So-called “drop in” barrels are so called because they’re supposed to drop into the slide, but the term “drop in” certainly doesn’t apply to the ability to drop a headspace gauge into the chamber. It didn’t apply to the Kart EZ-Fit or the cheap no-name barrel used in the last build, and it didn’t apply to the Storm Lake used for the current build.
The problem wasn’t one of chamber length, but taper. Reamers wear out from the front back, so each chamber after the first will be cut with increasing amounts of taper. Even with frequent resharpening, a professional barrel-maker’s chambers will acquire varying degrees of taper.
Fortunately, this is not a problem.
As always, the feeling of the reamer removing taper is distinctly different from setting the headspace. .45 ACP headspaces on the case mouth, so the flat lip at the front of the chamber must match up with the edge of the case mouth where it grips the bullet and the distance from this lip to the breech face of the slide must be long enough to permit the slide to go into battery, but short enough to permit the firing pin to reach the primer and fire the gun. Once the reamer removes the taper, only the very front edge of the reamer cuts, moving the lip forward. It’s possible to feel the instant when the last of the taper is gone.
That being said, I’m paranoid about headspace and checked frequently. Usually these neurotic tendencies just make everything take twice as long as it would for a more confident builder. This time, however, it prevented me from ruining a barrel.
My process was as follows: Turn the reamer a full revolution, remove and clean it, clean the chamber, and see if a GO gauge would drop into the chamber without catching on a tapered edge. Once the milestone was met, I added a step of removing the barrel, assembling it in the slide, and seeing whether it would close on the GO gauge. As the chamber approaches the correct length, the barrel hood will slowly start to rise to its locked position.
The problem was that the barrel hood didn’t seem to be going anywhere as the chamber got increasingly longer. After the third check with no apparent motion, I stopped cutting and took a look at the barrel and slide with the gauge in place. Strangely, it looked like there was light leaking through behind the base of the gauge–it wasn’t fitting flush with the breech. Further examination revealed why.
Incomplete machining left a tiny raised section on the breech face, effectively shortening the headspace. It could be removed, but had I caught it in time?
Only one way to find out.
I used a #2 Swiss cut pillar file with a safe edge to remove the bulk of the material, then cleaned up the top area near the curve with a series of needle files and some strong language. Although the results were surprisingly clean for something filed by yours truly, I couldn’t enjoy them right away due to concerns over whether I’d cut the chamber too deep before discovering the ledge.
I needn’t have worried.
The barrel was far from going into battery with a NO-GO gauge, yet closed perfectly on a GO gauge. The GO gauge had a very tiny yet detectable amount of slop, which was exactly what I wanted. Shorter chambers provide greater accuracy, yet are more sensitive to bullet irregularities and fouling. Given that this handgun would be used with a silencer, with commensurate backpressure and increased carbon buildup, a slightly longer chamber was desirable.
Thanks to the undetected ledge on the breech face, I’d finished reaming the barrel without knowing it at the time. In a way, the ledge did me a favor by preventing the inevitable dithering and nail-biting at the end of reaming. Was the chamber deep enough? Would one more turn of the reamer be enough–or too much?
No such concerns this time. The chamber was reamed and barrel fitting complete. The slide was far from done, though. Before I could stop work on the slide, I would need a stop–a firing pin stop.