Grendel Upper: Oversized or standard barrel extension tunnel

[Darryle]

I have several Aero XL uppers and several BCM uppers in the safe. I am building around the Liberty 16" medium weight fluted barrel and I want to squeeze every bit of accuracy I can out of this build. Do I use the BCM because of their thermal fit or do I truck on and build the Grendel like I did the 6mm ARC on the Aero XL upper and bed the extension with Loctite 620?

Thanks

[montana]

I have several Aero XL uppers and several BCM uppers in the safe. I am building around the Liberty 16" medium weight fluted barrel and I want to squeeze every bit of accuracy I can out of this build. Do I use the BCM because of their thermal fit or do I truck on and build the Grendel like I did the 6mm ARC on the Aero XL upper and bed the extension with Loctite 620?

Thanks

It is easier to lap the front of a standard receiver than a thermal fit upper..Lapping can improve accuracy if done correctly. I have witnessed this first hand..

When torquing the barrel nut to a standard receiver, the receiver will compress around the barrel extension..How much more a thermal fit upper improves accuracy is debatable and can be a pain to lap, requiring a lapping tool that will fit the smaller I.D. of the thermal fit upper...The I.D. of the upper receiver's carrier bore can effect accuracy if the I.D. is too sloppy with the bolt carrier O.D. I have pin gauges to match the best inside upper to my selection of bolt carriers O.D. Keeping the bolt as straight as possible with the bore can improve accuracy..That said, most commercial upper receiver's bolt carrier bores being manufactured today are smaller than the mil spec of 1", which means it is not a real problem in most cases..

Bedding the barrel is also not universally accepted as improving accuracy either, but I'm on the side it can be beneficial. Even with a thermal fit upper, the loctite will fill small imperfections in the barrel extension channel..

If my choice was between a thermal fit or a standard upper that I could lap, I would choose the standard upper..Be careful when lapping any upper as this can move the barrel feed ramps further into the upper receiver creating the over hang of the barrel feed ramps with the upper's feed ramps..

The quality of the barrel is my main concern, matched with a good BCG, trigger, optics, etc if accuracy is priority..

[js8588]

If the manufacturer's tolerances are tight enough for a thermal fit, they're likely also precise enough to not need lapping. I tend to avoid Aero and their subsidiaries whenever possible. They're all what would be considered "rack grade". Grab something OEM'ed by Zev and you won't have to worry about it.

[montana]

If the manufacturer's tolerances are tight enough for a thermal fit, they're likely also precise enough to not need lapping. I tend to avoid Aero and their subsidiaries whenever possible. They're all what would be considered "rack grade". Grab something OEM'ed by Zev and you won't have to worry about it.

You can quite easily tell if a thermal upper needs lapping using a .0020 feeler gauge.. After inserting the barrel in the upper, check for gaps between the upper and barrel extension lip..
High end parts are not immune to QC problems..Trust but verify..

[Bonas]

Consider using stainless shim stock to achieve a press fit between the barrel extension and the receiver instead of Loctite 620 or other gap filler. I shim anything that's not a press fit. I used to bed with 620, then shim and bed, but I just shim now. I've rebuilt two uppers with no shim or bed and found about .25 moa improvement with shims on the rebuilds. I don't remember whether I bedded both or just one. I don't think the bedding hurts anything, but I don't think it helps either. If you ever want to rebarrel, you might not get the bedded barrel out.

I'd use the BCM before the Aero.

[montana]

If you ever want to rebarrel, you might not get the bedded barrel out.

I have never had a problem removing any bedded barrel from an upper..

Many use a wooden dowel and tap it out but I found this to be the easiest method..https://www.youtube.com/watch?v=abxYM6ZQK_E&t=3s
I never used this for thermal fit barrels as a quick heat with MAP gas releases it quite easy..

[LRRPF52]

I have never had a problem removing any bedded barrel from an upper..

Many use a wooden dowel and tap it out but I found this to be the easiest method..https://www.youtube.com/watch?v=abxYM6ZQK_E&t=3s
I never used this for thermal fit barrels as a quick heat with MAP gas releases it quite easy..

Same here. I use my armorer’s receiver insert that I modified. It looks like a T shape with a shaft that fills the chart handle channel. I cut off some of the front section of the T handle so it could be used to punch out bedded barrels.

Last one I did tapped out with a hammer with much less force than I expected, very easy to do.

[Bonas]

Same here. I use my armorer’s receiver insert that I modified. It looks like a T shape with a shaft that fills the chart handle channel. I cut off some of the front section of the T handle so it could be used to punch out bedded barrels.

Last one I did tapped out with a hammer with much less force than I expected, very easy to do.

Was that upper bedded with Loctite 620?

[LRRPF52]

Was that upper bedded with Loctite 620?

It was 242 (Blue) if I recall correctly.

[Bonas]

It was 242 (Blue) if I recall correctly.

Loctite 620 and 242 are wildly different adhesives. 620 is a high-temperature retaining compound for cylindrical slip fits. 242 is thread locker. Thread locker isn't going to do much of anything between a barrel extension and receiver. Thread locker doesn't have any structural properties that could serve a "bedding" function. 620 does, though. 620 isn't coming loose unless it's heated to 500 degrees, when used properly. It's very strong.

[montana]

242 works well when bedding a barrels. I have used it in the past putting a lot of rounds down range..When I removed the barrels, the bedding held tight, yet was easy to remove.. Some prefer 620, 609,242,243,248,etc.. Pick your poison, I wouldn't sweat it using any of them...

[LRRPF52]

Loctite 620 and 242 are wildly different adhesives. 620 is a high-temperature retaining compound for cylindrical slip fits. 242 is thread locker. Thread locker isn't going to do much of anything between a barrel extension and receiver. Thread locker doesn't have any structural properties that could serve a "bedding" function. 620 does, though. 620 isn't coming loose unless it's heated to 500 degrees, when used properly. It's very strong.

I’ve used 242 to bed a lot of barrels, including ones that had already been installed in uppers and weren’t shooting, then tightened the groups up demonstratively after bedding.

It just acts as a filler/harmonic dampener. The Army Marksmanship Unit did a lot of testing with this back in the day over years and found that bedding the barrels absolutely tightened up their groups.

I’m not looking to form a strong adhesive bond between the upper and extension, just eliminating voids and the subsequent vibration and inconsistent behavior of a loose barrel/upper joint.

I’ve heard others suggest using Green Loc-tite 620, which won’t hurt other than when you try to separate them.

Some high-end builders have used one of the Red Loc-tite compounds as well.

Green 620 is better than Red-271 for the Gas Block and Gas Tube when bedding those due to its heat tolerance.

[Bonas]

I’ve used 242 to bed a lot of barrels, including ones that had already been installed in uppers and weren’t shooting, then tightened the groups up demonstratively after bedding.

It just acts as a filler/harmonic dampener. The Army Marksmanship Unit did a lot of testing with this back in the day over years and found that bedding the barrels absolutely tightened up their groups.

I’m not looking to form a strong adhesive bond between the upper and extension, just eliminating voids and the subsequent vibration and inconsistent behavior of a loose barrel/upper joint.

I’ve heard others suggest using Green Loc-tite 620, which won’t hurt other than when you try to separate them.

Some high-end builders have used one of the Red Loc-tite compounds as well.

Green 620 is better than Red-271 for the Gas Block and Gas Tube when bedding those due to its heat tolerance.

Joe Carlos with the AMU settled on retaining compound, specifically Loctite 620, not threadlocker. The max service temperature for Loctite 242 is 300 degrees. This is a link to an abstract of some of Carlos's work:

https://www.thefreelibrary.com/The+relat...0321900231

I've read Carlos's papers. Most people credit the idea of bedding barrel extensions with "Loctite" to Carlos's work, from what I've seen. He has migrated from bedding and shimming to using oversized barrel extensions. He discusses "red" versus "green" Loctite in the abstract linked to above. I think the idea that thread locker makes a useful bedding compound might arise from some people thinking of "Loctite" as just thread locker instead of what it is; a brand name for a full line of adhesives made by Henkel.

I recommend anyone who wants to bed an extension with a Loctite brand adhesive to use one of Henkel's products that the manufacturer recommends for high temperatures and is designed to fill gaps. That's 620 or one of the other retaining compounds in the 600 series. Threadlocker isn't designed to fill voids.

But, like I said before, I don't bed at all anymore. I just shim to get a press fit. My last good barrel appeared to come with an oversized extension, which was great.

[montana]

Joe Carlos with the AMU settled on retaining compound, specifically Loctite 620, not threadlocker. The max service temperature for Loctite 242 is 300 degrees. This is a link to an abstract of some of Carlos's work:

https://www.thefreelibrary.com/The+relat...0321900231

I've read Carlos's papers. Most people credit the idea of bedding barrel extensions with "Loctite" to Carlos's work, from what I've seen. He has migrated from bedding and shimming to using oversized barrel extensions. He discusses "red" versus "green" Loctite in the abstract linked to above. I think the idea that thread locker makes a useful bedding compound might arise from some people thinking of "Loctite" as just thread locker instead of what it is; a brand name for a full line of adhesives made by Henkel.

I recommend anyone who wants to bed an extension with Loctite to use one of Henkel's products that the manufacturer recommends for high temperatures and is designed to fill gaps. That's 620 or one of the other retaining compounds in the 600 series. Threadlocker isn't designed to fill voids.

But, like I said before, I don't bed at all anymore. I just shim to get a press fit. My last good barrel appeared to come with an oversized extension, which was great.

No disrespect to anyone, but the point of barrel bedding is to fill gaps to remove play..When tightening the barrel nut, the receiver bore thread area of the upper will compress around the barrel extension, similar to how a muzzle nut/devise will compress the end of the of the barrel bore, the higher the torque is applied to the nut/device..

Using a standard upper with no barrel installed and a greased mil spec nut, I have measured .001" compression at 35 ft lbs torque,
.002" at 40 ft lbs torque, .003" at 50 ft lbs torque..
After around 50-60 lbs, the compression seemed to stop..

Mil spec TDP upper receiver barrel bore is 1" +.002", but most modern manufactured uppers will run a little under..
Mil spec barrel extension O.D. is .9987" -.0008"

Upper receiver bores I have measured using + pin gauges were:

Colt .9994"
Wilson .9980"
Aero .9980"
Geissele .9975"
SOLGW .9960"
BCM .9960"

Considering barrel extensions can vary, the need for shimming varies from upper to barrel extension being used..
When Cerakoting the I.D. bore of the upper, the inside bore will also decrease.. I have had very tight barrel to upper fit after using Cerakote..
Considering some uppers are thermal fit to begin with, most modern uppers being under mil spec, the need for for a special bedding compound could vary from a (possible) maybe to mostly pointless IMO..
Sloppy machined uppers is a different story..

[LRRPF52]

I also Cerakote the insides of all my upper receivers in the extension tunnel. Some I have foamed-off the carrier raceway so the carrier rails don’t gall up on the Cerakote.

I’m not really concerned about heating in the barrel extension area since we know what the temperature measurements are from the M4 during extreme full auto testing done by ARDEC at Rock Island.

The barrel shank area only got up to 420 deg F after a 280rd full auto set of mag dumps in compressed time.

I’m not able to reach those temperature levels with any semi auto Grendel, nor would I attempt to do so on my barrels.

Grendel runs at much lower chamber pressure than 5.56 NATO and we typically don’t go cyclic with 6.5 Grendel.

This is probably why I see the same accuracy today as I did when I first built my uppers. Whatever I’m doing with bedding, it’s holding true over many years.

The sections of the barrel that seemed to accumulate the most heat in the ARDEC tests were midway on a carbine length profile before the step up, and right before the step up to the gas block journal and heavy forend, referred to TC-4 and TC-3 in the ARDEC tests. Look at pages 13 forward on that link.

Their main intent for those tests was to determine the surface temperatures between the smaller Colt Commando handguards that were common to the XM177E1/E2, GAU-5A/A, Colt 653, Colt 723, Colt 727, Colt 733, etc. and the new enlarged diameter, dual heat shield M4 handguards. They established in-depth 280rd Full Auto bare-naked barrel baseline measurements, before doing the same tests with both types of handguards.

I remember when all this was going on because there was a big deal about having real M4 handguards, vs carbine. I had carbine handguards on my personal 11.5” AR-15s, but saw the new Colt ads for the M4A1 and M4 with the fat handguards.

Ranger Regiment got Knight’s Armament RIS railed handguards shortly after, as did SF and NSW. The fat M4 handguards then were ridiculed and so “last season”.

When we got issued M4s in 1997, they were with the fat M4 dual heat shield handguards. We deployed to Panama with 2/75 November that year, and everyone in a Ranger Rifle Platoon had M4A1s with RIS, Aimpoint Comp Ms or ACOGs + Surefires and PAQ-4Cs.

The RIS was later replaced by the KAC RAS.