SR-4756 Powder in a 308 Win

[Pirate69]

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

[RickinTN]

My suggestion would be to skip the heat treat. Compared to what I shoot your bullets are too hard.
Good Luck to you,
Rick

[Pirate69]

My suggestion would be to skip the heat treat. Compared to what I shoot your bullets are too hard.
Good Luck to you,
Rick

OK, thanks.

[poppy42]

Ya might be better off trading that powder to someone who loads shotgun for a powder better suited for 308. I’ve loaded 38 special and 12 gauge with SR4756 and SR7625. Wish I had more of either one.

[TurnipEaterDown]

I can understand why you may not be thrilled with those group sizes.

What do other powders show for grouping with the same bullet, size and hardness?
Also, mixed brass: I often run into issue w/ varying neck thickness, and thus tension. Of course, this may be mitigated by jamming as you are doing.

While the heat treating is likely not required, I have been a contrarian - I used to cast bullets very hard and had them work fine (25 ish BHN on LBT tester, 6-8% antimony, and commensurate tin). Now I just find it often isn't required, and less alloying is cheaper.

[Pirate69]

I can understand why you may not be thrilled with those group sizes.

What do other powders show for grouping with the same bullet, size and hardness?
Also, mixed brass: I often run into issue w/ varying neck thickness, and thus tension. Of course, this may be mitigated by jamming as you are doing.

While the heat treating is likely not required, I have been a contrarian - I used to cast bullets very hard and had them work fine (25 ish BHN on LBT tester, 6-8% antimony, and commensurate tin). Now I just find it often isn't required, and less alloying is cheaper.

At this point, I am just trying to develop a load for the SR-4756 powder that I picked up. I may have to check some other powders.

[Pirate69]

There have been two comments that questioned the boolit hardness of the rounds that I am using. I am not about to question the experiences of others. So there has to be something that I am missing.

The Lyman 4th Edition lists a load very close to my load for SR-4756. The listed boolit is 3 grains lighter than my 173 grain GC-ed boolit. The seating depth is 0.088" less than my jam COAL. Lyman says a 12.0 grain SR-4756 load will generate 30,000 psi chamber pressure in a 308 Win. My 12.0 grain round should be slightly less than 30,000 psi.

The cast boolit is measured as a 17 BHN. Published data indicate that approximately 24,000 psi is needed to bump-up a 17 BHN boolit. This leads me to believe that the boolit would bump-up, if necessary to fill the bore and prevent gas cutting on its own. I tried the GCs just to eliminate any concern of shooting a GC boolit without a GC.

I don't understand why the boolits are seen as maybe being too hard. Please educate me.

[TurnipEaterDown]

At this point, I am just trying to develop a load for the SR-4756 powder that I picked up. I may have to check some other powders.

Point in my line of thought was: separating the bullet and its behavior from your rifle from the behavior of the powder under that bullet.

You question the powder choice.

Earlier question needs to be: how to separate that from your bullet choice, prepped the way it is.

[HWooldridge]

I have always tended to have more accurate results with harder alloys, so long as the bullet fit in the bore is correct. Of course, the good old scientific method sez to only change one variable at a time, so I might try a different mold design with the same alloy and 11.5 grain charge - but all of this assumes you have slugged the bore for the proper diameter.

I had a Ruger 77 in .338 Win Mag which simply would not shoot any lead bullet accurately - but it was a tack driver with jacketed slugs.

[trapper9260]

When you use military brass it is thicker then regular brass. The data will be different then the regular brass .Also did you slug your barrel ? Because the size you stated is on the small size for how it looks. I normally use .310 in my . Also you do not need to heat treat the boolits for what was stated already .

[Larry Gibson]

Some observations:

Your bullet is not too hard. I would add 1% of tin to your alloy which will better balance the antimony to tin creating more sub metal SbSn. that will keep the the antimony in solution in the lead during solidification. It also will produce better quality cast bullets.

Your lubed .309 sized bullets will not "bump up" in a probably .308 +/- groove diameter barrel. They will get swaged down and after riding a layer of lube they will exit the barrel at .305 - to maybe .307 diameter. Besides that, Lee's formula for obturation is based on a pure lead constant and you are shooting pure lead. I've pressure tested too many different alloy cast bullets of different designs in to many different caliber/cartridges to find any credence with Lee's obturation formula. It is a dog that won't hunt.

If all you are shooting at is 50 yards or even 100 yards I suggest starting at 8 gr of that SR 4756 powder and work in 1/5 gr increments back up to 11 gr.

You might also drill out the flash hole with a bit no larger than a #28. That will get the primer flash into the case better which will give better ignition and burn of the small charge of powder in the 308W case.

Also, if the scope on the rifle should be able have it's parallax adjusted to 50 yards. If not that can induce upwards of 1/2" +/- into the group size depending on the scope.

[Pirate69]

First of all, I want to thank everyone for the constructive feedback that I have received. This feedback is based on actual experience and is not taken lightly. If I ask questions, it is only to understand the reason for the suggestion. For me, loading and shooting cast boolits is a continuous learning experience. I realize I may be trying to fit a square peg into a round hole; but I am enjoying the challenge. I guess the purpose of this project is not to get the rifle to shoot at one MOA or less, but to get the SR-4756 powder to shoot at one MOA or less.

When I first started the project, I found some CTL312-160-2R boolits that I had cast many years ago for an M1 Garand. They were GC-ed and Lee Lubed. They measured 0.311". Did not measure the BHN, maybe need to go back and do that. I tested them at a powder charge range of 9.0-12.0 grains SR-4756, by 0.5 grain increments. These were the results.

9.0 grains........1.9 MOA
9.5 grains........1.8 MOA
10.0 grains......3.2 MOA
10.5 grains......2.6 MOA
11.0 grains......1.5 MOA
11.5 grains......2.0 MOA
12.0 grains......1.5 MOA

Since there seemed to be a potential at the 11.0 to 12.0 grain range, that is where I started the current project. Looking back, there may be a potential in the 9.0 to 9.5 grain range or even less. This has been one of the suggestions and I will try.

I think I am going to reboot this project. I have sized, 0.309", some 10.6 BHN boolits, installing a GC and PC-ed them. They are not heat treated at this time. Measures 0.310". They are of the same 0.5% Sn alloy as before. Next castings will boost the alloy to 1% Sn for a comparison. I want to get away from lubing if possible. The next testing will incorporate as many of the recommendations are possible. This is fun anyway and good trigger time as well.

Thanks again for your comments and suggestions.