Test Results
I could make little sense out of the reslting data
before plotting it. Any given load in the 6/52 gave lower
peak pressure and velocity, compared to the 6mm Rem. However
when the 6/52 charge was increased slightly to give the same
peak pressure, velocity was much higher. (See Graphs in figures 9,
10, 11 & 12 for effect of various loads.) This may have been due,
in part, to the fact that the SMc liked RWS 5341 primers better whereas
5333 primers were used in most tests. We were learning as we went.
Overall performance was gratifying with maximum velocities in excess of 3900, 3300 and 3100 ft/sec with 70, 105 and 115-gr. bullets respectively. Maximum chamber pressure was set at 65000 psi for the 6mm/52 SMc as those cases easily withstood that pressure with repeated reloading. Primer pockets stretched on the 6mm Rem. when the pressure exceeded about 62000 psi. It should be noted that the thickness of the brass between the primer pocket and rim relief is about 41% greater on 404 Jeffery basic cases than 6mm Rem cases so they could be expected to withstand more pressure before yielding.
Just to be sure, we checked the acceptance data on the lot of 404 cases used. They were very slightly (about 1.5%) softer than the 6mm Rem cases as determined by Vickers hardness. Norma runs Vickers hardness as an acceptance test on every lot of cases manufactured to assure uniform rim and base strength and hardness. Data was immediately available to us even though that lot of 404 cases was manufactured several years earlier than the 6mm Rem cases. Unless you have spent hundreds of hours searching for lot acceptance data in pursuit of a failure you cannot appreciate what a tribute this is to the Norma quality control system.