There are lots of ways to shoot rifle grenades. Some shouldered (ouch!) and some not (still ouch!). You're shooting a 1 pound projectile with enough velocity to go a few hundred yards. It really doesn't matter how you shoot it, there will be some recoil to deal with! Buttstock on the ground holding the rifle at an angle? You're still going to know it!
Before anyone tries to do this....1) be sure you've got the right ammo, and 2) get a little training. Talk to folks who've done this in the military (easy if you have IDF friends! everyone should have IDF friends
), And do some reading.
They are a blast! Target shooting is great, but playing mini artillery and trying to hit a target that way is a fun challenge. But, as always, be safe. And just because they are "training" grenades, treat them the same as any pojectile coming out of a firearm. Again, 1 pound rubber tipped thing with that much velocity...be careful.
Here, take a look at this:
http://www.youtube.com/watch?v=caKZ0bawcgQ
He's doing the "rifle under the arm" type shot. Watch how much that rifle moves. You can bruise your hand, get snagged by some mechanical bit sticking off the rifle, etc. Oh, rifles like FALs have a setting on the gas port for grenade use. Keeps from slamming the action. I never liked shooting RGs off my AR, always was afraid the action was being hit too hard.