I have an FM Products FM9 that is all factory parts except the muzzle brake. Recently FM Came out with a troubleshooting/break in guide. In it, for defensive ammo, they recommend 124 grain HST. Of course, all of my defensive ammo is 147 grain.

I have noticed my AR9 has a little more trouble feeding and cycling 147 grain ammo. Most of what I shoot is reloads and I typically go mid-range on the powder/power level. However, even when I go to full power the 147 grain loads don’t always lock the bolt back on the last round (new Magpul 12-round magazine) and have sometimes some Have trouble feeding.

I noticed that the recommended round, HST 124 grain has a higher muzzle energy than the 147 grain HST; does that translate into more force to cycle the action on a blowback like this? If that’s the case, if I reduce the weight of the recoiling parts or reduce the spring force, would that help reliability?

Thanks.