I'm getting miss fires in a 45-70 using 40grns of IMR3031 and large rifle primes. The primer will push a column of powder and the bullet into the rifling.
Factory loads work great, this is my first hand loading the 45-70.

Any ideas?

Just guessing
1 powder get wet or oil or something.
2. improper neck tension on bullet.

What weight bullet are you using.

let us know what you find out.

Your primers are not igniting the powder properly but still have enough energy to push the powder and bullet into the bore. Use a newer batch of primers if they are old or may have gotten damp. You could also try a different brand of primer as well as a heavy crimp. Try one change at a time so you get to the root cause of your problem and correct it, then let us know what you have found out.

Woody

You have only 2 things that can be wrong! Bad primers or bad powder. 3031 is great in the 45-70 and will burn just fine with almost zero neck tension and no crimp. Even, firm neck tension will increase accuracy and help the burn, but I have used 3031 with unsized cases, just finger seating the boolits.

Imr-3031 ismy favorite pwoder for the 45-70. I would suggest 2 things. You need a heavy crimp(Iuse a lee factory crimpdie) and try a winchester primer.

Had exactly the same thing happen with mine using 340gr cast boolits and H335. Even loads that went bang had half the powder in the barrel. I tried a winchester magnum primer and a moderate crimp and it became a real gem of a load.

I can't see the blame on the powders. I have used VV-N133, IMR3031 an currently am using H322. I don't use a mag primer. Have shot quite a few hardcast and some 4 or 5 hundred rounds of jacketed of various brands. I try to see whereing lies the problem outside contaminated powder-moisture or mishandled primers. Crimping or not is not an issue unless you have them in a tubular mag. Now my only "misfires" occur when a primer is seated .002" or deeper so I just chuck them brass in the trash. Keep us informed.