Ladder Test Opinion

[browninghunter86]

308 win
155 Sierra 2156
8208 XBR
30" 1-12 twist barrel

200 yard initial ladder while checking for velocity/pressure. 44.5 is #4(went in 0.5 increments)


Went back and re shot ladder with 44.5-46.0 in 0.3 increments.(1 being 44.5 and 6 being 46.0)



Found it interesting that at 350 yd seems 44.5-45.1 and 45.4-46 both were nodes. And at 200 seemed my node was 44.5 to 46. Didn't expect to see 2 possible options within that charge window

[Bart B.]

I don't think there's enough shots per test group for any load to be statistically significant as far as its accuracy is concerned. 2-shot groups have about a 15% chance of representing what accuracy would be shooting a 20 to 30 shot group.

It's my opinion that groups 4 through 6 of each load are statistically equal. If you shot a 20-shot group with each load, they may have a 10% spread across them. It would take 60-shot groups to see which one of these three was the most accurate with more than 95% confidence.

It's been my experience that with powders in the same burn rate range as N135 and N140 with Sierra's Palma bullets in my 30" barrels, there's little accuracy difference across a 1 grain spread of charge weight. As one of half a dozen US Palma Team members developing loads for Sierra's prototype 155's back in 1991, we didn't see any significant difference between 45 and 46 grains of IMR4895 across 6 different barrels. The load used for the first use of that bullet in competition used 45.3 grains of IMR4895 (average of the different loads used shooting 20-shot test groups at 1000 yards) and it shot very accurate across 25 or so rifles. Later, folks used different charge weights of IMR4895 and Varget (Australia's AR2208) in the 45 to 46 grain range all with very good accuracy.

[browninghunter86]

Right. I did one ladder at 350. Then came back and shot another one at same target to verify my first ladder. I am heading back today to do seating depth test(touch to 50 thous off) and see how it goes.

[Bart B.]

For accuracy tests, I suggest shooting at least 15 shots per test group.

[browninghunter86]

that would get very expensive shooting 15 shots each during load work up. Or are you talking about after you think you found a load?

I shoot 20 shots to verify load once I think I have it

[Bart B.]

If you've got two or three loads only 1/3 grain apart in charge weights, I think at least 15 shots per test group with each is important. 20-shot groups are better. Your single 20-shot group as proof is about 5 times better than figuring the average of a few lesser-shot groups.

Here's my reasoning. If a few multi-shot groups with a given load are shot and their extreme spreads are not within 10% of each other, you don't know to at least 90% confidence that the size of the largest one's a realistic representation of where all shots will go into.

But I base accuracy of shooting stuff on the extreme spread of several dozen bullets fired; not just 3 or 5 or even 15. The more shots of a given load shot into a group, the larger that group will get; except for rare instance that the first few-shot group was really huge.