So we're talking about big guns and recoil huh? Well, there was the rule that I used a while back which I think I got from arsenal which was, you take stun damage equal to the weapons uncompensated recoil for heavy weapons when firing unbraced, and for assault cannons you'd still get a bad shoulder bruise unless it was on a tripod.
The closest I can find to a modern "Assault Cannon" are the Mechem NTW, or the RT-20. Though I always assumed an "Assault Cannon" was actually just a large caliber anti materiel rifle in most cases, or the vigorous which is just a Barret.
A while back I statted out a 2070's version of the NTW, which pretty much just had a good sensor suite and similar capabilities to the RL one.