I dithered and dathered awhile on these choices; but now I've tried both the Stern (semi-permanent) mag block adapter and the Endomagand here are a few comments. The Stern is a very well built and finished unit with a seemingly solid hold-open device. A tip for installing- insert the unit with the upper and lower together to keep from seating it too high in the magwell (important for working well with my FM 9mm upper and bolt). Then snug your 3 screws (the larger one keeps the unit "glued" to the side of the magwell, so even if you hit your regular mag release by mistake, the unit won't be dumped on the ground). In any case I've fired about 150 rounds through several new and old Glock mags in the Stern block with perfect function including lock-back on empty.
The Mean Arms Endomag is a rather flimsy looking and feeling insert for 30 round gen 2 or 3 Pmags (the ejector is particularly flimsy). Only fired around 50 rounds so far. Have had perfect function so far including bolt lock-back on empty. However, examination of the plastic ejector afterward made me dubious of its long-term prospects. I think Mean Arms has a lifetime warranty on these?
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Proud veteran of the Naval Security Group
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