I went to the range today to try out some new loads for my 505 Gibbs. On my fourth shot, I got a click but no bang. I waited about 1 minute before opening the action and was surprised to see a case with powder spilling out and a bullet stuck in the first 1/8" of the rifling. The powder was yellowed and partially caked together indicating at least a partial burn. Load was 125grs of H4831SC with a 525gr bullet. Primer was CCI LR magnum. Brass was brand new and there was no contamination to powder or inside case, I'm positive. Bullet knocked out easily but I would still like to find an explanation on why this happened. I've been reloading for over 40 years and seem to remember this happening once before, also on a large capacity case of a 458 H&H or a 375 H&H. The powder that I used was new that I bought about a month ago. Anyone have this happen to them or a good idea what went wrong?