We could also argue a reduction of shock-induced barrel vibrations resulting from primer blast because the SMc design spreads out (in time) this shock wave impact on the case shoulder. Arguably, this should contribute to improved accuracy potential – chiefly, since it will reduce magnitude of barrel vibrations, it should reduce sensitivity to "barrel time".
At this time, it became necessary to develop a computer program to define shoulder contour, as a function of both case (interior) and bullet diameter and case interior length (within body portion). For any given bore size, each specific case capacity requires a unique shoulder design so that primer shock energy reflecting from case shoulder will properly focus behind bullet. Manual calculations (Smalley) required eight pages of math for each design – this invited error, whence the necessity for a computer driven algorithm.
As an aside, we might also mention that these concepts, particularly case shoulder reflection of primer waste energy and how simultaneously that shock wave hits the case shoulder are obvious explanations as to why certain conventional case designs have proven so excellent. A too-shallow shoulder angle allows this waste energy to reflect onto the bullet base, thereby encouraging heat loss and premature bullet movement. A too-steep shoulder angle design increases sharpness of primer blast impact on chamber shoulder and thereby increases barrel vibrations; it also reflects waste energy back toward primer, where that energy does little good.
In this regard, a quick analysis (Smalley) proved two things. First, the trial and error path that led to the relatively short, 30-degree shouldered case (PPC and similar), as are now used almost universally in benchrest competition, was predictable – in that class of cartridges, 30-degrees is a very good compromise. Equally, this analysis verifies the value of Ackley's 40-degree shoulder as an excellent compromise for the normally longer hunting cartridges to which it is usually applied.
Making Cases
(Currently, only Pacific Precision Grinding is
licensed to produce SMc reamers, which are offered for sale only through
Russ Haydon – 253-857-7557.) A note of caution, our patent is quite
broad and specifically includes any design utilizing any similar shoulder
configuration, whether elliptical, parabolic, spherical or some combination
thereof, as well as those with an inside case diameter that is two
or more times bullet diameter.
In general, owing to the unusually large case diameter, SMc cartridges of any given capacity are relatively short. Therefore, outside of 17- and 20-caliber designs, it is very difficult to form SMc cases with reasonable capacity. As case manufacturers create appropriate factory cases, this problem will disappear (we are now working with Norma). For now, a very efficient 17-caliber can be built on a 221 Fireball case by reducing body taper, a la Ackley, necking it down and incorporating the patented SMc shoulder. Similarly, we have created two 20-caliber versions, based upon a shortened and a nearly full-length 6mm Norma BR case and have acquired ready-to-install Pac-Nor barrels for a Savage rifle. Shortly after Newlon die blanks become available to make dies, testing will begin on those. Design of the smaller version is intended to afford 3500 fps muzzle velocity with Hornady's 33-grain V-Max (now obsolete!);