I've got a 1-8 twist T3x stainless LH that shoots 70 grain Barnes TSX very well! Fine shooting iron.
I think they only made a small run or 8 twist's. All I see now are 14's. Hopefully it will handle a 55 grain. My old Remington would handle a 55gr SP at .734' long, a 50 grain Nosler BT at .784" but not a 55 gr VMAX at .817" long. It didn't key hole, just didn't shoot well.