I recently installed factory 4.5 barrel on my sbr MPX Gen1. Using a Griffin 3 lug adapter and sico Octane 45 k with new 3 lug. I took a class this weekend and brought 115gr Federal range ammo and fiocci 124gr flat nose. The gun would not group consistently at all with 115gr. Like 6 inch group at 25yards! Checked everything and noted no issues with loose suppressor, barrel was properly torque by me with fat wrench and was still tight etc. Switched to 147gr fiocchi and worked as it should with nice little 1 inch groups off hand for the rest of the class. Anybody deal with issues with this ammo and short barrel before?

I have not had much experience with the ammo you mentioned. But that barrel I have.on a mpx gen 2and once I got the kinks worked out of the gen2 where it would work.

I took that weapon with that exact same barrel of coarse with a different suppressor. I used a SilencerCo HYBRID suppressor.

I fired 4 different kinds of 9mm ammo that day through it to test weapon. Even though they were all the same weight 124 grains. I got different results.

I shot as follows
TULA
WOLF
SPEER GOLD DOT
WINCHESTER NATO +P you could really tell a different in this ammo when shot suppressed.

You'd think the tula and wolf with them both being Russian ammo would have shot pretty close but they didn't.  Tulsa averaged about 3 to 4 in group
WOLF anywhere from 3 to 6 inches
The Winchester and golddot both shot pretty much exactly alike. They shot about a 1 to 1.5 inch group.

I was shooting from about 30 yds or so using a SIG red dot. I could have done better but I was primarily there that day to just see if that darn mpx gen2 would work after I applied my fixes to it.

Which it did with flying colors I might add and I'm extremely happy it now. It will shoot any thing I feed it without hesitation in the least bit. I shot another 150 rds through it the other day.

I still had not cleaned it from shooting the 540 rds through it. So that makes 690 rds so far without cleaning and not one ftf, FTE not one malfunction. I might just keep going just to see how many rds I can shoot till I get a malfunction

But my point is it just may be the nature of that barrel and rd being used. Shoot it some more to break in the barrel. Then see if that makes any difference. I bet after you reach say around 500 to 1000 rds with that barrel it will shoot like a laser with most anything you shoot through it.

The first 200-300 rds none of the ammo I shot grouped very well. But there at the end even the tula and wolf tighten up alot to around 2 inches. But I ran out of that ammo so I can't confirm it. I need to buy more to smooth everything out anyways

But keep shooting it. The more you do the better it will get and the more you'll get use to using it thus the better you will get shooting it.

The only thing I had problems with on 115gr federal was ftf a couple times. I couldn’t attribute it to the ammo itself as I piled and tried some more 115 then went to 124. Ran fine after that. Never had any grouping issues. I run a obsidian. Dunno if that helps.

Thanks for the replies. I will run it more to see what happens but I’ve never had such inconsistency with 115gr in any PCC.