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+--- Thread: How do you bed a barrel to the upper receiver? (/showthread.php?tid=17549)
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How do you bed a barrel to the upper receiver? - A5BLASTER - 12-17-2018
Huckleberry75 Wrote:And when you applied the Loc-tite, did you put on any release agent before bedding?
Nobody use's a release agent with locktite when they bed a barrel.
A short peice of broom handle cut flat on one end and a ball peen hammer works great for busting the connection lose.
How do you bed a barrel to the upper receiver? - Mad Charlie - 12-17-2018
Kswhitetails Wrote:Simple. Genius. Stolen.
Stolen twice.
How do you bed a barrel to the upper receiver? - LRRPF52 - 12-18-2018
Huckleberry75 Wrote:And when you applied the Loc-tite, did you put on any release agent before bedding?
Nope
How do you bed a barrel to the upper receiver? - Lastrites - 12-19-2018
Release agent would likely defeat the purpose of using Loc-tite which is lock the extension into the receiver.
How do you bed a barrel to the upper receiver? - Baroidengineer - 12-19-2018
grayfox Wrote:The other un-mentioned elephant in the room is to buy better uppers, ones that have tight or nearly (NEARLY) impossible fits, then heat them for a secure fit.
If there's a brand that gives loosey-goosey fits, get rid of it and don't buy that brand again (Klem has already discussed the good barrel, I don't bring that up again). Think of that upper as the extension to your barrel.
For me that means BCM and Aero, both of those have yielded tight fits from the git-go.
My $0.02.
I'll second this one. I only buy BCM uppers and am currently building with Aero Gen II lowers. (Aero Gen II has the tension screw to tighten upper fit.) Barrel goes in the freezer, heat gun on the upper prior to installation. Very tight fit. BCM runs a sale on their blem uppers from time to time and that is when I pick up a couple here and there for future builds.
How do you bed a barrel to the upper receiver? - LRRPF52 - 12-19-2018
We've beat this subject to death over the years. Starting with a square receiver face as a premise....here are the various methods you can use to make a more secure mating between the upper receiver and the barrel extension:
* Thermo-fit
* Shim stock and adhesive bedding
* Adhesive bedding
* Press-fit with matched tunnel ID to extension OD
* Metal epoxy build-up on extension, followed by turning to specific OD on a lathe
* Balanced torque specs with any of the above (harmonically-balanced torque specs depend on barrel mass, with lighter barrels getting more torque, and heavier barrels getting less according to extensive testing by AMU, talking ~40-65ft-lb ranges)
Then comes muzzle device balanced torque, and optic mount/rings balanced torque.
This is all under the assumption that we want a highly accurate, precise rifle/mount/optic combination. Each method has its advantages and disadvantages, with Thermo-Fit being pretty ideal, but requires very tight controls on surface thickness of whatever process is used to coat the aluminum upper receiver. Hard coat anodizing is very difficult to control thickness levels to that dimensional requirement for a manufacturer. There's a solution to that, albeit a very expensive one.
For the home-builder, you can buy several parts and hand-fit to find the ones that need thermo-fitting, or see if parts suppliers will provide you with minimum ID uppers with maximum OD barrel extensions, or just use the adhesive or adhesive with shim stock method.
By using the tight-fit parts and adhesive method on a square receiver face, I'm getting 8"x4" 6rd groups with a Wasp contour Lilja 18" barrel at 1000yds, and rapid-fired mag dumps into the heads of 18" silhouettes at 600yds on video.
How do you bed a barrel to the upper receiver? - cb4017 - 12-25-2018
For many years I used nothing. Lately, after finding out bedding was a thing, I've been using a blue Loctite type gel. It's made by Permatex. The gel is easy to work with and stays put. Truthfully, I haven't really decided if "bedding" makes any difference. I've built some pretty accurate rifles with no bedding.
How do you bed a barrel to the upper receiver? - bob4432 - 01-05-2019
LRRPF52 Wrote:* Balanced torque specs with any of the above (harmonically-balanced torque specs depend on barrel mass, with lighter barrels getting more torque, and heavier barrels getting less according to extensive testing by AMU, talking ~40-65ft-lb ranges)
Then comes muzzle device balanced torque, and optic mount/rings balanced torque.
This is all under the assumption that we want a highly accurate, precise rifle/mount/optic combination.
Mind expounding on this theory or give a reputable link that explains it?
Thanks in advance,
Bib
How do you bed a barrel to the upper receiver? - snarkscarbine - 01-06-2019
bob4432 Wrote:Mind expounding on this theory or give a reputable link that explains it?
Thanks in advance,
Bib
Also interested.
How do you bed a barrel to the upper receiver? - Mad Charlie - 01-06-2019
That's three of us...
How do you bed a barrel to the upper receiver? - NugginFutz - 01-06-2019
If I may, I believe what is being referred to is the balance of two masses on either side of a fulcrum. The barrel represents one of of the masses. The receiver, buttstock and scope form the other mass. The two masses meet at the fulcrum (the extension and upper receiver). At rest, a certain (minor) amount of torque applied to the barrel nut is necessary to overcome the two masses and bring them to equilibrium at the fulcrum. A properly trued and faced setup will then possess nearly 100% contact at the fulcrum, with nearly 100% parallel components.
Now, add to that, the function of a harmonic inducing shock (the weapon firing). These shock waves cause the barrel mass to flex and whip, which induces angular torque at the fulcrum. If the barrel nut is sufficiently tightened, it will prevent the shock induced waves from causing the two masses to move at the fulcrum, thus reducing the effects of harmonic torque on accuracy.
What can be counter-intuitive is that the lighter profile barrels tend to require the barrel nut to be tightened to a higher torque value. This is due to the nature of harmonic shocks. The lighter barrels can whip more at the muzzle end than the heavier profiles. This is because the thinner, lesser mass (pencil barrel) moves a greater distance when a given shock impulse is applied than would a thicker, heavier mass (bull barrel). This greater motion at the muzzle end of the mass translates to greater torque being applied at the fulcrum. To overcome this greater (albeit momentary) torque, the barrel nut must be tightened to a higher value to prevent motion.
In other words, light whippy barrels need to be tightened to a higher torque value than heavier, stiffer ones. The Barrel Nut is the component at the fulcrum which determines how much torque is coupled between the two masses, and how the shock waves are affected.
There are two ways to tune the system, with respect to system harmonics. You can try to mitigate their effects by dampening them (stiffer, heavier, bedding, etc.). Or you can try to tune them (shift the frequency) so that a given load (bullet, transit time, barrel length) all coincide to produce consistent, accurate exit alignments at the muzzle.
As this applies to the Barrel nut, (the one common knob we can all turn without adding anything new), finding the correct torque is the most direct way to minimize harmonics as a variable to the accuracy equation.
What is the correct torque? That is where some of the barrel manufacturers and upper manufacturers should come in. Most don't bother and go with what's been out there for decades. Others actually try and come up with some useful numbers specific to their own product. This can explain the different barrel nut torques being reported in the wild.
Hope this helps or, at the very least, doesn't muddy up the water.
ETA: My post does not address the portion of LRRPF52's mention of the muzzle devices or scope rings, but I think that the idea of resonance and torque coupling applies to them, as well.
How do you bed a barrel to the upper receiver? - Drillboss - 01-06-2019
bob4432 Wrote:Mind expounding on this theory or give a reputable link that explains it?
Thanks in advance,
Bib
http://www.65grendel.com/forum/showthread.php?15494-AR15-Barrel-Harmonics-3-Part-Video-Series-AMU-Gunsmith&highlight=harmonics
How do you bed a barrel to the upper receiver? - Bazinga - 01-09-2019
Retaining compound is what I have used. Loctite 620 equivalent i believe. Have to look a the exact brand tonight.
How do you bed a barrel to the upper receiver? - LRRPF52 - 01-12-2019
bob4432 Wrote:Mind expounding on this theory or give a reputable link that explains it?You'll have to bear with me since the way I learned to look at the world was from an aviation perspective when I was growing up. My parents (mainly my dad) worked on many different cutting edge military aerospace programs, and my dad would come home from working on those things to then pursue his real passion with theoretical physics.
Thanks in advance,
Bib
He would often explain some of the challenges they faced, or other programs faced, from a physics perspective when I would ask questions as a curious child. This often included explanations of video footage detailing things that had gone wrong in test flights, aerodynamic behavior of different designs, stresses, torsional loads, oscillations, vibrations, and the engineering solutions to those problems.
Things like buffeting seemed to always be a problem with airframes and missiles, where a certain resonant frequency in the buffeting would amplify itself at different speeds and flight regimes. They would have to re-design critical structures and shapes of the airframe and airfoil to deal with these problems, and you can see evolutions of specific structures on popular fighter aircraft if you were paying attention.
Anyway, some of the same principles apply to firearms when chasing accuracy, only instead of aerodynamic forces acting on the rifle, we're looking at the shockwave propagating from the detonation or explosive conflagration of the propellant acting on the chamber and barrel, and everything attached to that barrel.
Here's a great video that illustrates the various phenomena much better:
Go to 2:16 and watch the red colored gas block especially
[video=youtube_share;phwDMrau_7Y]https://youtu.be/phwDMrau_7Y[/video]
How do you bed a barrel to the upper receiver? - FW Conch - 01-13-2019
With all that going on it's a wonder we can hit anything. I guess the pill is out of the muzzle before most of that happens?