Detailed Information on SMc™ Cartridges
Encompassing an entire series, SMc ™ cartridges are designed to provide superior ignition
However, owing to unique design characteristics, parameters of SMc cartridges and chambers are more critical than those are in conventional cartridge designs; hence, specific loading precautions are important. For example, in SMc cartridges, the base of the seated bullet should never be located significantly deeper that flush with the interior junction of the case shoulder and case neck. Seating bullets so the base extends progressively more than about 0.015-inch below the base of the neck can progressively reduce SMc cartridge efficiency, in terms of reducing barrel heating and extending barrel life.
Bullet-specific throating might be beneficial for those requiring maximum accuracy. With conventional bullets and guns, ample evidence suggests that near optimum accuracy results when bullet-to-rifling jump is about 0.020 inch. However, we believe the combination of proper throat diameter (perhaps no more than about 0.0005-inch greater than bullet diameter) and sufficient bullet pull (neck tension, see below) eliminates most accuracy problems that have traditionally been attributed to excessive freebore. Generally, excepting benchrest applications, we have demonstrated that cutting a proper diameter throat that is deep enough to accommodate the longest bullet that might be used will not significantly harm accuracy potential with properly prepared SMc loads that happen to use a shorter bullet. Note that the unusually long neck of typical SMc designs significantly mitigates this potential problem with most bullets, regardless of weight. Unless you want to shoot, for example, both the 85-grain Berger and 155-grain Berger bullets in the 6.5/60 SMc, with proper throating for the heavier bullet, you can likely seat any reasonably lighter bullet close to the rifling and still have sufficient seating depth for good bullet pull.
We have adopted a useful cartridge designation system; SMc designation accounts for caliber and usable case capacity. For example, our 5/35 SMc is a 20-caliber (5mm) cartridge holding about 35 grains of water (to base of neck). One important patented design characteristic of all SMc cartridges is a powder column that is between about 2 times and about 2.1 times bullet diameter:
Consider the 22/40 SMc: Usable capacity is similar to the 22-250 but ballistics far exceeds feasible 220 Swift performance. The original reamer was designed to properly accommodate Sierra’s 69-grain MatchKing. With a 28-inch barrel, the 22/40 chambering safely launches that bullet at >3600 fps, it also safely launches the 40-grain Nosler BT at >4600 fps, both with benchrest accuracy. When necessary, we can design optimized chambers for use with longer or shorter bullets. However, only authorized SMc reamer manufacturers can legally make SMc reamers.
As a safety consideration, we strongly urge SMc cartridge handloaders NEVER to concoct any combination where the correct powder charge (amount of charge needed to generate normal pressures) fills less than about 95% of the available powder space. Secondary explosions have been observed with reduced charges in some cartridges and the SMc design might exacerbate this potential.
As noted, significant interference fit between the case neck and bullet is often helpful for best accuracy and ballistic uniformity. For the following calibers, expander ball diameter (inches) should be approximately: 22: 0.2215, 24: 0.2403, 25: 0.2541, 26: 0.2611, 28: 0.2808.
High neck tension is important to prevent bullet movement during handling, or from recoil, or in response to the primer blast; this aspect is particularly critical when using properly friction-proofed bullets.
Basic SMc™ Cartridge Principles
We hope you enjoy your SMc cartridge experience,
Byrom Smalley and Mic McPherson