My wife's .308 5R can shoot a ragged hole at 100 yds with Hornady 168 BTHPs zinging out at ~ 2700 fps. Typically, the round is loaded 0.020" from the lands in a type II alpha mag.
My friend bought a chrono, and she noticed that the velocity spread wasn't too good, with an ES ranging between 20-30 fps. I load her powder charges with a Scott Parker tuned beam scale, brass is cleaned using STM, and the primer pockets are uniformed. So, Rob01 suggested I try different primers and/or adjust neck tension.
Changing from CCI #200 to Rem 9 1/2 primers had no appreciable effect.
I FL size using a Forster comp. die, which results in a 0.333" neck OD. Seating the bullet results in 0.001" neck tension.
I've ordered some match grade Fed 210M's (still waiting on them), so in the meantime I wanted to see what the impact of adjusting neck tension would be. Rob was nice enough to let me borrow his Redding competition neck sizer with 0.332", 0.331", and 0.330" bushings.
I loaded a baseline lot, and a lots with 0.002", 0.003", and 0.004" (each lot had 10 shots).
All four lots were within 1 MOA at 400 yds on today's breezy morning. (Note: My wife and I took turns shooting, which I admit introduced some variation into the experiment).
Here are the results:
0.001":
Mean (fps): 2699
ES (fps): 33
Std D: 10.2
0.002":
Mean (fps): 2692
ES (fps): 54
Std D: 16.1
0.003":
Mean (fps): 2692
ES (fps): 32
Std D: 9.6
0.004":
Mean (fps): 2684
ES (fps): 59
Std D: 19.5
So, the results are not too impressive; however, both the 0.001" and 0.003" neck tension lots were 0.75 MOA and 0.50 MOA at 400 yds, respectively. The grouping of the 0.003" lot looked much better on paper; however, and I think I know why upon seeing the numbers when the the highest and lowest velocity values are discarded:
0.001"
Mean (fps): 2699
ES (fps): 22
Std D: 7.4
0.002"
Mean (fps): 2693
ES (fps): 27
Std D: 9.9
0.003"
Mean (fps): 2693
ES (fps): 17
Std D: 6.3
0.004"
Mean (fps): 2684
ES (fps): 38
Std D: 15.5
By discarding the extreme values, the 0.001" ES value decreased 50% and the 0.003" ES value decreased 88%, resulting in ES values of 22 and 17, respectively.
From this "experiment" I think it's safe to speculate the following:
- Changing neck tension has no appreciable effect on mean velocity.
- The primers I am using (Rem 9 1/2 and CCI #200) have a noticeable effect on chrono'd velocity in that they exacerbate variability. Despite the neck tension values I am using, it is not possible to reduce ES < 30 fps (unless discarding the highest and lowest values)
- Neck tension affects variability as well. Going from 0.003" (best) to 0.004" (worst) neck tension resulted in an 84% increase in ES.
- For her particular load, the best to worst neck tension values are as follows: 0.003", 0.001", 0.002", 0.004"
My friend bought a chrono, and she noticed that the velocity spread wasn't too good, with an ES ranging between 20-30 fps. I load her powder charges with a Scott Parker tuned beam scale, brass is cleaned using STM, and the primer pockets are uniformed. So, Rob01 suggested I try different primers and/or adjust neck tension.
Changing from CCI #200 to Rem 9 1/2 primers had no appreciable effect.
I FL size using a Forster comp. die, which results in a 0.333" neck OD. Seating the bullet results in 0.001" neck tension.
I've ordered some match grade Fed 210M's (still waiting on them), so in the meantime I wanted to see what the impact of adjusting neck tension would be. Rob was nice enough to let me borrow his Redding competition neck sizer with 0.332", 0.331", and 0.330" bushings.
I loaded a baseline lot, and a lots with 0.002", 0.003", and 0.004" (each lot had 10 shots).
All four lots were within 1 MOA at 400 yds on today's breezy morning. (Note: My wife and I took turns shooting, which I admit introduced some variation into the experiment).
Here are the results:
0.001":
Mean (fps): 2699
ES (fps): 33
Std D: 10.2
0.002":
Mean (fps): 2692
ES (fps): 54
Std D: 16.1
0.003":
Mean (fps): 2692
ES (fps): 32
Std D: 9.6
0.004":
Mean (fps): 2684
ES (fps): 59
Std D: 19.5
So, the results are not too impressive; however, both the 0.001" and 0.003" neck tension lots were 0.75 MOA and 0.50 MOA at 400 yds, respectively. The grouping of the 0.003" lot looked much better on paper; however, and I think I know why upon seeing the numbers when the the highest and lowest velocity values are discarded:
0.001"
Mean (fps): 2699
ES (fps): 22
Std D: 7.4
0.002"
Mean (fps): 2693
ES (fps): 27
Std D: 9.9
0.003"
Mean (fps): 2693
ES (fps): 17
Std D: 6.3
0.004"
Mean (fps): 2684
ES (fps): 38
Std D: 15.5
By discarding the extreme values, the 0.001" ES value decreased 50% and the 0.003" ES value decreased 88%, resulting in ES values of 22 and 17, respectively.
From this "experiment" I think it's safe to speculate the following:
- Changing neck tension has no appreciable effect on mean velocity.
- The primers I am using (Rem 9 1/2 and CCI #200) have a noticeable effect on chrono'd velocity in that they exacerbate variability. Despite the neck tension values I am using, it is not possible to reduce ES < 30 fps (unless discarding the highest and lowest values)
- Neck tension affects variability as well. Going from 0.003" (best) to 0.004" (worst) neck tension resulted in an 84% increase in ES.
- For her particular load, the best to worst neck tension values are as follows: 0.003", 0.001", 0.002", 0.004"
