Looking at progressively shorter cartridge pairs leads one to conclude that primary ignition depth is usually less than about 1 inch. Generally, this systematic error is directly related to cartridge length; the longer the cartridge, the greater the error; the shorter the cartridge, the lesser the error (with a short enough cartridge, the error disappears, specifically because the primer does ignite most of the granules is such loads). As those who accept the arguments given here might expect, in cylindrical cartridges, the powder column length where QuickLOAD predictions and actual performance coincide is near ¾-inch and shorter. (Those who run these calculations and compare results with actual performance must take into consideration that in such loads, the primer blast often moves the bullet significantly.)
Support from SMc Experimenting
Significant
testing of various SMc cartridges by Smalley and this author suggest
the validity of the above contentions. The SMc cartridge design incorporates
case body and shoulder features designed using first principles as
necessary to maximize both ignition and combustion efficiency, when
compared to a more conventional cartridge of similar capacity. In
order for these features to come into beneficial effect, the propellant
column must be long enough so that some significant portion of charge
is not included in primary ignition zone. Testing with three lengths
of 22-caliber SMc numbers shows a definite relative ballistic advantage
(compared to conventional case designs with similar capacity) only
when propellant column length exceeds about ¾-inch. In such cases,
the SMc advantage becomes worthwhile and it increases with progressively
increasing length, through typical magnum relative capacity.