45-70 - Soot on outside of spent shell case - basic question on charge burning

[tacotime]

Shooting some mild reloads in 45-70 last weekend (1300 to 1500fps), I noticed two or three cases came out of the single shot showing soot down the outside of the spent case.  The worst was a streak about 1.25 inches long and maybe about half way around the case mouth.  I am assuming this is from powder still burning in the chamber space left as the bullet leaves.  It was probably 4198 but maybe 4064 with a 405 grain JSP and standard primer.  What kind of issue is this?

And generally speaking, does the powder ever fully burn before the bullet exits the case, or is there always powder burning in the barrel?

Thanks.

[Blackhawker]

I have generally understood soot streaking down a casing having to do with a light powder charge that hasn't enough pressure to expand the cartridge open wide enough to fill the chamber.  If the cartridge doesn't fully expand into the chamber, gasses are blown back between the vacant space around the brass cartridge and a sooty residue is plated out on the cartridge.....and probably within the walls of the chamber as well.  I have found that a heavier load will prevent this from occurring.

As for your second question, YES, almost all powders leave residues behind in the barrel...some more than others.  I have found that Unique (at least the older version of it) was always a dirty powder which left lots of soot and particles in my barrel when using cast bullets.  Some of the ball powders also leave small particles in the barrel as well.  This is not uncommon at all.

[3leggedturtle]

Especially if you are using Starline cases, they take a few times fill out to your chamber. I only size the 1st 3/8 to 1/2" of the case or however far down the bullet seats.

[BBF]

Straight cases seem to do that more often then bottlenecks. My 44 Rem Mag did it as well even with almost full loads with some brass.
If I was to guess which powder it was I would pick 4064.

[torpedoman]

i reload the 45-70 with a universal decapper and a universal expander then use the proper die to seat the bullet in the case after the first firing no problems.

[tacotime]

Thanks