GI brass is heavier, and therefore thicker when sizing her down, You may have to inside neck ream, or you may not. I would check it none the less, as I found on necking down 30-06 to 25-06 with my rifle, the GI cases had to be inside neck reamed to get proper bullet release. It isn't that much of a step down with the 7.7, but check it any way. When fire forming 30-06 to 30 Gibbs, I loaded a practice round of 180gr. cast with a moderate load of 4831, They were loaded a little long where it was kind of a crush fit when the bolt was turned down. Perfect formed cases with the tapper remover and the sharper shoulder moved forward leaving a 7.62X51 (308n Win.) neck length. Never lost a case using this method, and it was a good practice round at 2500 fps. Most of your case forming is down sizing, and just the opposite, so you should not have a problem. Most likely you will just have to check case length, make sure that neck thickness isn't excessive. Load them up and shoot.