Hi, I asked this question, as a 45-90 pedersoli was up for sale, and the seller said that he shot 45-70 in the 45-90, but further research told me about loss of accuuracy, and possible chamber damage due to the shorter overall length. The possible chamber damage and accuracy loss, caused by the bullet not being far enough near the rifleing. Hope this helps, hope to be corrected if what I have said is wrong. I got a 45-70 instead.
You'll probably have some loss of accuracy due to the longer bullet jump (0.3 inches shorter case), but I would be surprised if you experienced chamber damage. It's common to shoot shorter cartridges in longer chambers (.45 Colt in .454 and .460, .38 Special in .357, etc. ). My experience in the past was simply that you need to make certain if you are shooting lead bullets that you clean the leading out of the chamber before resuming use of the longer round, or you may experience difficulties with chambering and potentially higher pressures.
One of our local guys created a short .45-70 round to be legally used in Indiana's deer season (1.6 inch max case length allowed) in his .45-70 rifle. It actually shoots quite well, and he took 3 (I believe) deer with it this year.
If you use heavyweight bullets (as in Lymans BPS 500 gr) and seat them farther out in the case to where the loaded length is about the same as it would be in a 45-90, accuracy loss would be minimal if at all. I do this all the time with trimmed 45/70's in a 45/70. I load these short cases with a 500 gr jacketed RN bullet, seated out. Just avoid using Hornady brass, as they're all shorter than R-P and W-W 45/70 brass.
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