Q&A for Cross Armory Safe Magazine — 2 Questions & 1 Answers

Depending on which type of upper receiver you have, they will have slightly different sizes. The Aero model is slightly smaller than the DPMS version. According to Cross Armory’s website:
Depending on your build, you will need to choose between the AERO & DPMS variations. Please see instructions below on how to measure when choosing between AERO and DPMS if you do not see your model listed here:
The AERO variation will work on the following models: Aero Precision, Anderson AM10, Armalite A4, Christensen DMR-10, Christensen CA TAC 10, Lewis Machine Tool (LMT), M&P 10 (needs extra spacer, please ask for in notes at checkout)
The DPMS variation will work on the following models: Alex Pro 308, Bushmaster BR 308 (may need fine tuning), CMMG MK-3, DPMS LR308, Fulton Armory 308, Jugernaught Tactical Lower, Noveski N6
HOW TO CHOOSE BETWEEN AERO & DPMS
Determine the width of the base of your upper receiver:
Separate your upper and lower receiver.
Use calipers or measuring tape to determine the width of the base of the upper receiver (measure are of base directly above the trigger)
If the upper width at the base is 7/8″ or 0.875″, choose Aero. If the upper width at the base is 1 1/4″ or 1.25″, choose DPMS.