I do not believe the pressures were high or the cause of the case separations.

I was using reasonable loads of IMR-4227 in a straight walled case.

I think the problem was caused by the case mouth being unable to expand to the side and being stretched forward by its grip on the bullet in the tight confines of the chamber throat.

BTW - My B.S.A. .310 Cadet Martini's bore diameter measured .317".
The chamber was original, not modified.

(IIRC - Kynoch factory ammo used a 'Heeled' projectile with an approximate base area diameter of .310". The exposed section of the bullet, in front of the case mouth, measured about .318").

I mention these details because some of Baltimoreed's brass has similar looking 'stretch' marks, (where the wall meets the web) to my brass that separated.