Some Observations on M1S Barrel and Converted Brass

[carcass]

I've been reading for a while about the problems that many of us have been having with Model 1 Sales' .300-.221 barrels, and I've experimented with a number of things. I think I've found some good information. I hope I'm not duplicating someone else here, but I thought what I've learned may save others some frustration.

Earlier this year I purchased a CH Dillon trim die for .223 Remington / 300 Whisper conversion after reading about them here. This has got to be the slickest way to convert brass ever, and if you have a Dillon case trimmer I wholeheartedly recommend it.

I had read about some people's problems with M1S barrels (rounds not chambering, requiring much shorter OAL, etc.) and was concerned that I had made a bad decision in getting one, especially when I loaded my first dummy rounds and could not eject them once chambered. In fact, I also experienced difficulty in getting the bolt carrier into battery. The gun would lock up solid, requiring the tried-and-true “pound the damn operating handle on a table” method to open up. Keep in mind that this was occurring both with cases that were just converted, as well as ones that had been successfully fire-formed.

At first I thought this was due to the bullet contacting the rifling lands, as some others had reported; however, I could not see any clear evidence that the bullet was being engraved. The weird thing was that seating the bullet to a deeper depth seemed to fix the problem for some cases, but not others.

Next I started playing with my crimping die, thinking that the case mouth was still somewhat belled. This moved a few more cases into the “go” pile, but my case rejection rate (based on chamberability) was still about 1 in 3. I was beginning to fear that I would have to carefully chamber check every case I ever made before loading it.

Third, because some here had written about case length issues, I tried trimming some cases shorter, which netted me a few more “go” cases, but not all. I hit a few cases with layout die, and started to see where the real problem was—rubbing from about the case mouth to about halfway back to the shoulder.

I had known that case wall thickness had been reported as a problem with military brass, so I started looking closely at the headstamp mix I was using. Lo and behold, all of my rejected cases were PMC cases with the “small letter” headstamp. The rest were Lake City of various vintages ('81 - '06), all of which worked great. Some PMCs worked well, but even some of them would hang up after a couple of reloads.

Now, comparing the LC and PMC cases, I think I have a good rule of thumb for rejecting a particular headstamp prior to making a bunch and function testing them. After trimming and resizing a single case with the CH die in the Dillon 1200, I then run it through my Redding resizing die with the expander ball in place. If, after this, the case mouth wall thickness measures less than 0.0125 in., the case will almost certainly perform well. All the PMC cases I had mic at 0.014”– 0.0165”. In addition, if the case mouth outside dimension exceeds about 0.326, it will likely be a problem.

I have been able to load out to 2.2" OAL with LC brass now with no problems. Loading long and allowing the bullet to contact the rifling shows a max OAL of about 2.215" in this barrel.

Last night I tried a few other headstamps including FC, RP, TZZ, and they measure within the specs I note above. I will go through the rest of the
headstamps that I have and try to verify each one.

I hope this information is helpful to have in one place. If other M1S owners would chime in I’d appreciate it to see if this information works for you too.