Mike Venturino is a big advocate for XMP5744 in old lever gun cartridges such as 38-55. I tried his loads in my 1893 Marlin with soft lead bullets, and was not impressed. In my case, I ended up with a lot of granules of burned powder residue in the action after firing. Maybe because she slugs with an oversize bore (.385 groove diameter) I am not getting a clean burn?
Anyhow, this actually created a safety hazard for me, as the granules built up in the full cock notch on the hammer. One day when I worked the lever, the hammer did not stay in the full cock position as it is supposed to, due to the cock stop being plugged with powder residue. Not thinking, I pulled the hammer back to full cock with my thumb and released it. Since the stop notch was filled up, the hammer did not stay put and instead dropped the hammer and discharged the rifle. I was essentially only holding it with my left hand on the forearm, and as such dropped the rifle in the sand, partly due to surprise and partly due to lack of control over recoil with one hand. Luckily I had the muzzle pointed in a safe direction and no one was harmed but it was upsetting.
Anyhow, I am not sure if my experience with XMP5744 residue is relevant, unless it could contribute to the neck expansion issue along with the accumulation of bullet lube, or if powder residue packed into the locking lugs and caused the bolt to not close as tightly as it should.
