303 Newmag: Picking Up An Old Idea

In 1920 Kynoch introduced the 303Magnum (bottom). This cartridge was based on the case of the 276Enfield (top) which was intended to become the new British service round when the First World War broke out and it seemed to be more reasonable to abandon this plan.

The case of the 276Enfield is of very interesting design, because base diameter to body length ratio is rather large and appears quite ‘modern’. The main proportions are nearly identical to the standard belted magnums. It would have been an excellent basic case for a lot of potent cartridges. Since the 375Ruger (a rimless case with .532 base dia) is available we can observe that phenomenom.

The 303Magnum existed in a rimless and a semi rimmed version. It served mainly as target round and for trials with boat tail bullets for the military.

Read more:    303-magnum-rimless


It would be interesting to get some information about the rifles in that caliber, too.

If it would be still available wildcatters would have modified it in several ways. If you believe that it could make sense to pick up this cartridge with its fat .30 caliber and convert it to a modern shape you will only find the 8x68S case and –due to it’s actual (Check it!) base dimensions- the 375Ruger for case forming. The .30Newton would be closer to the rimless original but it’s a collectors item, too.

A formed, trimmed, neck reamed and fireformed case has an inside volume -including neck- of 82grs. of water. When loaded to the same pressure it is as powerful as the 300WSM/300SAUM.

Gun writer Ken Waters necked down the 350 Remington Magnum to 312-caliber in the late sixties to get more energy out of surplus 7.65 Argentine Mauser rifles. It is called .312 Express:

It offers an inside volume of about 75grs of water and is slightly more powerful than the 30-06 Sprgf.