More info on Kynoch Ammo is at http://new-kynoch.apt-sites.com/6-5MS.htm. Barrel length was 26". They quote pressure at 17.5 Tons per sq inch. Assuming a UK ton, thats 2240 lbs per ton, that equates to 39,200 PSI. Haven't a clue whatt powder they are using, but probably something slow buring with lots of shove but not much pressure. The converse to short high presure cartridges from the 308, 260, 7mm-08, 243 type.

They also produce the 9.5 MS, or 375 Rimless.