The M3 version of the PMag adds an “over insertion tab” on the rear spine of the magazine. Unfortunately, it interfaces with an area of the lower that can vary substantially during the manufacturing of a milspec lower receiver group.
For this section of the weapon you have 2 sets of tolerances; forging tolerances and machining tolerances. When you add them up the Mil-Std TDP finished product has a range. For over 5 decades, the dimensions controlling the lower end of the magwell have been generous with tolerances. Allowing as much as +-.060” of tolerance in the M16, and even more within the M4 requirements, with the majority of the tolerances in the forging. That means lowers can vary by 1/8” in this area!
BCM lower receivers are built within the semi auto versions of those prints. Any AR15 lower built to the correct print could still have this issue with the over insertion tab.
It is not fully backwards compatible. Based on our analysis, the location of the tab is based on a lower built to the center of the tolerance range, which leaves the possibility of up to 1/16” of interference in some cases. We have gone over the documents many times to ensure this is not an oversight on our part. If it was, we would scramble to make it right. We have not found this to be the case though. Due to the allowable tolerances in the mil-spec prints, the tab will work with some lowers of any manufacturer, and not in others of any manufacturer. This is not specific to BCM and similar situations will arise from many other makers whose tolerances may even exceed the TDP allowances.
The PMag is obviously an excellent design, but this tab has the possibility to cause issues with any milspec lower. As a result it is possible even within the same manufacturer to have it fit some and not on others. If you want an inventory or load out of fully cross compatible mags we recommend you file off just a bit of that tab, because while it may fit in the couple of your lowers, and then it may not fit in the next couple of your buddies AR.
Thanks for reading,
Hope this info helps.
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