Posted: 12/26/2025 3:20:26 PM EDT
[Last Edit: thedownzero][Edited]
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Most of the time when building an AR-15, the upper receiver ID is either oversized or the barrel extension OD on barrel is undersized. This can be due to tolerance stacking of either part and/or out spec components. This play leads to a barrel that is only supported by the barrel nut torque on shoulder of extension agains the face of receiver, leaving that play/gap inside where barrel should be fully supported. There are two methods to rectify this: Option 1: Loctite 609 or similiar on the barrel extension. When pressed into the upper receiver, the 609 conforms around extension, filling voids and "bedding" the barrel extension. I've used this method previously with good results. However, I dislike the build up of excess Loctite between the receiver face and extension shoulder. Option 2: Using shim stock to fill the void and heating the upper to thermal fit around it. Why do I prefer this method? It's a more precise solution and it ensures the barrel extension shoulder is square/true to upper receiver; barrel nut torque evenly applied. Whereas Option 1 spills excess loctite in between, defeating purpose of truing receiver. Today I'll be installing a 11.5" Daniel Defense barrel on a Colt M4 upper. This upper was previously trued, so I figured I might as well shim it to maximize accuracy/minimize variables. Here is the trued receiver face, now it's perfectly squared... Supplies needed... First we take receiver ID and barrel extension OD, do some math and figure out your gap... You can find "shim packs" on Amazon and I opt for the 1" width (more on that later) Next we want to trimming shim length. Start by positioning one end against the pin... Wrap the other end tightly around, we're trying to cause the pin to "indent" the other end... If done correctly, we marked it... Next, cut the outside/left side of indent... |
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Once shim is proper length, we have to trim the width. As you can see, a one inch shim extends too far/past feed ramps. Some people use .75" shim stock to skip this next step, but I want maximum contact/coverage, so I trim the oversized shim to fit. Start by using a straight edge to mark about a couple mm's off. This isn't necessarily a precise cut or measurement and is more of an "eyeball" method. However, this ensures width ensures shim stock is away enough from feed ramps, but also easy enough to cut. Cut along line marked... With finished shim in hand, wrap it around barrel extension and zip tie in place. Zip ties act as a third pair of hands for fitting process. This is my secret sauce to make the process very efficient/simple. Don't make them super tight, you want them to spin/slide freely. Next heat your receiver with a torch. BE SURE TO ROTATE it constantly. As it heats, it expands, allowing you to fit it over shim... From there, you gradually twist the receiver into alignment with index pin and push the zip ties down. you many need to re-heat receiver as you do this, they cool pretty quickly. Have cutters nearby to cut the zip ties off as receiver slides further down. Eventually, enough of extension is encompassed and you can cut last zip tie off, fully seating barrel extension... If done correctly, there will be zero play between upper and barrel extension and no gap between receiver face and extension shoulder. You are ready to finish build and have a fully supported/bed barrel to upper. |
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Yup. Nice write up! https://thenewrifleman.com/barrel-bedding-for-the-precision-ar-15/ |
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General education should not be mere job training, but training in how to be fully human.
General education should not be mere job training, but training in how to be fully human.