A common problem that I have with Sigs is a slightly twitch to the right when the trigger breaks.
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I thought I was crazy.
My M11-A1 does this also.
Or - I do it ONLY with that particular pistol.
It shows on the target.
I did not notice it untill I practiced dry firing.
I do not do i with my P-220. Thats a hugely different grip though.
Thanks for the comment