There are specific conditions that signify a non functioning firearm. What you describe as the rear end of the frame blown off may or may not meet the BATFE criteria. beetr to ask them what they consider non functioning and non repairable.

If they consider it repairable (forget cost factors) it would still be a firearm.

Remember these are the guys that say you have a machine gun if the semi auto weapon you have can be be readily converted to a full auto weapon with 8 hours in a machine shop .
Do not go by your own opinion as to whats inert/deactivated etc.
then get the agents name etc for future reference.

Brian
LTC, USA Ret.
NRA Patron Member
AHFGCA Life Member
USPSA Life Member