They are typically in the range of T3-T6 depending on the diameter. T-6 is very common on knives and hardware the manufacture doesnt want you to remove. And T-5 has become more common on firearms lately. Sear's is a good place to pick up a set of small Torx drivers, available in both screwdriver style and bit drivers.
Just don't use hex (allen) wrenches to try and side-step buying the correct tool. I have ruined many Torx screws on old knives doing this; you can get close, but never close enough for any torque without stripping
