BARREL BEDDING
Frequently Asked Questions — Mid-Range & Long-Range Precision Uppers
The barrel doesn’t float in the receiver. It’s bedded to it.
// The Short Answer
Every AR barrel is built to slide into the receiver. That clearance has to go somewhere.
An AR-15 barrel is held in place by a barrel extension that slips into the front of the upper receiver, with the barrel nut clamping it against the receiver face. For the barrel to install at all — and for barrels to be interchangeable across receivers — that fit has to be a clearance fit. There is intentional space between the outside of the barrel extension and the inside of the receiver bore. The barrel nut controls how hard the barrel is pulled back against the receiver. Nothing controls where the barrel sits inside that clearance, or whether it stays there. Bedding fills the gap with a high-strength retaining compound so the joint becomes a single rigid unit instead of two parts held together by clamp load. On a duty carbine it makes little practical difference. On a rifle expected to hold a small group at 600 or 1,000 yards, it is the difference between a joint that returns to the same place every shot and one that only mostly does.
// Why It Shows Up At Distance
Nothing about the joint changes. What changes is what the joint costs you.
Group size is angular. A rifle that disperses a quarter-minute from a shifting barrel joint disperses a quarter-minute at every distance — that is roughly a quarter inch at 100 yards, where it disappears into the noise of wind, ammo, and the shooter. The same quarter-minute is two and a half inches at 1,000 yards, and it is the difference between a hit and a miss on a steel plate.
The mechanism is small movement that doesn’t repeat. Recoil, barrel heating, a loaded bipod, sling tension, and the twisting load a suppressor puts on the muzzle all push on that joint. When the barrel is free to move within its clearance, some of that movement doesn’t come back to the same place. It shows up as a group that opens as the string goes on, a first-round flyer after the rifle cools, or a zero that shifts when the rifle is carried differently.
Bedding does not make the barrel better. It removes a variable. The barrel extension and the receiver stop being two parts with a gap between them and start behaving as one. Every other precision feature on the rifle — a match barrel, a rigid handguard, a good trigger — is measuring against that joint. A reference that moves is not a reference.
| The standard | As applied at Second of Ten |
|---|---|
| Compound | LOCTITE® 620 — methacrylate ester, green |
| Type | High-strength anaerobic retaining compound for cylindrical slip fits |
| Service temperature | −65°F to 450°F (−55°C to 232°C) |
| Shear strength, steel | ~3,800 psi |
| Gap fill | Up to approx. 0.010″ |
| Cure | Anaerobic — cures confined between close-fitting metal, activator used to drive cure on aluminum |
| Applied to | 6mm precision uppers on the SOT billet upper receiver · CMT UPUR-2 billet upper receiver |
| Not applied to | SOTM4 & SOTM5 MOD-0 thermal-fitted uppers — interference fit, no compound |
| Not applied to | SOTM4MB Monobloc® barrels in any upper, billet or forged — fit is tight enough without it |
| Effect on headspace | None |
| Reversible | Yes — controlled heat releases the joint for barrel changes |
// Questions We Get Asked
Is this the same thing as bedding a bolt gun?
Same idea, different joint. On a bolt-action rifle, bedding means filling the gap between the action and the stock so the action sits in one repeatable place. On an AR, the stock isn’t the problem — the receiver is aluminum and the barrel bolts to it directly. The joint that needs the same treatment is where the steel barrel extension meets the aluminum receiver:
- Bolt gun: action bedded into the stock, so the action stops moving relative to what’s holding it.
- AR precision upper: barrel extension bedded into the receiver, so the barrel stops moving relative to what’s holding it.
- The purpose is identical: take a joint that has clearance in it and make it behave as one piece.
Isn’t the barrel nut supposed to hold the barrel?
It does — in one direction. The barrel nut pulls the barrel extension rearward against the receiver’s face and holds it there under a lot of clamp load. That controls the barrel front-to-back very well. What it does not do is control the barrel side-to-side inside the receiver bore, because that’s a clearance fit by design. Bedding addresses the direction the nut was never able to.
Why do you only do this on precision builds?
Because that’s where it’s worth what it costs. A general-purpose carbine firing at defensive distances will never show the difference, and bedding adds process steps, cure time, and a barrel change that requires heat later. On a rifle bought to hold small groups past 500 yards, the same variable is one of the few left that hasn’t already been engineered out.
Why LOCTITE 620 and not something else?
620 is a retaining compound, not a thread locker and not an epoxy. It’s formulated specifically for bonding cylindrical slip-fit parts, it fills the clearance rather than just wetting the surfaces, and its 450°F service rating covers what a barrel extension actually sees. It also resists the fretting corrosion that steel-in-aluminum joints develop over time under vibration.
Does bedding change my headspace?
No. Headspace on an AR is set between the bolt and the barrel extension — both steel, both forward of anything bedding touches. The compound goes on the outside of the extension where it meets the receiver bore. Your rifle is gauged before and after assembly regardless.
Can the barrel still be changed later?
Yes. Retaining compounds release with controlled heat, and the joint comes apart without damage to either part when it’s done correctly. What it is not is a barrel you swap in a hotel room with a wrench. Plan a bedded barrel change as bench work — ours or a competent shop’s — not a field task.
Which of your rifles get bedded?
Our 6mm precision uppers — 6mm MAX® and 6mm ARC — built on the SOT billet upper receiver and on the CMT UPUR-2 billet upper. It’s a standard feature on those builds, not an upcharge or an option. It is not used on the SOTM4MB Monobloc® line or on MOD-0 thermal-fitted uppers, for the reasons in Q 08 and Q 09.
Why aren’t the MOD-0 uppers bedded?
Because there’s no clearance left to fill. The SOTM4 and SOTM5 MOD-0 uppers are thermal-fitted — the receiver is expanded with heat, the barrel is installed, and the receiver shrinks onto it as an interference fit. The joint is already metal-to-metal with zero gap. Adding a retaining compound to an interference fit accomplishes nothing and interferes with the fit itself. Two methods, one objective.
Why aren’t the SOTM4MB Monobloc barrels bedded?
Because that particular pairing doesn’t need it. We do not bed any Monobloc® barrel in the SOTM4MB line, in a billet or a forged upper. In our billet uppers, the Monobloc barrel’s fit is tight enough that the joint is already controlled without compound; the forged MOD-0 versions are thermal-fitted instead. That is specific to the Monobloc line — our 6mm precision uppers on the same billet receivers are still bedded as standard. We match the method to the joint rather than applying one process everywhere.
Isn’t this just gluing the barrel in?
It’s a specified engineering adhesive applied to a verified clearance in a controlled process — surfaces degreased, fit measured, activator applied for cure on aluminum, full cure before the upper is assembled or fired. Retaining compounds do this job in machine tool spindles and transmission bearings. The dismissive version of the description isn’t wrong about the chemistry; it’s wrong about the standard.
Will it hold up under sustained or suppressed fire?
The joint sits well behind the chamber, at the receiver, which runs substantially cooler than the barrel forward of the gas port. 620 carries a 450°F service rating, which covers normal precision use including suppressed fire. We won’t claim it’s indifferent to abuse — sustained high-volume automatic fire is outside what a precision rifle is built for, and outside what we’d stand behind on any bedded joint.
// What Bedding Does Not Do
It removes one variable. It does not fix the others.
- It won’t make an average barrel shoot like a match barrel. Bore uniformity, chamber concentricity, and the crown decide what the barrel is capable of. Bedding only keeps you from losing what the barrel already has.
- It doesn’t substitute for correct barrel nut torque or proper assembly. A bedded joint assembled wrong is still assembled wrong.
- It doesn’t address handguard flex. Rail deflection under bipod and sling load is a separate problem solved by a rigid mounting system, not by the barrel joint.
- It won’t quiet an inconsistent load. Ammunition velocity spread is usually the larger contributor to vertical dispersion at distance. Bedding a rifle fed inconsistent ammunition changes very little.
- It is not a repair. Bedding does not rescue an oversized or damaged receiver bore. If the fit is out of tolerance, the fit gets corrected — we don’t fill a bad joint with compound and call it bedded.
// What You Get
Bedding is a standard feature on our precision uppers, and it’s documented on your build record. Receiver and barrel identified, fit verified before assembly, compound and lot recorded, cure completed before the upper is assembled or fired, and the initials of the person who did the work. When you send the rifle back for a barrel change years from now, that record is what tells the next armorer how the joint was made and how to take it apart.
One joint. One piece. One reference.