Some other techniques that I use for long gun retention:
-over/under grab: continue forward pressure and muzzle strike the subject in the sternum
-under grab: rotate the weapon, continuing forward pressure, and strike subject with receiver area of weapon
-over grab: continue forward pressure as you direct muzzle down and strike subject high with butt end of weapon
I favor the use of heavy forward momentum to increase the balance advantage in favor of the weapon bearer, decreasing the possibility for the attacking subject to establish a solid base.
A good exercise for any form of weapon retention is to get a partner to do a grab at reduced power/speed, then try to feel the energy he's giving you to find a way to redirect the weapon to your favor (either with a strike or by discharging the weapon). For those familiar with the Wing Chun kung fu exercise of Chi Sao (sticky hands), it's the same sensitivity/energy drill with a weapon.