OzFclass

I had an interesting one the other day that I haven't yet had a chance to follow up on so I thought I'd put it out here and see if anyone else has found a similar issue.

I was checking powder batch variations using a magneto speed with a known load and velocity tested around 12 months ago.

When I dusted this barrel off for re testing I got 50fps less using the same powder batch.

The 2 differences are a new primer batch (same brand etc) and new batch of bullets. I was only looking for a comparison between powder batches so I haven't yet done a control with all the original components bullet, powder and primer as I wasn't expecting to see that sort of difference.

has anyone seen that much variation caused by changing primer or bullet batches?

has anyone seen that much variation with the mag speed bayonet positioned closer or further from the bore axis? distance from muzzle was identical. OR battery charge affecting the reading?

I did the other day. Just moving the Bayo position 25mm resulted in a step change in my velocity readings.
So I would put it down to that.
I also delete the first velocity reading in a string, as I think this can be off by quite a bit also which will effect your average velocity when only firing a few shots.

The bayonet was in the same position to the mm in relation to the crown BUT there could have been a slight variation on how far the bayo was positioned below the bore.

Also only averaged after deleting the first 2 shots our of 10 with the remaining 8 giving an es of 12.

The ammo temp was within a few deg of last test.

Projectiles can differ in diameter and bearing surface length between batches which could give you a difference. Primers too can be hotter or milder. Might be a combo of the both. You could have to adjust your seating depth too if you didnt check it before hand with that lot of bullets.

Yeah Brad I'm leaning toward bullet pressure ring diameter or something along those lines but I'm not home at the moment to check.

I check my seating depth every time I load and these were within .002 of the previous batch to the lands but I didn't get a chance to check the actual bearing surface.

I have had up to 40 fps difference between different projectile lots and I think it was Shane G found 80 fps between 2 batches. I have also found similar differences in velocities depending on how many spacers were on the bayonet.

Rod did you find any actual dimensional difference between the batches?

did the tune return after matching velocities with a previous load?

Rod has a great memory
My experience was 3 years ago and 3 batches of Berger .308 gave me a 50 fps variation!
Same load; powder lot; primer lot; new cases sorted by weight. Verified scales - Sartorious.
All tied to bearing surface variation and thus pressure variation = velocity spread!
I never buy projectiles in smaller lots than 1000 now.
If I could I would buy in 10K lots but even then for 1000 yd they need to be individually checked.
Powder varies little - ADI anyway and my primers seem to be consistent.
BUT each lot of powder has to be verified!
My case prep is anal so presumably that is out of the equation?
FWIW
Shane

Shane, can you remember if the issue was diameter or bearing surface length or both? David.

BATattack I can answer your question re "battery charge affecting the reading". I did a stupid thing with my V2, I put one cell in back the front. I am normally anal about checking this but was in a hurry on the day. At the range I kept getting a low battery warning which I just ignored as for once I didn't have spare batteries in my gun bag. I also run mine with the back light forced on. The back light was quite weak as well. Any rate when I got home I found the reversed cell. I set up 4 cells on the bench (wasn't game to put a reversed one back in the controller) and IMMSMC got under 4 volts output. Went back to the range next day to repeat my velocities with the same loads, this time with cells in correct orientation, and got virtually the same velocities. So, the V2 would appear to be quite tolerant to low battery charge without accuracy being affected. Amazing.

ShaneG wrote:Rod has a great memory
My experience was 3 years ago and 3 batches of Berger .308 gave me a 50 fps variation!
Same load; powder lot; primer lot; new cases sorted by weight. Verified scales - Sartorious.
All tied to bearing surface variation and thus pressure variation = velocity spread!
I never buy projectiles in smaller lots than 1000 now.
If I could I would buy in 10K lots but even then for 1000 yd they need to be individually checked.
Powder varies little - ADI anyway and my primers seem to be consistent.
BUT each lot of powder has to be verified!
My case prep is anal so presumably that is out of the equation?
FWIW
Shane

yep Shane I'm also pretty anal about brass prep and powder charges and also buy everything in several thousand batches so on the same page there!

when I get a chance I'll check the bearing surface lengths and see if there is much variation between batches and correlation to the speeds.

I'm lucky I didn't actually do the test with old and new projectiles on top of the old and new powder as I would have first assumed the variation was caused by the powder.

6.5x55ai wrote:BATattack I can answer your question re "battery charge affecting the reading". I did a stupid thing with my V2, I put one cell in back the front. I am normally anal about checking this but was in a hurry on the day. At the range I kept getting a low battery warning which I just ignored as for once I didn't have spare batteries in my gun bag. I also run mine with the back light forced on. The back light was quite weak as well. Any rate when I got home I found the reversed cell. I set up 4 cells on the bench (wasn't game to put a reversed one back in the controller) and IMMSMC got under 4 volts output. Went back to the range next day to repeat my velocities with the same loads, this time with cells in correct orientation, and got virtually the same velocities. So, the V2 would appear to be quite tolerant to low battery charge without accuracy being affected. Amazing.

another good piece of info to check of the variables list!

BATattack wrote:Rod did you find any actual dimensional difference between the batches?

did the tune return after matching velocities with a previous load?

I never did measure the bullets but, yes, with a slight load tweek, my accuracy came back and the node was in exactly the same velocity. When I do switch to a different batch, I just fire a few rounds of each bullet lot over a crony and match the speed. I also check that the bullet is jammed the same amount as it previously liked and make sure the seater is set up to give the same jam.

Well back at the bench and I've had a chance to measure some samples.

There is no definitive difference in bearing surface length with both samples varying by around .007"

I did find a slight variation at the pressure ring at the base of the bullet.

The new slower batch measures .2843 at the pressure ring and .28415 at the front of the bearing surface while the old faster batch measures .28450 and .28415 respectively.

I don't know if this helps ! But I found with our hot summers here in Chinchilla . I found that my 4 kg tub of 2208 would lose velocity as the older it got! I think with my recent experience of leaving the powder in the hopper just over a few days had a severe affect on velocities of about 40 Fps.
So I think the good stuff can evaporate with heat when sealed or just fume off and also take in moisture when sitting in the open!

Have you tried fresh powder ? Just a thought!
Graham.

ADI powder is suppose to get faster (burning rate, not velocity) as it gets older according to their tech blokes. 10 year life span at specification.
If it smells like vinegar then it is starting to deteriorate. High temperature speeds up the deterioration process so store it in a cool place.