At 12:46 AM 06/11/2001 -0700, Tim May replied:
Well said, but: In _The Irish War_ there's a description of IRA improvised recoilless 'rifles' which, like their .mil-industrial analogues, toss an equal mass out the back end. The reacting countermass is a bunch of flakes which dissipate the KE against the atmosphere.
How this Irish makeshift recoilless rifle actually works is unknown to me, but the dissipation of KE by the chaff is not germane. The expulsions of some mass (M) at some velocity (V) is germane, as above, but not the way the mass behaves once it has been propelled backward.
The military recoilless rifles are more or less bazookas - their objective is to fire a relatively large and usually explosive shell to blow up tanks, trucks, and other big hard targets, while still being conveniently portable. I'm also puzzled by the "flakes" comments - rapidly expanding gasses are plenty of reaction mass, though perhaps there's some sort of wadding to provide increased gas pressure that gets flaked in the explosion.