45/70

I don't have a 45/70 but I do have a 450 Marlin - similar but not quite the same. I've never had a problem with my 450 Marlin but I have had a few experiences with hang fires and also with just the primer going off and shoveling the bullet a couple inches into the barrel in other calibers. Creates quite a mess with all that un-burnt powder. Each and every time this has occurred I was loading well below minimum recommendations for the powder. AA1680 is one powder you don't want to go much below the minimum suggested starting load.
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I think that is your problem - 45 grains of H4198 with that 300 grain bullet isn't developing the pressure necessary to ignite the powder - going to a mag primer such, as a Federal 215 may work but I wouldn't recommend going that route. Hodgdon recommends a starting load of 53 grains with pressure being around 40K CUP - 45 grains in my opinion is a might low.

http://www.hodgdonreloading.com/data/rifle
 
My Hornady 8th Edition has H4198 minimum at 42.3 and max at 51.2 grains for the Marlin loads. Being a highwall I'm under the impression I can use Marlin loads in my rifle. Sure wish all loading books were closer to being the same.

Actually, if you look at Hodgdon's loads for the trapdoor, their starting load is 45 grains w/ H4198.
 
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Ya, loading books/references do have differences. The Lyman 49th edition doesn't even list H4198 - just IMR4198 for the 45/70 with 300 grain jacketed. Barn's list 49.5 to 53.5 for their 300 grain jacketed using H4198.

When I first experienced problems using below minimum AA1680 loads and experienced hang fires and just the primer going off using standard primers as called for - but with magnum primers, I seldom experienced that problem - I called Western Powders with my concerns. The powder technician guy advised me that I wasn't developing enough pressure to set off the powder column in the cartridge. The primer starts the reaction but with the 1680 powder characteristics, I needed more volume to create the pressure necessary for complete combustion. The magnum primer usually assisted in developing the minimum pressure to start the combustion but was not a fool proof fix and not recommended for my problem.

This made no sense to me, as I'm not educated in that field, but when I went up in powder weight to the minimum suggested with the standard primer - everything worked as expected and I never experienced another hang-fire or just a primer pop lodging the bullet in the barrel and powder all over the place.

I've messed around with some surplus powders such as BR-28, a powder completely unknown to most reloaders. Starting low, I had a few primer pops but as the powder was increased, the primer pops went away - I was able to increase those loads to the point of heavy bolt lift and ruined primer pockets.
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Accuracy for my particular rifle was found to be on the top-side of that powder and my cases lasted for 3 firings before ruining the primer pocket shooting a tight neck chambered 6BR - I purchased a bunch of that powder, as it was inexpensive, couldn't give it all away, so I still have a couple pounds to remind me of my folly. Been meaning to give it a go in the 223 for the past 30 years but just haven't gotten around to it yet.

I would still recommend you increase your powder charge with that lot of H4198 until your problem goes away or change over to IMR-4198 or another powder of your choice. Myself, I prefer H322 but H4895 is a close second for those 458 diameter bullets.
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Thanks JoeyJ for the advice. I'm really new at loading for the Highwall so its all a learning adventure for me. The rifle is just going to be a gun club range rifle so I have been trying to keep the loads light and the recoil to a minimum. If thats possible with a 45/70.
 
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