similarly, the larger version is intended to produce 4500 fps with this bullet.
Moving to larger calibers, the 284 Winchester or 376 Steyr case are the correct diameter for use with 22-caliber SMc designs. Trouble is, when we sufficiently shorten either of those to achieve a reasonable capacity, the case wall becomes too thick at the shoulder (thereby preventing the desired two-to-one powder-column to bore-diameter ratio). Case shoulder and neck also become increasingly difficult to form and excessive neck turning is then required.
One correct solution is redrawing of cases, to shorten tapered portion of case body wall and to reduce minimum body wall thickness (because SMc cases are necked down so far, case neck thickening is unusually great, beginning with a thinner case wall mitigates this problem). We could solve this with the help of Buffalo Arms (208-263-6953) who specialize in redrawing cases. They can modify cases to suit our needs, see below.
Similarly, the new WSM and Short Ultra cases are ideal for 6mm hunting cartridges. It looks likely that we will be able to simply neck and shorten one of those basic cases sufficiently to provide a case with capacity similar to the 6mm Remington. Simple conversion to a case with capacity similar to the 6mm-06 is almost a certainty.
These cases will also work (marginally) for a 25-caliber version but we have not given that bore size much consideration, despite our personal biases toward the quarter-bore, bullets now available in 6mm and 6.5mm sizes suggest very little real need for a 25-caliber cartridge, either in hunting or target work. (A preferred deer chambering of Smalley's is the 257 Ackley Improved; his 120 load in his rifle outperforms many typical 270 Winchester loads.)
Cartridge designation in the SMc system is as follows: bore diameter (caliber or mm), followed by a slash, then case capacity in grains of water – as measured to interior juncture of shoulder and neck. Since, in SMc designs, this juncture is where the bullet base should be located in the loaded round, this number essentially equates to usable case capacity (excepting the issue of a slight variation depending upon bullet base configuration – e.g., boattail).
Because bullet base location is relatively critical in the SMc design (too-shallow seating is not particularly harmful but too-deep seating is quite detrimental), chamber throating must match intended bullet length (or bullets of similar length) else some bullets that might be used in the chambering must have a significant jump before engaging the rifling. This is essentially opposite the practice in traditional chamberings, where cartridge overall length was generally held as the important issue. For this reason, SMc reamer design is complicated.
Our 6.5mm testing forced use of the largest diameter case that is readily available, the 416 Rigby. Our initial foray into building such a rifle (McPherson) is the 6.5mm/60 SMc, a long-range target design. As the name implies, this is a 6.5mm cartridge utilizing a case holding 60 grains of water to the base of the neck.
To make these very short cases, we had Buffalo Arms redraw a batch of 416 Norma cases to shorten the tapered section so that it ended at a point just over 1-inch from the base of the case and so that the case wall from that point forward was 0.010-inch thick. Since we did not know exactly which 6.5mm cases we would form from these stretched cases (we have also designed