Marlin Cowboy 38-55 case separation ?

[Dom]

Testing some loads with a new Accurate 300gr cast bullet mold, in my Marlin Cowboy 38-55. A mild load using 14grs of a powder to achieve 1345 fps. I have fired many of these , no problem. Using cases from a batch of Win brass I've had for years . Some cases from this lot have been shot 20 times. Other than annealing the case mouth every 10 shots , perfect. Today a case from this same lot , on the third firing ( I still have new unfired brass from this lot ) separated. It appears the front portion of the case & the bullet went down the bore & out. After carefully checking the chamber & the bore I went on to fire a few more of the same bullet/ powder combo without incident & great accuracy. I have to assume a bad case. I bought this rifle new in 1999 or 2000. Acquired this lot of brass back then. Never a problem till now. Picture is of the rear of the separated case , normal case & the bullet I'm shooting. 289893289894

[Bent Ramrod]

That shiny, grainy ring around the base of your shells could be an indication of incipient head separation. Straighten a paper clip, bend 1/8” of the end 90 degrees, and draw it up and down inside the shells. If you feel a groove around the circumference down there, the shell is likely to pull apart on future firings.

Occasionally, though, one just “lets go,” through the vagaries of mass production. I had a new .44-40 shell do that on the first loading. The rest of the shells have given no trouble after a dozen loadings.

[Dom]

I already checked for the separating ring in the base of the case. None there. I have been shooting this same lot of brass for years & have never had a case separate. As stated before, some of these cases have been shot 20 times. At this point I have to assume a bad case.

[John Taylor]

Looks like the pressure may be a bit high and also you may have some headspace problems. First step would be to check headspace. Most of the lever guns are not designed to handle pressures over 42,000 ( 30-30 factory loads). If there is a headspace problem the brass usually will not stretch at this pressure but the primer will back out. When the pressure gets much higher the brass will stretch so the base is against the bolt and the primer has not backed out. I had an old Win. 94 come into the shop with .025" headspace and it was still being used by the owner. Primers would back out but no sign of problems with the brass. A sure sign of pressure and headspace is the shiny area about 1/4" to 3/8" in front of the base.

[Eutectic45]

Are you shooting the bullets un-sized? What is the diameter of the coated bullets? Outside diameter of the loaded rounds?

I have had this happen with 357 Mag. The variations in casting diameter and case wall thickness combined to equal the chamber diameter on a few rounds. The result was no room to release the bullet and the cases separated on a few rounds with a normal load.

The hint was some rounds did not drop into the chamber. Just a little push as needed, not even noticeable on the lever.