“Ok, so tried to chamber .243 win and it (almost) chambered, I see that the case length is just barely longer than .308 so I am now 99.9% certain. .308 is my man, but I'm doing a casting just for good measure”
Basically and actually are two words I avoid, if the chamber was a 308 W, the 243 W would chamber. Both have the same case dimensions with the exception of the neck, it could be said the 243 is a necked down 308W.
Then there is the feed system, many shooters with a lot of experience feed rounds from anywhere, I only feed from the magazine box. If the rifle has a claw I treat the rifle like it was control feed (Mauser design), if there is a chance your are chambering rounds into the chamber and then closing the bolt like the rifle was a push feed design the extractor could be wadding and not allowing the bolt to close.
Wadding up as in the extractor claw can not jump the case head extractor groove.
The chamber, just because I can not reach ‘it’ dies not mean I can not measure it, but if I had to I would use silicon, lube the chamber, plug the bore at the beginning of the rifling then fill the chamber with an extended tube.