A couple thoughts about non-factory parts, gathered from armorer instructors for both Glock and S&W ...

Aftermarket companies usually aren't given the engineering specs for the major components, parts and assemblies as produced by the gun manufacturers (for obvious proprietary reasons).

That means they have to examine parts from guns they usually buy. (Or maybe is loaned to them by a customer?)

To paraphrase one instructor, if an aftermarket company only measures a few guns, and not a thousand or a couple thousand guns, they may not get the whole picture when it comes to various allowable specs and acceptable tolerances. That's probably going to affect the tolerances and clearances of some aftermarket parts and how they fit and function in different guns.

Now, FWIW, we were told that the primary purpose of the plastic sleeve used on the longer Shield magazines is to prevent over-insertion of the magazines and damage to the ejector. Removing the plastic sleeve from the longer magazine and over-insertion can break an ejector.

The buttplates on the standard length mags serve the same role.

So, it's important not to be able to over-insert a magazine ... whether or not it's a factory magazine, or one modified from factory condition, I'd imagine ...

Just a thought.

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Ret LE Firearms inst & armorer