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Installing/Fitting a Compensated Barrel

By Bob Londrigan, First Published in Front Sight Magazine, September 2020

There is very little information available on fitting bull barrels and, in particular, compensated bull barrels, so this article should come in handy for those trying to do it themselves. I will cover installing a one-piece barrel/comp instead of a screw on type compensator. Fitting for the-one piece barrel/comp is slightly different and you can install a screw in barrel using these instructions if you screw it on and clock the comp to the proper alignment prior to starting.

The first step is to take some measurements:

  1. Width of the hood on the barrel.

  2. Width of the hood slot in the slide.

  3. Distance on the slide from the breechface to the first barrel locking lug – measured where the hood contacts the breechface. The easiest way to get this measurement is to measure from the end of the slide to the breechface, and then from the end of the slide to the first locking lug. Subtract the second measurement from the first and you have the distance from the locking lug to the breechface.

  4. Distance on the barrel from the end of the hood to the first locking lug.

  5. Distance on the slide from the first locking lug to the end on the slide.

  6. Distance on the comp from the first locking lug to the back of the comp.

Be careful taking these measurements and do them several times until you get something consistent. The first thing you have to do is cut back the end of the slide to allow for lockup of the barrel comp. The barrel when in lockup actually points down slightly (about ½-3/4 degree), so you will have to cut the end of the slide to allow for this. The slide also needs to be shortened a little. Here is an example of some typical measurements:

Slide first locking lug to end of slide – 3.575 Barrel/comp first lug to back of comp – 3.565 Slide breechface to end of slide – 4.890 Calculated slide breechface to first lug 4.890-3.575=1.315 Barrel/comp hood to first lug – 1.325

In this example you would cut 0.010 (3.575-3.565) off the end of the slide at the top, at a ½-3/4 degree angle, cutting more off the bottom of the slide than the top. You would cut 0.010 (1.325-1.315) off the hood. This will get you close to where you want to be and should leave a little material to do the final fitting. Once you have done these preliminary cuts you can start fitting the hood width. Put your slide on the frame and slide the barrel in and move it back until you can see how much needs to be taken off each side. You want to do this with the slide on the frame because that will align the lower lug in the slot in the slide and center the barrel properly. Take material off each side until the hood will move back in the slide and contact the breechface. You are now ready to fit the hood length.

One of the key things to remember in fitting the one-piece barrel/comp is that there should be clearance between the end of the slide and the back of the comp. You want to end up when you are finished with 0.003-0.005 inch. While you are fitting as long as you have some clearance you will be ok. You can even it up at the end with a file. I take a 0.0015 feeler gauge and make sure I can fit it in between the comp and the slide all the way around the barrel. If it sticks anywhere take a little off the slide until I have clearance, then go back to fitting the hood.

Once you get the hood length close you will need to start taking material off the bull barrel wherever it has excess contact so that the barrel moves up into the locking lugs on the slide. Measure the lockup as you fit the barrel. You want 0.045-0.050 inch lockup. This should center the firing pin strike in the center of the chamber. You can put the barrel in the slide, center it, and then tap the bottom lugs with a plastic mallet. Remove the barrel and you will see marks on the barrel. Remove material in these areas until the barrel starts to move farther up into lockup. Keep checking clearance between the comp and the slide as you gradually take material off. As you get the barrel to go higher into lockup you will notice less clearance at the bottom of the interface between the comp and the slide (Remove material as needed.) This is because the barrel is rotating up into lockup. Go slowly and work back and forth adjusting wherever the fit is too tight. You will have to adjust a combination of hood length, bull barrel upper surface contact, and contact in the locking lugs on the barrel.

Check for comp to slide contact often, otherwise you may take material off the wrong surface. Comp to slide contact can make you think the hood length is too long if you are not careful and you will take too much off the hood length. Keep working at it until you have the barrel high enough in the lugs to center the firing pin strike. After this is done run a comp alignment reamer through the comp just to be sure there is clearance for the bullet.

Once you are happy with the barrel fit in the upper lugs then go back and even up the gap between the comp and the slide to 0.003-0.005 gap with the barrel fully up into the lugs. At this point you can blend the comp to the slide. Most comps are a little wider than the slide for fitting purposes. You need to lock the barrel in the slide with a barrel alignment block and blend the sides. This can be done with a file or if you are careful with a belt sander. If the belt sander scares you, use sandpaper on a flat surface and work the comp/side unit back and forth until the sides match. Usually the diameter of the comp is a little larger than the radius on top of the slide. You need to blend this to match also.

At this point the fitting that is left to complete the barrel install is the same as if you were fitting a standard non-comped barrel. Fit the bottom lugs, chamber the barrel, and tune the ramp for optimum feeding and you are done with your project and ready to test fire.


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