Out of Spec lower receiver

Jig Stick

Gunny Sergeant
PX Member
Aug 27, 2010
Pittsburgh PA
How can you tell if a lower receiver is out of "mil" spec? I have two Bushmaster lowers that I would like to register as SBR's, but have heard that Bushys have a reputation of being "out of spec". I have used both of these lowers on 3 different uppers without any problems fitting them together or functioning. What should I be looking for?


PX Member
Oct 25, 2013
Blaine, MN
I tend to look at it this way:

If the lowers are functional with the parts that you would use on an SBR, then they would be good to go for the Form 1 process. If you have malfunctions that you can trace back to hole tolerance or spacing issues, then ditch them and pick up something else. Malfunctions might be things like trigger issues (i.e. no reset, doubling, pins walking out), mag catch, takedown/pivot pins not working as intended, messed up threads, etc.

Lowers are cheap and plentiful now so you would easily be able to find replacements.