Jackal,
Same issues here with my K running a stock Sig 4.5” barrel. 115 Critical defense shoots best. 147 Lawman opens up to ~2” plus at 50 yds and throws 2-3 fliers out at 4” in a 10 shot string.
I’m running an SRD9-MPX fixed thread on mine which makes things even worse from what I’ve tested. This past weekend I tried the following loads:
147 gold dot
147 HST
147 Vcrown
147 superVel
My MPX w SRD9 was throwing them all over the place at 50yds. By that I mean ~12” either side of an 8” plate. Two shooters, off a bench, different loadings.
115 Fiocchi 9AP hot stuff shoots like garbage.
115 Aguila ball runs ok, better without the can, but not great.
I will be checking torque on all of the screws and plan on playing with the baffle orientation to try 50/50, all the same, and alternating baffles to see what that does.
I’m curious about how the gas port sizing may be impacting things.