I have a homebuilt, it consists of a Stag lower w/ DPMS parts kit, and a RRA complete upper.  My mag block is the VM HYTECH.  So, I seat the mag block correctly, give it an extra whack, and start firing.  After a while, the mag block will work itself out and start to come out.  I can't figure out why it is doing this.  Any ideas?  BTW, after I seat the block, I do tug on it forcefully to see if it will come out.  It never does.  

Bueller...Bueller

Tip the upper open....

Install block to the magwell and verify it is caught by the mag catch  ----> I have already personally seen the block not install fully to the magwells' of spme lowers because of varying magwell depths (STUPID MANUFACTURER VARIATIONS)  from the deck of the lower to the bottom of the magwell... ArndCtzn had his block trimmed on a mill, rather than cut lower, and mine required a few swipes with file to the bottom of the front of the magwell to allow block to sit deep enough to latch via the mag catch.  My block fits fine in a friends factory Colt M16A1, GEE, wonder why...


Close the upper to the lower, and install the takedown pin,  install mag and fire..  the portion of the block that sticks up above the deck rest in the cut-outs of the upper, and the upper applies slight downward pressure against the mag catch, and you find the catch in a slight bind, so you cannot readily dump the block without a bit of efort to the mag relaese button.