Powder batch changes

NigelM

Well-Known Member
Half way through a work up on a new bullet for the 280AI I ran out of powder and had to buy a new tub.

The powder in question is IMR7828.

I have been putting every shot over the chronograph and recording everything very carefully. All other factors have been absolutely consistent, same batch of primers, weighed brass and weighed bullets. Same day so no changes in temperature, atmospheric pressure, altitude etc.

Just to check the changes in velocity with the new powder batch I loaded the last 4 with the old powder at 60.4 grains and the first 4 with the new powder at the same charge weight. Average velocity on the old batch was 3031fps with an SD of 10. Average velocity on the new batch 2984fps and an SD of 11.

That's a huge change in powder burn rate on the same powder. Has anyone else experienced the same thing?
 

jb1

Well-Known Member
47 fps not too much difference is it in the grand scheme of things. As long as it's consistent :)
 

Yorric

Well-Known Member
That amount of velocity change is not unusual between powder batches.
It has always been advisable to check & rework precision loads when a new batch of powder is started.
It pays to buy powder in big lots to minimize the frequency of this. If buying multiple tubs, check the batch number is the same on all containers.

Ian
 

srvet

Well-Known Member
Was there any variance in atmospheric temp between the days of testing. If the second batch was colder that could contribute. The main thing to me is the ES is pretty much the same which is good!
 

Muir

Well-Known Member
Four rounds is not an adequate sample. SD certainly isn't valid with so small a sampling. Go shoot it.~Muir
 

NigelM

Well-Known Member
Was there any variance in atmospheric temp between the days of testing. If the second batch was colder that could contribute. The main thing to me is the ES is pretty much the same which is good!
As I said in the OP, same day, a couple of hours between them.
 

NigelM

Well-Known Member
Four rounds is not an adequate sample. SD certainly isn't valid with so small a sampling. Go shoot it.~Muir
Agreed on the statistical validity of the SD but I had 36 shots chronographed on the old batch and velocities on 0.4 grain increments were very predictable. They have been very predictable on the 16 shots fired of the new batch as well, but the new batch is averaging about 45fps slower.

Little doubt in my mind it's a batch difference with a slower burn rate on the new batch, I had not appreciated that changes between batches could be so high.
 

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