A 125 gr boat-tail, be it from Nosler or Sierra, has a pretty short shank length, so I doubt if seating short enough to run in the magazine will be an issue. The .30/30 has a very generous neck that will accomadate bullets that are quite long, I even load 195 gr cast spitzers in mine, without any excessive neck trimming, although I single load them due to the tube magazine. For your 125 gr loads, you can use the 130 gr data from the Hodgdon site without getting into trouble.