I traded for a 25-5 (made in 1979) a couple of months ago and with a calipers (not the best way to measure but that is all I have) the throats measure about .454. when I find a set of plug gages (not a high priority) I will get a little better measurement. the cast bullets I shoot are .452 and they seem to to just fine. I shoot mostly off hand so whether it shoots 1 inch groups or 3 inch groups at 25 yards probably won't make much difference. I'm not sure it is anything to get excited about. I might buy some .454 bullets (not a high priority either) sometime to see if it makes a difference that I can. it sure seems to shoot good just that way it is.