Irratic change in muzzle velocity

[Pwhite11798]

I couple weeks ago I posted about inconsistent change in muzzle velocity with my loads. A fellow shooter replied and said he was having the same issue with the powder I was using, so I switched and started load development on imr 4166 in .308. I noticed the same thing that was happening with power pro 2000. I would shoot 2 or 3 shots that had low es and then I would have a shot that would be 30+ fps different. My load consisted of 40-43.5gr 4166 stepped in .5gr increments every 5 shots. Barnes mb 175gr. Sellior and ballot sr primer. Bullet seated .02 off lands.
Now what I have checked case diameter of loaded rounds. All but a few .338 + or - .0005. I've tested different primers when shooting pro 2000. Checked the diameter of the bullets all the same. The only the I can tell a considerable difference is in the seating. Some bullets have almost 0 feel while others have what feels to be more pressure to seat. I know this can make a difference in how rounds shoot. But would it be that much of a difference. And is there a way to prep and individual case to change how the bullets seats. Besides the normal steps ,brush or burnish with steak wool.

 

[rardoin]

Higher neck tension results in higher velocity in my loadings. If you have inconsistent seating pressure, thus tension of the brass neck on the bearing surface of your projectiles, I would expect velocity variation. I tested different NT on 6.5 Creedmoor rounds for grouping. I tested 0.0005". 0.0015", 0.003" and found up to 45fps average from the lowest to the highest. From 0.0005 to 0.0015" there was not a difference I would consider significant from a statistical standpoint. Disclaimer...only 10 rounds per neck tension group were measured. I did not attempt to quantify seating pressures but I did observe that the lower the neck tension, the less compression on the Belleville washer stack as observed on a dial indicator while seating a bullet...nothing unexpected. (K&M arbor press with seating 'pressure' indicator used).

To make a short story long...yes, a change in seating resistance will likely change your velocity. Yes, you can treat the necks with a brush/steel wool to get more consistent seating 'feel' but if your neck wall thickness and brass hardness is not uniform, you can expect velocity variations.

 

[Pwhite11798]

I think I having a variance in neck hardness. I don't know how I got this. But if that is the case would annealing solve the issue. On my case prep this reload I annealed and could still feel a difference While seating bullets.

 

[Tempest]

The only the I can tell a considerable difference is in the seating.

When I feel that, those do not get shot for score or group when testing. You need to find out what is causing that. Neck thickness or variation from one side to the other would be my guess.

 

[Pwhite11798]

I went and tried something different I took out my .337 bushing and installed my .336 and the button pull on the recapping pin. I resized 50 cases and loaded 30 to test against the thirty that were loaded with the .337 bushing. I did notice that the in difference in the feel of tension when seating bullets had pretty much disapered. I'll shoot them tomorrow and find out.

 

[Martin52]

The rounds referred to in your OP - were these in cases resized with just a bushing, or, per last post with the expander ball on the decapping rod ?

 

[CharlieNC]

ANNEAL. Fixed the same issue for me.

 

[Pwhite11798]

Bushing only and it was .001 smaller than the loaded round. With some of the loaded rounds being .3375 instead of .338 and a few being .3385 I think this is were my fliers were coming from not having enough tension. Theoretically if my my cases were perfect @ .338 and I used a .337 bushing then I should yield .001 neck tension but that's not the case. My cases are not absolutely perfect and some are 0005. Up or down from. .338. All this created conditions were I was not getting almost any tension on some bullets.

 

[rifleman700]

Can you post the average velocities you were getting with each charge from 40-43.5?

What brass are you using?

What primers have you tried?

.001" is not very much neck tension unless you are soft seating in the lands which you are not.

 

[Pwhite11798]

Lapua Palma brass. With 4166 I've only tried Sellior and bellot primers but the other powders like h4895, rl 17, pro 2000 I've tried br4, #41, & 400's. The Sellior and bellot gave identical performance to br4 at half the cost. The velocities I was seeing was 2550 fps-40gr. To 2775 fps-43.5. 43.5 was just starting to show some cupping on the primer. No other pressure signs. Barrel was a 27" 3 groove 10 twist lilja. The nodes seemed to be around 41.5 and 43.5gr. In my setup I am doing further testing at 43.5gr. Today.

 

[Homerange]

The annealing suggestion makes perfect sense to me especially if you are using traditional dies. I don't do any annealing of late and use a Lee collet die and yes while there are small differences in seating effort required they are much less than if I used traditional expander dies. I don't use lots of ammo just hunting but if I was to shoot lots more annealing would be necessary.
My best loads have 6 fps ES.

 

[Pwhite11798]

Shot some loaded rounds yesterday. I did a comparison between .001 neck tension and .002 neck tension. The .002 neck tension seemed to do the trick also tested seating depths at the same time. Got my groups down to 3/4" moa looking to improve on that from there.

 

[Martin52]

Do you find a difference on the target between the cases that are a) resized with a bushing only and b) resized using a bushing and the expander ball ? Assume the bushing is the same size in each case.

Whidden Gunworks have expander balls in various sizes that can be selected to minimise the amount the neck is expanded when the ram is lowered. The advantage seems to be the case body is held in the FLS die, the bushing ensures minimal reduction from fired case neck dimension, the expander ball (if correct size selected) will ensure minimal expansion and the result may be greater consistency in internal diameter before the projectile is seated. I have yet to buy a set and try this out but will be doing so shortly.

You seem to have found good results with 0.002 NT, there just might still be further gain with the expander ball option if minimal reduction/expansion can be achieved.

Across all this though, I agree with the above comment - anneal the brass.


Martin

 

[eric32]

The expander ball is used to push out all the variance in neck thickness. IF the brass has not been neck turned.

 

[Pwhite11798]

Do you find a difference on the target between the cases that are a) resized with a bushing only and b) resized using a bushing and the expander ball ? Assume the bushing is the same size in each case.

Whidden Gunworks have expander balls in various sizes that can be selected to minimise the amount the neck is expanded when the ram is lowered. The advantage seems to be the case body is held in the FLS die, the bushing ensures minimal reduction from fired case neck dimension, the expander ball (if correct size selected) will ensure minimal expansion and the result may be greater consistency in internal diameter before the projectile is seated. I have yet to buy a set and try this out but will be doing so shortly.

You seem to have found good results with 0.002 NT, there just might still be further gain with the expander ball option if minimal reduction/expansion can be achieved.

Across all this though, I agree with the above comment - anneal the brass.


Martin

I use Whidden dies. The difference between having .001 nk tension and .002 nk tension w/ expander ballwhen shooting was less flyers. The .001 nk tension had a horizontal spread of 2.5" with some vertical flyers. The .002 nk tension grouped as well as .75" with very little vertical. I shot all groups on my f class bloods and rear bag and stayed very focus on my shooting fundamentals. I did not use my crony. While doing this test. I tend to get a little sloppy while shooting with the crony on the barrel cause I get focused on the ouput of that device instead of the fundamentals. So I'll work on it till I find a load that shoots well. Then I'll crony the load again.

 

[LCazador]

Neck turn your cases and get rid of the expander ball.