I put a new scope on my Marlin Guide Gun 45-70 over the weekend. I took it out Monday to sight it in.
I had fired about 5 rounds and had it almost where I wanted it. A bit a readjusting and when I touched off the next round all I got was a mild 'pop'. I waited a few seconds and opened the action and the unburned powder fell out all over the action and the bench. Some of it was caked an all of it was a orangish brown color. (H4198). Luckily I was shooting my muzzleloader too and my jag fit nicely down the bore and I was able to knock the bullet out.
With the powder into everything I couldn't camber another round so I had to take it home and strip it down and give it a good cleaning. I pulled all the remaining bullets from the cases and emptied the powder into a container. It all looked fine. I resized 10 of the cases and reloaded them with the same powder. Took them out and fired them without any problem. Actually at 50 yds the first 3 rounds went almost in the same hole an one just outside. Did some fine tuning of the scope and shot off the remaining rounds with no problem.
Its a mystery what happened with that one round. I am very careful with loading. The only thing I could see happening is that something got into that one charge to contaminate the powder. But what I don't know. I never have gun oil out when I am loading, (it would have had to be a decent amount of oil to contaminate the powder, but the primer was fine). I use Hornady One Shot case lube.
Any ideas????

That IS strange. Sounds almost like a load with no powder, and just the primer went off enough to lodge the bullet in the barrel. The powder color just doesn't make sense. It must have had some kind of ignition to make it turn color, I'd think, and I really don't understand why all the other shells you pulled the bullets from had propre colored powder and it worked on the successive reloadings.
I had a powder experience with my 45/70 this summer. When using factory (Rem.) 300 gr. ammo to get it sighted in with and get some casings to start loading with, I noticed powder granules sticking to the patch when I cleaned the bore after each shot during the break-in. I tapped the casings on the bench, and a few more particles fell out. It was not the same color as the factory powder, as I cut a factory shell apart when I got to the house. This happened every shot with the factory ammo. I also reloaded with H4198 and have not had that happen since, but this is not the same scenario you describe......strange.... Hope someone on here has a better idea.....

Sounds to me like something contaminated that one powder charge. Also sounds like that primer fired OK, but the charge did not ignite completely. Very odd occurrance.

I am new here and this seems to be a good forum to join as I was searching for information.

The same thing happened to me using 45-90 cartridge I had reloaded with IMR 4198 powder. The gun jammed and had to blow out the powder with air gun.

Out of the batch of powder I purchased, so far I fired 100 rounds and only had that one problem. Could it be the primer?

Was it a new set of brass or some you may have cleaned. There may have been something lodged in the casing you never saw and caused it.

Do you tumble with corn or walnut? I have to think something stopped the ignition and pressure didn't hit where that powder likes. But 4198 seems to have a wide range of pressures from 28gr with 300gr to 55gr.

I hate these problems where your shaking your head.

It happened on the second or third round with the fired brass. And I do tumble with crushed corn. But never really inspect the brass closely. Just tap it against the tumbler when taking them out.

I had a very similar thing happen to me and it was due to media being trapped in the flash hole. I had it happen once too but the bullet actually made it about 10 yards and hit the dirt, and that one I chalked up to using the One Shot lube. I role my cases now with RCBS case lube now.

Sorry, I've been away for a while. My cases all get deprimed then tumbled in crushed walnut shells. Then I go through each one individually and clean out the primer pocket and the flash hole.
About the only thing I can think of is that I previously attempted to load the 45-70 with RL7 and had this happen several times. (I was talking to an acquaitance from work the other day and he had the same thing happen to him with his 45-70 and RL7. I would have been informative to see if this was the same lot of RL7.) It could be possible that this one loaded cartridge with the older RL7 got somehow mixed in with the H4198 loads. I never mix loads but I guess anything is possible. I've fired hundreds of rounds of H4198 loads out of this rifle without any problems previously.

Sounds like a plausible explaination.... I was just thinking something else....If you took a primed case and intentionally placed a piece of tumbling media over the flash hole and then finished loading it as normal and fired it, I'm wondering if you would get the same result as you did initially...??? Just a thought.....