February 15, 2013
By Gus Norcross
As we've noted before, the most important factor in 1911 accuracy is barrel fit. In the last article we fitted the hood, and in this one we delve into the fit of the bottom lugs to the slide stop. When the slide is in battery, the barrel should be wedged between the slide stop and the locking grooves in the slide with zero movement. In fact, if you function the pistol by hand without the recoil spring, you should feel a slight resistance as the barrel locks.
The lugs on our Kart barrel are oversized and must be cut to fit the slide stop. Fitting can be done by hand with files and stones (very time consuming), with a hand-turned lug cutter such as sold by Brownell's (080-000-068WB) or with machine tools.
I used a manual lug cutter for many years before I had a milling machine and they work fine, but there is no control over the size of cut. You get a .195" or .186" cutter and they have to work. I believe machining the lug with a dedicated fixture is the best way and that is how I do it now.
We begin by verifying the size of the slide stop pin. Check the size of the hole in the frame with pin gauges. The Colt frame I'm using for this demo is .202" and our new Ed Brown slide stop is .200". That fit is reasonable. Slide stops are available with pin diameters up to .203" for worn frames. Use the largest pin that will fit the frame and still rotate freely.
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Install the barrel and Kart bench bushing in the slide and assemble it to the stripped frame without the slide stop. You will need pin gauges in sizes ranging from .180" to .200". Insert a .180" pin gauge in the slide stop hole and try to move the slide forward into battery.
If the barrel locks up on the pin, keep trying larger pins until you reach the largest size that will allow lockup. Let's say the largest pin the pistol accepts is .180". OK, that means you've got to remove .020" of material from the lugs for them to lock up on your .200" slide stop pin. Remove the barrel and take it over to the milling machine.
In our mill vise we have the Weigand barrel fitting fixture. This tool allows the barrel to be clamped in a V-block while the lugs are milled to size and the barrel can be removed for checking fit and returned to the fixture without loss of zero.
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The bottom of the fixture has a step milled into it so cutting is done at the proper lockup angle. A digital readout is highly recommended when using this fixture. The cutter we have mounted in our mill collet is a four-flute HSS 5mm end mill (closest size available to our .200" slide stop diameter). Take a test cut, removing a few thousandths of material from the lugs.
Now we reassemble the slide, barrel and bushing to the frame and select the largest gauge pin that will allow the barrel to lock up, let's say .184" in this case.
An important check at this point is determining if both lugs are bearing on the pin equally. Blacken the .184" gauge pin with a marker, install it in the frame, function the slide a few times and remove it. Good contact by both lugs should be visible on the pin.
If not, adjust the jack screw under the barrel lugs on the Weigand fixture so the cut is slightly heavier on the lug that shows the most contact and take another test cut. Repeat this process as necessary to produce even contact by both lugs. When lug contact is correct, continue cutting until the lugs will lock up on a .196" or .197" pin.
Now, reduce your cuts to only .001î at a time until the barrel will lock on the slide stop with just a bit of thumb pressure applied to the rear of the slide. Finish fit with lapping compound and you should be done.
Judge the length of cut with the thumb safety when the barrel is locked up. It should easily enter the notch in the slide without a big gap at the front. If you are fitting the barrel to the original slide that came with the pistol, the rear edge should be even with the edge of the frame. If you are using a new slide, it may have to be dressed off after the barrel is fitted so there is no overhang.
Our barrel is not completely fitted yet. We still have to finish cut the chamber and install a bushing. Stay tuned for part 3.
Tools Here are the tools needed for successful barrel lug fitting on a 1911: pin gauges from .180" to .200", calipers, Kart bench bushing and a marker.
Slide stop shaft measures The slide stop shaft measures .200". If it won't fit easily, you'll need to open up its hole in the lug until it operates correctly, without binding.
Weigand lug barrel fitting fixture The Weigand lug barrel fitting fixture. . This tool allows the barrel to be clamped in a V-block while the lugs are milled to size with a milling machine.
Lugs To test fit the barrel lugs with a gauge pin, first blacken the gauge pin with a marker to check lug contact. Note the good contact here from both lugs.
Lugs To test fit the barrel lugs with a gauge pin, first blacken the gauge pin with a marker to check lug contact. Note the good contact here from both lugs.
Lugs To test fit the barrel lugs with a gauge pin, first blacken the gauge pin with a marker to check lug contact. Note the good contact here from both lugs.
Cutting Lugs Cutting the lugs with a 5mm end mill and the Weigand fixture. A digital readout for the X and Y axis is highly recommended for getting the very best cut.
Digital Readout Cutting the lugs with a 5mm end mill and the Weigand fixture. A digital readout for the X and Y axis is highly recommended for getting the very best cut.
Thumb Safety The fit of the thumb safety will help you judge the length of your cut. It should easily fit into its slide notch without a gap at the front edge.
Rear of the slide The rear of the slide should be close to even with the rear of the frame when the thumb safety fits correctly in its notch. Don't cut any farther here.
Perfect fit A perfect fit. Note the even lug contact on the slide stop after lapping. Both legs of the lug need to tough the slide stop evenly for best accuracy.
