Shoulder Position and Accuracy
Date: Aug 01 2008
Every handloader knows the shoulders of cases must be set back slightly during the reloading process so cartridges will chamber without a hitch in hunting rifles. But how little or how much set back of the shoulder is the correct amount for a rifle to shoot its smallest groups possible?
I hadn’t given much thought to how the minuscule thousands of an inch of difference in shoulder position related to accuracy until a couple of years ago while developing match loads for a Cooper Firearms Model 22 .243 Winchester. The rifle would shoot three Sierra 70- grain match bullets in a group as tightly as .3 inch at 150 yards with the shoulders of the cases set back .002 inch. But every once in awhile, a cartridge sneaked in that didn’t have its shoulder set back quite far enough and required a bit of downward pressure on the bolt handle to close the bolt. The bullets from those cartridges always flew outside an otherwise tight group, usually high and sometimes right.
Those cases with too long a shoulder were the result of cases being fired 8 to 10 times. After that many firings and sizings, they had become work-hardened. After firing, some of the cases were so rigid their shoulders failed to spring back fully from the chamber walls. When these hard cases were run through a sizing die, they also resisted sizing.
The problem was solved by using cases that had been fired only a few times and a Stoney Point Headspace Gauge (now made by Hornady and called the LNL Headspace Gauge) to make sure shoulder positions were all the same. With cases having all the same shoulder setting, fliers disappeared when shooting the Cooper .243. The lesson here was consistency.
I still didn’t know, however, what case shoulder setting a rifle shoots most consistently. Many target shooters prefer to leave the shoulder in the same position as the fired case comes from the chamber, only sizing a portion of the neck to hold the bullet. A few consider a shoulder a tad long the best, as pressure from turning down the bolt handle provides a near crush fit that removes all play of the cartridge in the chamber. Some shooters prefer to set the shoulder back somewhat so cartridges freely chamber without placing any stress on the action.
I tried these various shoulder positions to find which one shot best in a Cooper Firearms Model 22 .22-250 Remington. For a load I used Berger Varmint 50-grain Match Grade bullets with 36.0 grains of Hodgdon Benchmark. Winchester cases that had been fired once in the Cooper were used for the unchanged shoulder position and the setback shoulders. For the slightly long shoulders, I sorted through Winchester brass that had been only neck sized and fired six times.
The cases with a long shoulder were about .001 inch longer in the shoulder than once-fired cases. Half the length of their necks was sized to hold a bullet. A lot of pressure on the bolt handle was required to chamber a cartridge with this shoulder position. All that force no doubt removed any play of the cartridge in the chamber. However, it may have also put undue strain on the action. The two groups fired with these cartridges had a definite horizontal string.
Cartridges with their shoulders set back required running the cases through a full-length sizing die. To position shoulders back .002 inch, the sizing die was set about a full turn from touching the top of the shellholder. Setting shoulders back .006 inch required turning the die all the way down to touch the shellholder, lowering the shellholder and turning the die down another half a turn. Cartridges with .002 inch setback printed groups with a slight amount of horizontal spread. As the load table shows, groups were nice and tight.
The first group shot was one ragged hole with cartridges with .006 inch setback. The second group, though, opened quite a bit. Only a sliver of light is visible between the jaws of a micrometer opened .006 inch. However, that space is enough to allow a cartridge to tilt in the chamber, especially if the bolt face contains a plunger ejector, like the Cooper. Some accuracy experts remove the plunger, believing it puts pressure on one side of the face of the head that can misalign the cartridge in the chamber.
To keep shoulders of fired cases in the same position as they came from the chamber required setting the RCBS full-length sizing die in the press just enough to size half the length of the case necks. This setting narrowed the junction of the case body and the shoulder .002 inch, but all other dimensions of the case remained unchanged. These cartridges with no shoulder setback shot the tightest average group. The first group was nice and round with the bullets nearly overlapping each other. One bullet in the second group landed a bit to the left of the other two bullets. Other than that one bullet, the two groups indicated there was no undue stress on the action, and the cartridges were positioned in the chamber straightly with the bore.
After these cases have been shot another time or two, though, I’ll have to keep an eye on their shoulder movement with the Headspace Gauge, then size and bump them back accordingly. But for now, the Cooper .22-250 has 50 cases with a consistent amount of set to their shoulders that will provide a lot of accurate shooting.