That's unfortunate. Wonder if you just got a bad barrel? I've got 450 rounds through the 4.25" 9mm Storm Lake conversion barrel on my M&P 40 FS. It eats everything I have thrown at it 115 & 124 gr FJM, JHP. WW, Remington, Federal, Fiocchi. Even mixed the various rounds in a single mag. Not a single FTF, FTE. Groups every bit as good or better than the factory 40 barrel. (Though both shoot better than I can hold
)
Hope you get it sorted out DanTana.
Be well,