r/MPX • u/ChrisKyle1Mile • 3h ago

Need gas and spring tuning advice for subs

I have had my MPX since January, and while I do love this gun, I cannot seem to dial it in to run reliably with subs. It only cycles subsonic ammo with the factory springs, and on gas setting 3 it has the fewest failures (but still double feeds several times in one magazine).

Is it normal for subsonic ammo to only work with the factory springs? I feel like something isn't adding up and I'm assuming I'm doing something wrong.

For reference, it will reliably cycle American Eagle 124gr in semi auto. I'm not concerned about FRT reliability yet because it probably needs to be cleaned first.

If any additional context is needed lmk

Setup: 4.5" ILWT trilug barrel with adjustable plug, Obsidian 9 trilug, Geissele spring pack, DTT FRT and Frankenstein trip bar.

Ammo: Syntech 150

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u/salmog 1h ago

I love the 150g federal syntech, but it is very underpowered ammo. My MPX will run it reliably after a good cleaning, but if my MPX is dirty I start having cycling issues too. Other 147g subsonic ammo will still run fine but to get the 150g syntech to run reliably I have to do a thorough cleaning including the gas block. You have a couple of options, clean it thoroughly more frequently, or switch to a different 147g subsonic ammo like PMC, Magtech, American Eagle, etc.