IMR 7383 is a very strange military surplus powder that I was introduced to in early 2002. An internet search for this powder will yield tons of information concerning its original use, so I won’t waste your time with a history lesson.
I bought my first four jug case of 7383 in the fall of 2002 and have been burning it ever since. It is limited in its versatility, but because it is economical, I continue to use it and experiment with it.
The most important feature about 7383 that the handloader needs to be aware of is it has a propensity to create very dangerous high pressure spikes in bottleneck calibers when the charge density approaches 100%. Oddly enough, it doesn’t begin to perform well in tapered straight wall calibers (.375 Winchester, .45-70 Gov’t., etc.) until the charge density exceeds 100%. Go figure, make up my mind.
On the other hand, it performs very poorly at charge densities below 90 to 95% in bottleneck calibers. It would seem that this powder must be used within a very small window of charge density. That’s about the size of it.
All my research with this powder has revealed that the best charge density to start with in bottle neck calibers is 95%. Please refer to my article “Establishing Load Density” for my method to achieve this.
Most of the time, I’ve found that 95% is not the maximum, but a good place to start in load development with 7383. I have found, however, that in larger bottle neck calibers, 95% is very close to the maximum. I will say that the largest bottle neck caliber I’ve tested 7383 in is .30-06.
Oddly enough, I have seen times when the pressure did not manifest until the charge density was almost at, if not all the way up to 100%. 7383 is a very quirky powder.
If you decide to experiment with 7383, I would strongly recommend you not start with a charge density in excess of 95%. I would also strongly recommend you use the method I described in establishing that density. A mistake of a few grains can make the difference between a safe, successful start and a locked bolt or worse.
With respect to the percentage of charge increases, I would recommend 1% at a time. If your calculator shows a 1% increase in between whole numbers in grains, round back to the nearest grain. 7383 is NOT a powder to crowd. Better to go slow than wreck your rifle and/or need a ride to the ER.
On determining the stopping point, it’s been said many times, but it bears repeating. Using a bolt-action rifle makes this very easy. Fire the shot and with the rifle still in the rest, pull the bolt. If you see unburned granules of powder in the barrel, you’re not there yet. Make note of the charge when you find no burned granules in the barrel. That’s your safe maximum charge for the combination at hand.
Contrary to what you might think ( “let’s go one more grain” ), DO NOT exceed the charge at which the powder is completely burned. Going past that point will most certainly put your chamber pressure in the red zone. The question is at what charge will it spike. Trust me, that’s not a question you want to find the answer to.
7383 can be made to work. However, there’s a limit to what you can do with it. If you take that into account, use safe reloading practices and go slow, you will find it can be used successfully.