In wearing out three .30-.338 barrels shooting matches, I learned what everyone else who shot them well in competition did.
New cases made by full length sizing .338 Win. Mag. cases down to 30 caliber with about 5 thousandths headspace clearance shot the most accurate. Both the military teams and civilians winning matches and setting records tried conventional full length or neck only sizing and it never worked. The military teams just used new cases; they often gave their fired cases to civilians 'cause they never shot well reloaded. New case bodies had a ridge 1/32 inch in front of the belt after they was fired. That ridge caused interference with the chamber at that point when fired again; it wasn't swaged down by any case sizing die.
But a couple of folks found a way to reload fired belted cases getting rid of that ridge (or step) such that their performance equalled what new cases would do. It required fired cases to be resized back to virtual new case dimensions. To do so required a second body sizing die used to reduce body diameters all the way to the belt swaging that ridge back down to virtual new case dimensions, not 1/16th inch in front of it which normal full length sizing dies die. They took a regular full length sizing die then cut off its top just below the shoulder and cut its bottom off about 1/16th inch above the belt clearance counterbore. After depriming and cleaning the fired cases, they would first be sized in a standard full length sizing die setting the shoulder back about 3 thousandths. Then used the body die set to size the case all the way to the belt; the bottom of the body die barely touched the case belt when the ram was at the top of its stroke. This eliminated the interference between chamber and case at this point that regular full length sizing dies left.
If one doesn't do this, then any fired case sizing that makes the case head-to-shoulder dimension a fraction longer than the chamber so the bolt binds a bit when closed will force the case head hard against the bolt face. This lets that swelled part of the case right in front of the belt have minimum, but still some interference with the chamber. It gets worse as the firing pin drives the case head off the bolt face pushing the case a bit forward in the chamber. Accuracy will not be as good as with new cases or double-sized as explained above.
Larry Willis (
www.larrywillis.com) now has a collet die that does the same thing. If one wants best accuracy with belted cases, using one of these dies is essential.
As far as the load's concerned, the popular load was 65 or 66 grains of IMR4350 under a Sierra 190 in standard SAAMI chamber dimensions for the .338 Win. Mag body and normal chamber neck diameters at .344 to .345 inch. Nobody got consistantly good accuracy with tight necks. Here's what my latest .30-.338 barrel did with new full length sized cases shooting 200-gr HPMK's and once fired double-sized cases shooting 190 HPMK's. 15 shots with each alternating between each load; all 30 shots fired in 20 minutes. Ten ring's 10 inches: