I had an interesting observation when loading cartridges for a Thompson Center .223 Remington barrel I acquired a few years ago. Mike Bellm has written about these barrels for decades and identifies several issues with production of them over the years at Thompson Center. Throating is one of the major burps that can arise and varied over the years with same caliber barrels. Off center chambers and ovoid throats were a major problem on some calibers and these issues too, varied over the life cycle of production years.
Knowing how to evaluate headspace in a T/C barrel is misunderstood by the vast majority of T/C users. Barrel to Frame gap is NOT headspace in these barrels. Mike Bellm has multiple videos on The internet showing this if you are interested. My concern when working up the loads for this barrel was the ever so slight projection of the cartridge from the end of the barrel. It was about half a thousand (.0005") and almost imperceptible. I noticed it when gently closing the barrel and it would not lock-up properly. Forcefully closing the barrel, accomplished barrel/frame lock-up but I knew something was amiss.
Above: Resized Federal Case with RCBS SB Size Die. There is an almost visually imperceptible protrusion. A metal ruler edge shows it clearly.
A factory cartridge inserted fully into the chamber with a noticeable depth to it and clearly deeper than my resized cases. Fired cases scrounged off a range floor would not seat fully due to expansion in another chamber. As said above, my cartridges sized with a RCBS Small Base sizing die almost fit correctly but missed the mark by an almost imperceptible degree. I have said many times before that half-a-thousandth or one thousandth of an inch in a firearm is like the grand canyon to a gun.
Above: Factory Ammunition Fully Seating
Above: Scrounged Brass Prior To Resizing
The obvious place to start was with the cartridge base. A precision micrometer revealed that the base of my RCBS sized case was smaller by a few thousandths that a factory new cartridge which seated correctly in the chamber. Of course my notes are in the workshop right now, not in my office where I am typing this. If memory serves me correctly, my cases were .003" smaller (narrower) than the factory case.
I then recalled an issue I had with resized 38-40 cases not chambering in some of the Uberti 1873 revolvers. To make a long story short, I had .020" turned off the bottom of the Lee sizing die to set the shoulder back allowing for chambering in my family of 38-40 firearms. The issue was not the die, it was the differing chambers not accepting the apparent SAAMI correct die specifications with regard to shoulder location. As some of you may know, a factory 38-40 cartridge has almost NO shoulder evident to make chambering in a wide variety of guns possible.
Remembering the shoulder setback issue mentioned above, I carefully inspected my resized cases. The bases were getting resized properly but I noticed the neck was resized about 2/3 of the way down. There was no contact with the shoulder. Could this be the issue? Since I had a few extra shell holders, I decided to take a few thousandths off the top of the shell holder allowing the case to enter the sizing die a little farther to get the neck/shoulder sized properly.
My first case ran into the die perfectly, and testing it in the chamber revealed I was on the right track. The case inserted fully into the chamber. My second case, when run into the sizing die, ripped the top off the shell holder where I had thinned it on the withdraw stroke. My suspicion was correct, they are thick there to withstand the enormous pressure of withdrawing the case from the die. Now I had a stuck case. Darn.
I have another set of .223 Remington dies and broke out the Lee Precision die set. I normally use this die set in the Dillon machine but haven't run off large batches of .223 for 3-Gun matches in probably 20 years. These cartridges were used in a semi-auto rifle to hit targets much closer than the T/C would be used for.
I adjusted the die base to touch the shell holder and sized a few cases. Bingo! They all fit into the T/C chamber as perfectly as new, factory fresh cartridges. Problem solved. The Lee sizing die formed them with small bases, and fully formed the neck while kissing the shoulder, forming it back to SAAMI spec. There are many among us who love to Lee-ment about the lack of quality of Lee products. I am not one of them since my experience with Lee products has been positive. That's not to say everything Lee is perfect, like the hand prime tool which I gave up on after 3 of them broke. Lee did send new parts but my thumbs could not stand another 10 years of swelling. I now use the RCBS Auto Prime Tool and my thumbs have returned to their normal size.
So here is a case where the "better" tool needs modification to size cases correctly like the Lee tool did. Rather than spend $60 to have the RCBS size die faced off on a lathe, I instead ordered a new die set. Perhaps the new set will size the cases fully when adjusted to kiss the shell holder. We shall see.
Ultimately my issue was headspace, caused not by the cut chamber, but instead by the incorrectly sized case. One must know however how to evaluate and measure headspace in a T/C barrel. Headspace may be .003" with a barrel/frame gap of .50". Watch Mike Bellm's video about this. An important side note here is that a high primer in one of these cases could have led to an out-of-battery firing on the gun when the action was forcibly snapped shut. That's not an event I wish to experience with a Contender.
I mention all this to point out that our first guess as to why something won't work may not be the root cause at all. Careful thought is usually required along with knowledge of how things "should" work in a particular situation. That takes careful observation and learned knowledge. Correcting handloading and firearm issues are not usually done with a quick fix or a Dremel tool.
Safe Shooting!!
With rifle in hand, I confidently go forth into the darkness.
