I've got one that I bought a couple of years ago. It came with about 5 different caliber inserts for a firm fit in the muzzle, ranging from .22 to .50. It works quite well, and will certainly get you in the ballpark before you've fired a round. Just make sure you remember to take it out of the bore BEFORE you try shooting! IIRC, I think I paid something like $39 for it, and it was money well spent.