The SMLE magazine dictates the type of cartridge applicable to the action. The magazine has significant taper from front to back and needs a cartridge with taper to feed...hence Lon Paul's choice of the .348. Of course, if used as a platform for a single shot, none of this applies. My choice for a larger caliber for the SMLE is the 35 Win., although the OAL must be shortened a tad to fit the magazine. I've done a couple of these and the rifles feed well without magazine alteration. A 250 .358 bullet at 2200fps mv is an adequate load for most hunting and shoots plenty flat for the iron sights typically used. Dies, brass, and bullets are readily available as well. Old Western Scrounger has 45-70 single stack magazines for no.4 SMLEs for $75ea, which may make larger caliber straight case cartridges a viable option...but I've not tried this. Since so many great bullets are available in 358, including a plethora of pistol bullets, I'm a strong advocate for the 35 Win option.

Otto