Still it’s an interesting topic from a survival(ist) perspective. You wouldn’t want to try it if there’s no necessity to take the risk but someday or in some circumstances it may be necessary to try it. I would hazard to guess that if the story about the guy who routinely fired factory 30-30 rounds in a 303 were true, it’s probably because the larger dimension of the Savage chamber at the base caused the case to instantly shorten as it expanded so that catastrophic pressure at the case mouth was avoided.
Cartridge dimensions in loading manuals and reference books are chamber dimensions (don’t forget) and loaded cartridges are always some measure narrower and shorter than chamber dimensions. The 30-30 chamber is additionally .020 narrower at the base and .025 longer (1/50” and 1/40”) respectively than the 303S. Either is roughly the thickness of the paperboard used to make bandaid or cigarette boxes. The rims are effectively the same, .505 and .506, they do not expand so are not a factor.
I haven’t tried it but I predict that the 30-30 shoulder is further forward enough that you couldnt chamber and fire a factory 30-30 round in a 303S chamber (being a lever action). I do think that you could take a 30-30 case and essentially neck size it by running it thru a 303 sizing die, trim it to 2.015, load it using the 303 die and then you could fire form it. There would be a substantial pressure ring and you could probably only do it once but I think that it could work IN AN EMERGENCY
