I had a spare 1/4 of an hour yesterday so I dug out some old Winchester 45 - 70 cases that i'd retired, annealed the s***e out of them, Imperial waxed them and put them through a CH-4D FL size die. I took a couple of photos this time just to reassure the old dear.
In the photo below you see from right to left:
Full length size die with decapping stem removed.
Buffalo Arms reformed 45-70 brass with stretched necks
Annealed Winchester 45-70 case. The batch that this case is from had been shot over 20 times and was retired.
Similar case after forming in the die.
As you can see it is shorter than the stretched BA case.
The shorter neck should not be a problem but a chamber slug should be taken of the rifle just to be sure.
The necks will stretch somewhat on firing, Winchester brass is very thin so a thicker case like Starline or R-P would give a little more length. 45-90 cases could also be an option but I've never tried that.
You can see that the reformed cases chambers OK and just need the rims reducing to finally fit.
7.62 x 54R can be fire formed in the chamber as the rim does fit but the case head diameter is a tad less so you just need to check the final fit after fire forming, the case will come out around the same legth as the reformed 45-70 cases.
Both should get you shooting though subject to checking your chamber.
