When factory 1911 barrels are produced, the chamber is cut measuring from the rear edge of the hood forward. As you remove material from the hood to fit the barrel to your slide, you are shortening headspace. If you drop a round into a barrel with short headspace, the rim will be above flush with the rear surface of the hood. This condition may prevent the barrel from locking. I have seen "drop in" barrels from major manufacturers with short headspace.
When the barrel is locked in firing position, the chamber end is higher than the muzzle and the cartridge base is actually making contact with the breech face at a slight angle.
My Clymer .45 ACP Go gauge measures .900 inches but we will need a bit more headspace for reliability so the round will not bind during chambering and also to allow a little room for crud.
Years ago I made my own Go and No/Go gauges for barrel fitting measuring .903 inches and .906 inches, respectively. The SAAMI No/Go spec is .920 inches, and I have only seen one pistol that would chamber a .920-inch gauge in decades of working on 1911s. The .45 ACP is a low pressure cartridge with generous allowable headspace dimensions, but if you are chambering a more intense round like .38 Super, the specs will be tighter.
Install the barrel in the slide with the Kart bench bushing, drop your Go gauge in the chamber and attempt to push the barrel up into locked position. The likelihood is the barrel will not fully lock. Remove the barrel and clamp it vertically in a vise.
The chamber will have to be reamed with a headspace reamer a little bit at a time until the barrel locks. Use cutting oil and go slow. You will note from the pictures I have installed a sleeve on my chambering reamer to control the depth of cut.
The depth of cut is controlled with feeler gauges between the sleeve and the barrel hood. Start with a .020Ã® gauge, take a cut, check headspace.
If the chamber needs to be cut deeper, I go to the next smaller feeler gauge and take another cut. In this way, I closely control cutting depth. No mistakes, no overcut chambers.
Once your "Go" gauge will chamber normally, add a .003-inch feeler behind it. When the barrel will fully lock up with the "Go" gauge in place and a .003-inch feeler gauge, you're done. Try a .006-inch feeler as a No/Go check, but don't use the actual No/Go gauge. In fact, there really isn't any reason to buy a No/Go .45 ACP gauge.
At this point, we have fitted the hood, bottom lugs, link and cut the chamber. The last fitting step is the bushing. Stay tuned.
Here are the tools needed for headspacing a .45 automatic: chambering reamer with sleeve, feeler gauges, Kart bench bushing and headspace gauge(s).
Short headspace may cause the rim of the cartridge to protrude above the barrel hood, preventing the barrel from fully locking into the slide.
Check headspace by attempting to press the barrel into full lockup with the Go gauge chambered. Most of the time, it won't quite go into position.
Lengthening the Chamber
Adjust headspace by carefully lengthening the chamber with a chambering reamer. This sleeved reamer uses feeler gauges to control depth of cut.