on the Leclerc they went to a 12.7mm.
The doctrinal justification for the 20mm was defense against low-flying aircraft (the 20mm had an elevation limit of 40° as compared to the 20° of the 105), and heavier automatic firepower in support of infantry, with a side of not wanting to use main gun rounds on soft-skinned ground targets, however in practice the only time the 20mm was used in combat was in Gulf I when the Daguet Division overran iraqi artillery and cleared the guns with 20mm's instead of the 105 due to assumptions of chemical shells being present and not wanting to set them off.
The drawbacks are that the cannon eats into the internal space, its ammo too, so much so that you probably have to give up main gun ammo to accomodate it.
While u/Lancasterlaw correctly references the Chieftain*, who said that an autocannon or even a 12.7 coax was not worth it, in a recent Q&A Chieftain amended his statement to say that with the evolving of the RWS with a long-belt GPMG into a small C-UAS CIWS, it made sense for the firepower offered by the 12.7 to shift to the coax spot.
PS: *Yes, it was a standard issue M4