Why Some Mag Wells Have No Rear Grooves And...

[BrianQ]

The grooves are for the locking lugs on the rear of the magazine. Why some trigger housings have full length grooves and some have partial length grooves is due to the tooling and milling process the particular manufacturer used. It's the same reason why some receivers have a square mill cut on the right side of the receiver and others have a round mill cut. There are a plethora of differences in like carbine parts that are directly attributed to the tooling the various manufacturers had on hand or could obtain.