That's what I spent the afternoon doing .

Customer #1. Will this work in my DPMS? (20rd AR magazine)
Salesperson. (picks up package examines, checks all fine print) No.
Me. Excuse me, what caliber is your DPMS?
Customer #1. .223
Me. Yes, it will fit.
Salesperson #1. Be sure to keep the receipt in case it doesn't fit...