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I screw cut the barrel of my Sako Varmint 75 22-250 (5/8" x 24tpi) Friday and took it to the 100m tunnel range to zero it with a new sound moderator this afternoon. Last time out I zeroed it with 55 grain Vmax and it was spot on, this time it scattered the 55 grain bullets over a 1.75" group around my old zero. Somewhat horrified I sat back and the Viking tried it with some of the last of my 50 grain Vmax and promptly shot a small thumbnail sized group. OK, my turn and I did the same so it would appear it does not like 55 grain bullets now the sound mod is fitted yet performs flawlessly with the 50 grain.

Any thoughts on this anyone? The sound mod was tight for the duration, if anything the fit is a bit on the snug side thread wise however it bottoms on the shoulder. I clocked it in at both ends of the barrel to as good as it gets before screw cutting and the loaded ammunition are known and proven loads.

Barrel harmonics spring to mind, there's a change of POI on all my rifles with the Mods removed, not found any difference in grouping though.

My 222 wont shoot the 52gr amax for poop with mod, no mod it shoots fine. 50gr vmax shoot great through mod or not.

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My immediate thought was barrel harmonics and it is interesting that the point of impact with and without the mod remains the same or within a 1/4" for the 50's whereas the 55's are awful. Luckily I prefer the 50 over the 55. Someone at the range did suggest tying something in the 40 grain bracket however the rifle is in constant use so the throat is well on the move now.

I never could get the 55s to group with my 12 twist barrel with moderator but ragged holed using the 52 amax's
To be fair I never tried it without the mod as it was always going to be a night shooter on the edges of built up areas
Would say it's a harmonics issue and will require a revisit to the load development if the 55s are to be used, tweeking seating and powder

Duey

Based on what I've been told, it's just down to the timing of when the bullet leaves the barrel and where the shock-wave is at that point. If the shockwave is at the crown when the bullet leaves the barrel, it will scatter the POI around the target. IIRC and on average, the shock-wave bounces between the chamber and the crown 60 odd times before the bullet leaves the barrel.