6-groove rifling. You might be able to tell cut or button rifling from the surface finish at the muzzle end (usually minimal erosion on that end). If you can see traces of super small "grooves" in the grooves of the rifling, we're talking nearly microscopic scale here... then it's likely cut-rifled, and that may narrow it down a little bit. HS-Precision makes cut-rifled 6-grove barrels. Others will, too, but typically you see 4-groove (brux, Krieger, etc.) or 5R (Bartlein). If it looks like a mirror, it might be button rifled and it's anyone's guess. Wilson, Douglas, Benchmark, Mcgowen, etc... Most of them honestly are 6-groove.