Malf

This will sound more combative than it is meant.

If a gun requires, "some effort and diligence to learn how to properly lube and tune them, and to develop handloads with a specific combination of quality components that would run each gun flawlessly," then it will garner a relatively bad Malf number in GURPS.

The GM should let the player make a Guns/TL roll based on IQ to do the proper work on the weapon to alleviate some or all of the Malf number.

The SEALs with the Stoner are an example of where this works.

The much maligned Chauchat is another, believe it or not.

A person intimate with a temperamental gun should be able to run it better than a novice.

It'd be up to the GM to decide if a gun is a poor design or just a temperamental one that needs more TLC.

Coming back to the Chauchat, the magazines are fragile.  No amount of TLC is going to bring a broken one back.  That would take Armory/TL (Small Arms) to repair.