Since I have an abundance of SR-4756 powder, I decided to see how it would function in 308 Win. I loaded rounds that varied in powder charge between 11.0 grains and 13.5 grains; varying each charge by 0.5 grains. The Lyman 3rd Edition suggests a starting charge of 12.0 grains for my boolit weight.
The cast material used is a pure lead/lino blend (97.4/1.95/0.65) of a calculated 10.6 BHN. The cast boolits were heat treated at 425F and water quenched. After 12 hours, the Lee Hardness Tester indicated a 17 BHN. The boolits were Lee Lubed and a copper GC applied while sizing the boolits to 0.309”.
The cases were a mixture of military and commercial; what was available. They were resized using a turned-down oversized neck expansion ball (0.310”). There was enough spring back to provide enough neck tension that the boolits were held in place when seated.
The boolit jam COAL was determined and the boolits seated about 0.050” longer than the jam COAL. Each round was seated against the lands when chambered; minimum pressure required.
The rifle used was a VG24 Mauser action with a E.R. Shaw barrel (0.5” muzzle) installed. Rifle is scoped. This barrel has never had a copper jacketed bullet fired through it. Not sure what it is capable of with jacketed bullets, but I hope it is better than what I am seeing with cast boolits.
Five rounds of each load were fired at 50 yards. The groups MOAs are pretty consistent up to the 13.0 grain load. Seems the 13.0 grain and 13.5 grain loads opened up significantly. The best group was the 11.5 grain load. I watched the first four shots form a nice tight group. The 5th shot was out of the tight group; oh well. If the 5th shot is discounted, the remaining four rounds constitute a 0.93MOA group. The muzzle was checked after each group and there was a good lube star and no indication of any leading.
Results
11.0 grains…..2.3MOA
11.5 grains…..2.1MOA
12.0 grains…..2.3MOA
12.5 grains…..2.1MOA
13.0 grains…..3.5MOA
13.5 grains…..3.1MOA
There is nothing to "write home about" with these results. Just sharing information.
The next step is to see how P-ed boolits perform in the 11.0 grain to 12.5 grain range. Pressure and velocity may be low enough I can eliminate the gas checks and still keep the same level of accuracy.
Anyone have any suggestions on how to improve using this powder. I have other powders that I could use if necessary. I also have other molds that I could try if the responses indicate that they may be a better application.
C309-170-F
Accurate 309235M
NOE 311180K31
Lee C312-155-2R
Lee C309-200-R