I have a cartridge (about .312 caliber) that has a headstamp of ELEY'S SIMPLEX. The case is .648 inches long and is .332 inches at the base. The strange part is that the rim is .446 inches in diameter.

Can anyone help me identifiy this cartridge?

Thanks

the_captn

I'm not certain (I'm away fom my collection at the moment) but it sounds like the .303 adapter round. This was used for short-range practice with .303 Lee Enfield rifles and was designed to fit into a special adapter which was shaped like a .303 round on the outside, and had a short chamber and barrel on the inside.

Sounds like you have the cartridge for the Mullineux Patent .303 Adaptor aka .303 Simplex. It comprises a .303 case shaped steel insert chamber with no rifling , and a cut-away part way round the rim circumference to allow the fired case to be extracted with the fingers . The cartridge has the wide flange type base and a .311 cal lead projectile .. It was available as a kit from Kynoch for the 303 Regulation Rifle , and contained a short rod for prying out stuck cases as well .
Hope that helps , it is one of several .303 adaptor type cartridges available .

Thanks for the info

the_captn