Mine is a 1:12 also, but I did stumble across a bit of a heavier bullet that shoots OK out of it- the Win 64gr Power Point. I mean, it's not great, but they manage to stay right around 1" @100 with Varget, CCI Small Rifle Primers, and full length sized PPU brass. I kind of doubt I'll be buying any more of them any time soon, as I mostly just stick with 55gr V-Max for what I do. So, there is a chance you can find something that you can live with that's heavier.
Also, check to make sure your action screws are tight.