the 6.5mm/70 SMc, which is a hunting cartridge), we had Buffalo Arms leave these blanks quite long.
To form the 6.5/60 case, all that was required was to shorten those cylindrical blanks, then carefully square and deburr case mouths, anneal mouths, run each through press (using appropriate neck-bushing dies) eleven times, re-anneal necks, slightly expand necks to fit K&M Services outside neck turning mandrel, turn necks, trim all to final length, deburr each, load and fireform all. (We omitted various critical steps from this short list!) One might conclude that breeding carbide toothed beavers and training those to whittle productively on hardened brass might have its advantages!
By minimizing body taper and using a slightly loose chamber base, we can also use the modified 416 case for 7mm versions of the SMc cartridge. For a 30-caliber SMc, we would need a ready supply of cases with a base diameter similar to the 505 Gibbs. For this and larger calibers, it seems unlikely that any currently available mainstream receiver would be appropriate. We recognize that this is a legitimate problem but it can be solved.
Meanwhile, until we can get Norma to produce special, short, thin-walled cases (in the style of the 6mm BR Norma) factory-necked to the desired caliber few shooters will want to get into this game. For now, it is far too much like copious quantities of hard work! Moreover, with the difficulties and extreme measures required in case forming, significant accuracy penalties (resulting from limitations in fireformed case quality) seem inevitable.
Preliminary
testing
One of us (Smalley) has considerable experience with two 22-caliber
SMc designs. We throated the 22/28.5 SMc for use with 69-grain bullets;
we designed the similar 22-29.5 for use with 52-grain bullets.
We intended the former to compete on an equal, usable capacity, basis with the 223 Remington. In this number, we have found several loads that easily launch a 69-grain bullet (sans moly) at well over 3200 fps from a 24-inch tube. Considering the unconventional use of the 6mm BR case, where case head is fully 0.030-inch smaller than chamber diameter, and the fact that these Norma cases are not necessarily the hardest ever produced, this performance level seems sufficient to prove the ballistic merit of this design.
The latter, a similar version intended for benchrest, easily generates more than 3600 fps with 52-grain bullets, using powder charges near 28.5 grains, depending upon powder type used. To date, accuracy is disappointing owing, we believe, to problems with case quality.
Preliminary results with the 6.5/60 are interesting. This number easily launches the 130 Norma Match bullet (moly-plated) at 3400 fps from a 28-inch Pac-Nor tube. These loads resulted in precisely zero additional case-head expansion (fireforming load had induced between 0.0010- and 0.0015-inch case head expansion). Christer Larsson (Chief Ballistician at Norma) believes that lack of such expansion in these particular Norma-based cases suggests that peak chamber pressure must have been less than 70,000 psi. While 70,000 psi is certainly significant pressure, it is not far above pressures used in many modern cartridges. QuickLOAD suggests that loads at 3300 fps will generate perfectly normal pressures.
Unfortunately, as a first test toward drawing these case walls so thin, Buffalo annealed the case sidewalls twice. Our hardness testing shows that in doing so they created an unusually soft case body. For this reason, the case body does not spring back from the chamber walls