powder lot variances

[stubblejumper]

After using an old lot of IMR 4831 to develop a load for a new rifle I settled on 59gr which produced 3070fps.Since the old lot of powder was nearly used up,I bought three more cans of IMR4831.Before loading any large amount of rounds,I decided to compare the new lot# of powder to the old lot#.I loaded up five rounds with each lot# both using 59gr.I fired all ten rounds over the chronograph,and the old lot of powder was averaging 3060fps,while the new lot# was averaging 3165fps.It is not unusual to see differences of 25fps to 50fps between lot#s,but 105fps is much more than I am used to seeing.

When you change lot#s of powder,do you just keep loading rounds with your old recipe,or do you actually compare both lots of powder first?

 

[Eagleye]

SJ; I DO compare, because there is quite often a surprise waiting if you fail to do so. That being said, I have been known to buy 8-10 pounds of powder of one lot number to keep the redevelopment at bay!! I have also bought several pounds of one powder [different lot numbers], dumped it all together, mixed it thoroughly and then repackaged it in the original containers, scribbling out the lot number on the cans. Either system reduces the amount of time I spend comparing burn rates of different lot numbers. IMR 4831 has shown some variance, as has Varget, H4831, IMR 4064, and a few others. Regards, Eagleye.

 

[Silverado]

I know a number of precision reloaders who will mix quantities of old and new lots of powder to minimize this.

~100fps is quite a difference; it would equate to a big part of 1MOA at distance!

 

[Boomer]

Stubblejumper, did you observe this 100 fps difference from a single cartridge, or was it an average over a number of rounds? Many variables come into play here, and a single cartridge case that might have slightly more or less volume combined with a lot of powder that might be slightly faster or slower and perhaps a single bullet that is slightly lighter or heavier than the advertised weight can all effect the velocity of a single fired round on a day that was warmer or cooler than on your previous test day. What is your normal velocity deviation over 20 rounds, and did the loading with the new lot of powder change this?

Edited to add . . .
If your new can of 4831 is acting more like 4350, perhaps notifying IMR is in order.

 

[stubblejumper]

Stubblejumper, did you observe this 100 fps difference from a single cartridge, or was it an average over a number of rounds?

As previously posted.

I loaded up five rounds with each lot# both using 59gr.I fired all ten rounds over the chronograph,and the old lot of powder was averaging 3060fps,while the new lot# was averaging 3165fps.

If your new can of 4831 is acting more like 4350, perhaps notifying IMR is in order.

It is possible that the recipe has been changed over the years.The velocity with the new lot is closer to matching the current load data,than the velocity produced with the old lot.