In the JACKETED BULLET TEST, 223 and 22-250, 3 barrels, 7 powder charges, 3 bullets; 63 combinations each, 223 avg = .945"; 22-250 avg .935".
7.12 CARTRIDGE SIZE AND ACCURACY
After finding that the 308Win Savage Striker shot reasonable group sizes with tiny charges of powder and 200 grain bullets at velocities down to under 600 fps, I began to wonder about some of the trends in cast bullet shooting. Specifically, with cast bullets, does the size of the cartridge case affect accuracy, and if so, how much? Are small cartridge cases more accurate than larger cases?
Many years ago John Ardito wrote about his experiments with the 308 Win. case shortened to various lengths. Others wrote about 32/20, 32 S&W and other cartridges as offering great accuracy advantages.
I had three single shot rifles barreled for short small cartridges around this time, but experienced no accuracy tsunami.
In modern benchrest, the 30BR with slow twist has become popular for score shooting.
In Cast Bullet Association, (CBA), shooting, the 30BR / 308 X 1.5” has become the most popular cartridge case in Heavy and Unrestricted classes and the 32 Miller Short, (32 MS), is most often seen in the PBB, (Plain Based Bullet), class.
In the American Single Shot Rifle Association, (ASSRA), matches, the 32 MS is frequently the winner.
These cartridges and their near relatives are small; substantially smaller than the 308 Win or 32/40.
Neither the 30BR/308 X 1.5” nor the 32 MS is available in a factory-built rifle. ASSRA rifles in 32 MS are often built on the DeHaas/Miller action with custom barrels; and CBA Heavy and Unlimited class rifles are built on Remington 700 or custom actions with custom barrels.
It is difficult to separate the contribution to reduced group size that is made by each variable; the smaller cartridge case, the action, the barrel, the stock, the bullet or the other variables.
Would single shot rifles with DeHaas Miller actions, Smith barrels, and all the other bells and whistles, in 32-40, shoot as well as the same rifles in 32 MS?
Would Heavy or Unlimited class rifles chambered for 308 Win shoot as accurately as the 30 BR or 308 X 1.5?
Testing would require more effort and expense than I, or probably anyone, would care to expend. However, there’s a way to look at some data and make at least some statements.
The CBA publishes match results and equipment lists that record groups, some shot with 308 Win in Production class, and 30BR/30X 1.5” in Heavy and Unrestricted classes and 32 MS in PBB class.
The CBA group events are for 5 and 10 shot groups at 100 and 200 yards. There are four 5-shot groups and two ten-shot groups fired at each range.
I took the 2004 through 2009 CBA National Match results, selected the Production class entries shooting 308 Win, Heavy and Unlimited class entries shooting either 30BR or 308 X 1.5”, and PBB class shooting 32 MS, and analyzed the dickens out of the data.
The PBB results should be of some special interest to the ASSRA members.
Since we’re dealing with a lot of possible variables here, and small numbers of data points, we’re not absolutely sure of much if anything. However, the data tell us the following:
(Let 1 moa = 1” @ 100 yards.)