Much has been written about IMR 7383, the quirky milsurp powder that is so unpredictable and difficult to understand. I’ve been testing and experimenting with this powder for 12 years now and I’m still discovering unusual characteristics about it.
I’ve used it to push 55 grain NATO ball bullets through a bolt action 223 Rem. all the way up to hard cast water dropped and gas checked 540 grain lead alloy bullets in a .45-70 Gov’t. bolt action. The two things I’ve learned about this powder are completely oxymoronic. It can be made to work, but it can’t be predicted.
I’m going to try, as best I can, to refrain from repeating that which has been repeated many times over. However, some of what’s been said bears repeating for the sake of safety.
My test results and conclusions lead me to believe there is a window, very small albeit, where 7383 works. And then, just when I get a grip on this goofy powder, it makes a fool and a liar out of me. Make up my mind for cryin’ out loud!
A recent test with a new caliber is what motivated me to write this article. Last week, I took possession of a new Savage Axis Youth model in 243 Winchester. Among the powders I have to work with, I decided to try 7383. Using 55 grain jacketed soft point bullets and knowing 7383 works best at high pressures, I decided to try it.
I started by die forming some 2005 Lake City 7.62X51 brass to 243. While I’m at this point, I’d like to thank my good friend out in Texas, Trey Meyer, for the brass. I weighed and sorted all the cases to within two grains of weight. I checked each case for length and found they were all within spec, + .000″/- .002″.
When it came time to find the top of the pressure window, I used the method I developed specifically for this powder. The notes from my journal are as follows:
55 GR. DOG TOWN
42.5 GRS. 7383
100% CHARGE DENSITY
40.4 GRS.- 95%
41 GRS.- TOP LOAD!
The tests I’ve conducted over the years have always indicated 95% charge density is a good, safe place to start load development toward the top of the pressure window. I started at 40 grains with this test and it’s a good thing I did, as 41 grains proved to be the top load.
Once I established the top of the pressure window, it was time to shoot for group. The first of five rounds went off the paper because I did not have the scope correctly set. Once that was rectified, I put four rounds in .600″ at 50 yards.
With the scope set to maximum magnification, 16 power, and a gusty right to left crosswind, I had a bit of difficulty holding steady. Had it not been so gusty, I feel sure I could have done better.
When I’m shooting load development ammo, I do a visual inspection of each and every case as they come out of the rifle. Since I was shooting top pressure loads, I checked each case carefully looking for pressure signs. The first four came out with slightly flattened and printed primers and no chamber prints on the case walls. The fifth case came out looking like the chamber pressure spiked.
Now remember, I checked each case carefully before I began loading. Both cases were + .000″ and both were within – .002″. The left case weighed 179 grains and the right case weighed 181 grains. That puts the right case at only 2% heavier.
After firing, the left case was still within length spec. The right case, however, grew by .008″. However, neither case gained any diameter at the head.
While weighing out the charges, I manually trickled the powder into the pan of my Hornady Auto Scale. I was very careful with this as I know that four to five granules of 7383 equates to .1 grain. I wanted to make absolutely sure that I didn’t go to 41. 08 grains as the scale would not indicate that.
Before loading, I checked ten bullets for weight variation. One was 55.1 grains, one was 55.2 and the rest were 55.0. That’s less than 1% variation. When seating the bullets, I checked the COL of each round as they came out of the seating die. Each one was within .001″, +/-.
Why all these careful measurements? Chamber pressure can spike with little or no notice with 7383, especially at the border line of the top of pressure window. and I’m not too fond of having to beat the bolt open with a rubber mallet.
Long range shooters go to great lengths to weigh every bullet and every case and choose only those that match perfectly. But I wasn’t shooting long range, I was shooting loads that were at the top of the pressure window. Knowing what I do about 7383, I was confidant that the loads I had assembled would not spike. But look what happened with the last round, the case on the right.
I am absolutely positive that every component of every round of that batch was in spec. So what caused the pressure in the last round to spike? I have no clue. The primers of the first four rounds appear to indicate a pressure somewhere in the mid to upper 40Ks range. The primer of the fifth round looks like the pressure went to 60K, maybe a touch higher.
So, what’s the point of all this lengthy blather? If you’re going to experiment with 7383, especially with top of the pressure window charges in bottle neck calibers with relatively small case mouth diameters, you sure better know what you’re doing. One minor mistake and you might be wearing a bolt.