[CowTowner]

could 30-40 krag brass work?

Yes and here's some info: http://www.303british.com/id37.html

Many reloaders trim their brass to minimum dimensions according to SAAMI specs. A better method is to make a chamber cast and accurately determine the maximum case length for your particular rifle. I have two Lee Enfield No.4s that have chambers of 2.26" in length. That's a full .05" longer than the standard trim to length for the .303 British of 2.21". For these I select over length brass and trim cases to a uniform 2.25. On way to get longer than average .303 British cases is to reform .30-40 Krag brass. The Krag cartridge is nearly identical except about .2" longer than the .303 case, and can be run through a .303 sizing die, then fire formed in your chamber. I often fire form untrimmed Krag brass using a light charge of pistol powder such as Unique and then fill the case with an inert filler like Cream O Wheat. Cases formed in this way will show the actual length of chamber (impressed into the expanded case neck) of your rifle and can then be trimmed to fit it precisely. The longer cases are more accurate with lead slugs because they support the bullet better as it enters the rifling's leade. (Krag brass is slightly larger at the head, and reformed as .303 British will also eliminate the unsightly bulge that .303 cases often show in large-chambered rifles) Cast bullet loads do not usually cause much case stretching, (especially if you have tightened up your headspace) but I check my extra-long cases every time I reload them, just to be sure.