Older Guns Still Work

Danger Room has a piece about the Syrian rebels using old (or copied) American recoilless rifles.

If you watch the second video in the clip, you'll see them using the spotting rifle that is attached to the weapon. The spotting rifle uses a .50 cartridge that is ballistically matched to the 106mm round. So you'd shoot the spotting rifle, get on target with it (the projectile generated a puff of smoke on impact), and then let fly with the recoilless rifle.

IIRC, the projectiles had pre-engraved driving bands to match up to the rifling. Might have been something about how if they didn't do that, they couldn't be counted on to be able to balance out the recoil and counter-recoil effects, as the amount of effort it took to normally engrave the rifling on the driving bands had too much variability. Maybe they also needed to match the vent holes in the chamber with the perforations in the cartridge casings.

One issue with shooting them a lot was that the jets in the recoil compensator part would wear out, allowing even more gas to fire to the rear. When that happened, the gun would start to move forward on firing.