Starting loads are a good thing

[dok7mm]

I'm glad I used starting load data, instead of saving a little bit of components. The rifle is, of course, a STW on TAC 300 action, 26" Bartlein 5R, nine twist. Barrel is nitrided and I'm using HBN coated 180 Berger hvld pills. I was gonna use IMR 7828, but fill was only about 80%, so I opted for Retumbo, Nosler brass and Fed 215M primers. Rifle has .316 neck and brass is turned to .014" for .004" clearance. Loaded nk is .312".

I started @ 75.0 grs. and loaded to 78.5 grs. in .5 increments. I shot 5 foulers @ 75 grs to stabilize velocity. COAL was 3.786" and CBTO was 3.090, with .010" jump. Groups started to tighten up @ 76.0 grs. with sd- 5.4 and ED of 11 fps. But average was running well over Berger manual prediction @ 2964 fps. Chrono was LabRadar.

Hit another node ? at 77.0 grs and average velocity of 3032 fps. SD was 3.1 and ED of 6.0 fps. This was slightly above my target of 3000+/- for the 180. At 77.5 gr, group opened up and started getting a pretty good ejector smear. I was a full grain below Berger's usually conservative max load, but running about 200 fps faster than max load speed.

Temperature was in the low to mid 90's, but ammo was in ice chest and was close to 60 degrees according to thermometer.

I'm just wondering why velocity was so much higher, compared to manual, than it should have been. Was it the nitride, the HBN, the low round count (37), the 5R rifling, the new brass (headspace increased by .010" after firing), an out spec powder lot, or a magic barrel ???

Anyone run into this or is it just one of those "rifles are different" things?

 

[MudRunner2005]

I'm glad I used starting load data, instead of saving a little bit of components. The rifle is, of course, a STW on TAC 300 action, 26" Bartlein 5R, nine twist. Barrel is nitrided and I'm using HBN coated 180 Berger hvld pills. I was gonna use IMR 7828, but fill was only about 80%, so I opted for Retumbo, Nosler brass and Fed 215M primers. Rifle has .316 neck and brass is turned to .014" for .004" clearance. Loaded nk is .312".

I started @ 75.0 grs. and loaded to 78.5 grs. in .5 increments. I shot 5 foulers @ 75 grs to stabilize velocity. COAL was 3.786" and CBTO was 3.090, with .010" jump. Groups started to tighten up @ 76.0 grs. with sd- 5.4 and ED of 11 fps. But average was running well over Berger manual prediction @ 2964 fps. Chrono was LabRadar.

Hit another node ? at 77.0 grs and average velocity of 3032 fps. SD was 3.1 and ED of 6.0 fps. This was slightly above my target of 3000+/- for the 180. At 77.5 gr, group opened up and started getting a pretty good ejector smear. I was a full grain below Berger's usually conservative max load, but running about 200 fps faster than max load speed.

Temperature was in the low to mid 90's, but ammo was in ice chest and was close to 60 degrees according to thermometer.

I'm just wondering why velocity was so much higher, compared to manual, than it should have been. Was it the nitride, the HBN, the low round count (37), the 5R rifling, the new brass (headspace increased by .010" after firing), an out spec powder lot, or a magic barrel ???

Anyone run into this or is it just one of those "rifles are different" things?

Tighter spec'd chambers don't allow the brass to expand as much as most sloppy factory chambers. So sometimes they achieve the higher pressures at lower load volumes. Which in turn means you will equal higher velocities at lower load charges than your average SAAMI spec chamber.

Also, I've never used HBN coating, but I have use Moly bullets, and those did produce slightly higher velocities once you got a few down the bore and got some Moly in the rifling. So, I'm sure the HBN coating helped a little bit, as well.

Also, 5R barrels seem to be a bit faster from my experiences, and they also clean easier and last a bit longer between cleanings. I think this has to do with the design of the rifling and the fact that being an odd-number of grooves there is no pinch-point that is created because where there is pressure from a land on one side of the bullet, directly adjacent there is a groove.

 

[dok7mm]

Thanks, MudRunner. You hit the points I had considered. It was chambered with a match reamer. I'm not gonna gripe about the velocity, but it kind of freaked me out to see it running much faster than the charge would normally dictate.

The HBN is similar to moly, except you mop the barrel with a slurry w/alcohol to kinda pretreat, so it doesn't need as many foulers. In my other rifles, the gain is usually only 35-50 fps.

I thought I might have had a " senior moment " at the bench and mislabeled the charge weight on the cases.