>>536995395
it's closer to the autocannon than it is to the recoiless rifle. it's got a breach and an entire recoil dampening system rather than just venting the exhaust gasses out the open back to counteract the recoil of firing the projectile.

also here's the barret M82A2, a shoulder fired variant of the M82. shoulder fired 20mm autocannons are not outlandish or unrealistic