Another possibility is a .218 Mashburn Bee. The blown out Bee case should give over 3000 fps with a 40 grain bullet, a bit less with a 45. With the boattailed polytipped bullets of today, the ballistic coefficients on such bullets are significantly higher than the earlier styles with which the Bee was originally loaded. Not a .22-250 or a .220 Swift, but a reasonable 250 yard varmint rifle.