New bullet jam / jump test next step?

[Lefty Trigger]

I recently switched over to a new bullet for my 308 since my supply of the 175gr A-max's ran out. I got an awesome deal on 178gr HPBT Match bullets at Graf's and set up my jam/jump test going along with Bergers test for VLD bullets, its worked for me in the past. However on this bullet the results were not so "cut n dry" these shot really well at .080 off and .120 off that's a huge spread. What would be my next logical step to help narrow this down? I thought about a retest from .060 to .100 in .010 steps. In the past with other bullets there was just one clear winner in the test.

 

[Ccrider]

I would work with the .012 in .003 increments in and out.

 

[Texas10]

That is a wide window, I'd put a chrono on it and see if one depth is more stable than the other. One guy I met at the range said he was jumping .180 ( he was loading 125 gr soft nose) and getting terrific accuracy and speed.

Are you crunching powder at -.120?

 

[CharlieNC]

That's a really long jump for those bullets, how did accuracy compare in the .020 range?

 

[Lefty Trigger]

That is a wide window, I'd put a chrono on it and see if one depth is more stable than the other. One guy I met at the range said he was jumping .180 ( he was loading 125 gr soft nose) and getting terrific accuracy and speed.

Are you crunching powder at -.120?

There is no way I would crunch powder as the loads are on the lighter side for a 300 yd range.

 

[Lefty Trigger]

That's a really long jump for those bullets, how did accuracy compare in the .020 range?

The Berger test is .010 jam .040 jump .080 jump .120 jump as a rough test to see what it likes. From the grouping the .080 and .120 was the clear winners but they are similar in group size.

 

[CharlieNC]

Those types of bullets usually shoot well with something in the .020 jump range. I would explore that region before looking at finer increments.

 

[Dusty Stevens]

Good jumps repeat. What youve seen sounds right and you can find a couple more if you want to keep on.

 

[Lefty Trigger]

Those types of bullets usually shoot well with something in the .020 jump range. I would explore that region before looking at finer increments.

So, something like .010, .020, .030 range? The A-max's I was using really liked .032 but these (178 HPBT) do not, I have tried several times and keep getting 3 to 4 inch groups at 200yds.

 

[CharlieNC]

So, something like .010, .020, .030 range? The A-max's I was using really liked .032 but these (178 HPBT) do not, I have tried several times and keep getting 3 to 4 inch groups at 200yds.

That should do it.

 

[T-shooter]

I did a test at 100 yards with a .308 several years ago. These were 5 shots each done round robin and about 30 seconds between shots to cool. Also 3 warm up shots before starting the test.

 

[mikecr]

I thought about a retest from .060 to .100 in .010 steps.

I like this plan.
That should lead to a good coarse seating, with which you would then move to powder testing.
Then with best coarse seating, and best powder, tweak seating in it's window(which is revealed here) for tightest group shaping.

 

[Link]

The Berger test is .010 jam .040 jump .080 jump .120 jump as a rough test to see what it likes. From the grouping the .080 and .120 was the clear winners but they are similar in group size.

If this test was done at a longer range than 100 would it reveal more info?
My nearest 100 yd range is 10 miles but my 200 to 400 yd range is 70 miles. I would like to test at the closest range .

 

[Lefty Trigger]

If this test was done at a longer range than 100 would it reveal more info?
My nearest 100 yd range is 10 miles but my 200 to 400 yd range is 70 miles. I would like to test at the closest range .

All my testing is done at 200 yards, we have a 300 yard range but it is only available to me on Thursday evenings only since it shoots across our 100 yard range...

 

[Lefty Trigger]

Here is a follow up test since a lot of you think I missed the actual "area" the bullet likes. Well you all are right, I went back and retested at .010 jump, .020 jump, .030 jump up to .060 jump and it looks like a place to fine tune from is .020 jump. Here is the actual test groups shot at 200yds.

 

[Texas10]

My suspicion falls on the powder you're using. According to the burn rate chart I have, it has a poor temperature stability rating of 1.1 as compared to Varget's 0.13 rating.

Without a chronograph to monitor velocity changes as the barrel heats up, not sure your data is worth a hoot. I recently put my Magnetospeed on my Criterion barrel in 223 to check a load that was shooting well. Starting from a cold barrel and using Varget, every one of the first 8 rounds was faster than the last. This taught me that even with temperature stable propellants, warming the barrel up to operating temperature prior to load testing is critical to achieving accurate load data.

So now I load at least 10 warm up rounds, middle of the charge weight range if doing an OCW, to get me and the barrel settled in. Then shoot and cool methodically during the test. Temperature strips or a IR temp gun can keep close track of barrel temps.

One other consideration is combustion efficiency with a light load may not be as reliable as it could be with less free space in the cartridge case. For that reason a slower powder and a fuller case might yield better results.

Notice how your groups are moving around from 8:00 to 6:00 and different distances from the POA?

 

[Lefty Trigger]

I agree with you on some of that, I never shoot any test with a cold bore. I make up a few extra rounds as well to make sure I'm still close to aim point and warm the barrel. Also while I will agree the test is slow and that powder choice is not a great choice either I always shift my scope down 2 inches to avoid hitting those orange dots, bullet holes in the dots are harder to see at distance.