SMc case designs are unusually forgiving as to primer choice. I tested 5/35 loads with the 7½, WSR, 205M and BR-4 in one loading (Ramshot 1720, with the old 33-grain Hornady V-Max) with one seating depth. In that limited test BR-4s produced the smallest groups. 205Ms showed the best consistency and therefore have potential; however, 205Ms tend to blank (pierce) primers if the firing pin does not fit the bolt well and when one pushes the pressure too high. I was not concerned with reducing ballistic variation as all tested 5/35 loads are sufficiently consistent for shots beyond 400 yards! We have seen many loads generate single-digit standard deviations!
Note: Usable capacity, loads and performance are similar between the 5mm/35 SMc and the 20 BR. The SMc has the significant advantage of a longer neck – usefully longer barrel life – and likely slightly less barrel heating due to superior trapping of unignited propellant within the case – owing to a superior shoulder design. The 20 BR is a very good, conventional, case design but has no advantage over the 5/35 and is likely to show reduced barrel life and increased barrel heating so is therefore less desirable. (Reforming 6mm BR cases for either design involves precisely the same amount of work – two simple necking steps.)]