As stated above, if the action is locking up completely and there is a good firing pin strike, the issue may be a high velocity variation. 43 grains of H110 is listed as maximum on Hodgdon's site with 350-grain bullets, but your rifle may not like H110. I used H110 in two of my .45-70s (Sierra data) some years ago and got terrible velocity spreads on 5-shot strings.
I would try either H4227 or LilGun - both are a bit easier to ignite than H110. Make certain you are using a hot primer - Hodgdon uses a WLR, a pretty hot standard primer. Alliant recommends AA#9, which may work better too.