Looking for recommendations

Here are my criteria: 

Co2 powered
semi-auto

metal slide (prefer metal frame, but polymer is ok too)

6mm/airsoft (8mm ok too but I plan on using metal bb’s so 8mm might be too heavy)

preferred metal internals or upgradeable to metal

easy to modify for optimum performance (looking for something I can get to shoot steel or lead at 800-1000 FPS)

14mm threaded barrel

field strippable

smooth bore
adjustable hopup 

I can not legally own a firearm, but I have a family to protect so this is going to be a home defense aid. It is not going to be my only weapon. But I figure a few 6mm metal bb’s to the face ought to cause enough pain to distract and allow me to grab another weapon to cause further damage to an intruder if necessary.