Actually, the priming mixture in the corrosive primers was mostly Potassium Chlorate, which, in itself is not
particularly corrosive.
However, when the primer is detonated, the residue left behind is Potassium Chloride, which is a salt that
attracts moisture vigorously, and can quickly cause pitting from rust in the iron of the barrel as well as
other areas of semi-automatic firearms. Eagleye.
This is true.
There is one other thing that we seldom come across but is very relevant. There are times when the powder in the case deteriorates. When this happens in our storage containers we notice it either by the smell when we open the container or in the case of the older metal and paper/metal containers by the rust that formed on the inside surfaces and the little rust particles in the powder itself.
When powder has deteriorated to a certain point and is fired down your bore, it can be extremely corrosive. I had some surplus Bofors #44 powder from Higginson's a few decades ago that did exactly that.
There was a very slight difference in point of impact that I didn't take seriously at the range one afternoon. I didn't clean the rifle because I would be taking it to the range again the next morning and I had only fired a few fouling rounds since the last cleaning.
To give an example of just how corrosive this "bad" powder was, when I went to the range the next morning and shot the first round there was definitely more recoil than I had come to expect from the little FWT Mod 70 in 257Roberts. The primer was flattened way more than just a little. This raised some flags but I decided it was a one off and chambered another round. This time almost no recoil. That was enough to get me to stop. Something was seriously wrong.
I took the rifle home and CLEANED it. Back in those days, Wipe Out wasn't available. So I used Sweets and sometimes Young's 303 oil with Motty's bore paste for tough jobs like stuck on cupronickel that didn't respond well to Sweets. The homemade ammonia mix used by the armorers was only used as a last resort.
What I found after letting the Sweets to its work was a badly damaged bore.
This bore had less than 300 rounds down it. It shot 85 grain flat base bullets and 100 grain flat base bullets extremely well with the #44 powder.
The bore wasn't pitted but looked like someone had taken a ball peen hammer to it. That must have been why the heavy recoil with the first shot. It pushed the rust/etc out the bore ahead of the bullet. Must have been relatively loose/soft because there wasn't a bulge. When the second shot was fired there was almost no rifling left and the grooves were enlarged as well.
At first I though it was the Winchester barrel. I had purchased the rifle new and it was one of those push feed models that was right in the serial range of a few batches that had poor barrels.
I called the Winchester rep I knew and he told me that I could send the rifle back but turn around times would almost be forever. He knew his stuff though and deduced that if the barrel shot well initially there was another issue. This was my first run in with powder that had deteriorated. He did suggest it may be the powder but it was only a suggestion.
I went to what was left of the 50 round container of cartridges and pulled the bullets. They had only been loaded a few weeks previously but the bases of the bullets were stained green and bits of powder were stuck to them. The powder in most of the cases had bound together in a solid block. When I was able to dig the powder out of a few that insides of the cases were stained green as well. When I went and looked at the once fired cases they were badly pitted on the inside as well.
This was an extreme condition. I had purchased the powder back in the early seventies from old Tom. I had been using his powders for quite a while and trusted them. By the time I had issues with it the powder was 20 years older and for whatever reason had broken down. The only other powder I've had break down was also a surplus lot of Bofors powder from Higginsion's as well. They were selling it as OEM 4350 but in reality it was a surplus lot of N160. After ten years and using close to twenty pounds of it in my 338-06 and a very accurate 7x57, I came into the powder magazine to be greeted with a smell similar to HCL.
At first I didn't know which powder lot was causing the offensive odor and it was so strong I didn't want to stay in the room until the breeze cleared it out. I was even afraid of turning on the lone electric bulb.
Once there was enough light to see, the metal shelves the boxes of offending powder were stored on were dripping rusticles. Sadly I had to throw close to 10 pounds into the fire pit.
Eagleeye can attest to this as well. He had some of the OEM 4350 go bad.
Sooooooooooo, given the proper circumstances powder can cause rusty bores. Given that surplus ammunition is USUALLY sold as surplus for a reason, often just a best before date, you can never be to careful with the powder in those cartridges. Yes, the powder can and will deteriorate even in sealed containers and cartridge cases and they can do a lot of damage to your bores.
I'm in no way even suggesting that you stop using the Mexican Reload method. I use it myself. I am suggesting that you pour the powder into one container after weighing several charges and to get an average and blend it all together. Then, weigh out the average and fill your case. While doing this, inspect the powder for rusty/dust or rust flecks. The chances of there being a problem are slim at best.
By the way, it doesn't matter whether powder is single or double base. They both will break down eventually.
I have a container of 4831 that is unopened with a early 1940s date. The container beside it is just about empty now. The sister cannister contains powder that performs exactly as it did when I first purchased it from Hodgdon's back in the early seventies. It's always been carefully sealed after some is taken out and it's never been frozen or over 65F and it's always in the dark.
Ganderite is really the one that could answer this far better than I have. Hopefully he will put his two cents worth in.
