Detailed Information on SMc Cartridges

Encompassing an entire series, SMccartridges are designed to provide superior ignition
and a more efficient propellant burn. Compared to conventional cartridges generating
similar performance, the patented SMc design provides many advantages, chiefly:

  • Increased velocity potential (superior ballistics)
  • Reduced barrel vibration (increased accuracy potential)
  • Reduced barrel heating (extends shooting strings before heating disrupts
    accuracy and becomes a significant impediment to reasonable barrel life)
  • Reduced bore damage (increased barrel life)

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
  • The SMc ™ Cartridge designation specifies caliber and usable capacity; for example, 6.5/60 SMc indicates a 26-caliber (6.5mm) cartridge holding 60 grains of water
    (to base of case neck) — similar in usable capacity to 6.5-284 Norma but with
    superior performance, 7/75 SMc indicates a 28-caliber (7mm) cartridge
    with a usable capacity of 75 grains of water, etc.
  • Choose propellants so the correct charge fills the available
    powder space or is somewhat compressed by the seated bullet
  • We have often demonstrated the highest velocity using ball-type propellants
    but exceptions exist and we expect the newest extruded propellants from
    Alliant and IMR are likely to change that situation, testing will tell that tale
  • Use relatively high neck tension, as necessary to create good bullet pull
  • Primer choice might significantly alter velocity, pressure, and accuracy potential, but, our testing has demonstrated that primer choice likely matters far less in
    SMc cartridges than in conventional cartridges, which is exactly what SMc
    case design theory predicts, making this just one more demonstration
    that the design does what we designed it to do
  • Because SMc cartridges work best when the bullet base and the base of the case neck are approximately aligned, optimum reamer design (chamber throat length) might depend on bullet length, without such bullet-specific throating:
    • Use of shorter bullets results in minimal loss of accuracy or ballistic
      potential in a chamber throat designed for use with such bullets
    • Necessary deeper seating of heavier (longer) bullets in a chamber
      throat designed for lighter (shorter) bullets can significantly
      compromise performance

We hope you enjoy your SMc cartridge experience,

Byrom Smalley and Mic McPherson