I recently purchased a 300 h&h mag mod 70 pre 64. A factory load or handload won’t chamber or really come close. It lacks a good 1/8 in to 1/4 in for the bolt to close.  I took the gun apart and the underside is stamped 300 M 5 on the barrel. The receiver has a proof mark the barrel doesn’t. It says Winchester proof steel and 300 h & h mag on it. The gun was made in 53. I don’t think it’s been rechambered to 300 weatherby or anything bigger being the h & h case won’t drop in or come close to chambering. I’m no expert by any means. I would think the h & h would drop in to anything it could have been rechambered for. Does anyone have any idea what the problem could be? The serial# is hard to read but it appears to match the guns serial # .  The rifling looks very good.