I concur, wrong twist rate is probably the culprit. What powder/charge are you using? Have you cronyed the rounds? Also, there is the possibility of the threading for the suppressor being off center of the bore (as opposed to the barrel centerline). If your twist rate is the 1/8 advertised, take the can off and see if your prefered load still keyholes at 75+. If it doesn't, send the barrel to a more reputable smith, I'd suggest Noveske for AR work, and have the barrel threading re-done for the suppressor. If that's not the issue, find someone with a bore scope and look for burs in the rifling or throat (pay special attention to the gas port area and muzzle). If that isn't the issue, try some shorter pills and get the velocity up close to 1050--if you're not already there.
Outside of that, I'm not aware of anything else that could affect stability.
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