What Grit Lapping Compound and which Loctite for Bedding?

[Hansel&Grendel]

I have been sending AR-15 uppers to a specialty shop for lapping for better barrel fitment, at a good price point. I have quite good accuracy by using quality components that equates to happy end users, but I want to do better to include bedding the barrel extension to the upper.

Criterion Barrels has a 4-part AR-15 accurizing video series. I was a bit dismayed to see the CB armorer lap an upper receiver while holding the upper in his left hand and working a lapping tool (Brownells) on a keyless chuck hand drill. I would think that vertical lapping of an upper secured in a vise block along with a chuck-key drill (no lathe at my "shop") would be best to avoid slop. Also, the CB armorer used Loctite 609 to bed the barrel extension to the upper.

So, 3 questions:
1) Is Loctite 609 (the green stuff) suitable or is there a better Loctite formula for bedding the barrel extension to the upper?
2) Instead of outsourcing, what lapping tool is most recommended? (I know that Brownells and Pacific Tool & Gauge have them).
3) Lastly, what grit lapping compound is best for lapping a vertically placed upper receiver when using a lapping tool in a chuck-key drill?

Thanks,
Hans

[A5BLASTER]

Get the brownells tool, it comes with the lapping compound. I think mine is made by caldwell, can't really remember but it comes with everything needed.

You don't have too chuck the upper in a lath you can hold it in one hand a lapp it with the tool in a cordless drill. All your doing is evening out the surface coating. Your not taking any metal off.

Leave the locktite out of the action, use steel shim stock to bed the barrel.

[grayfox]

Better idea than loktite is to get a BCM or Aero upper, those are tight fits to start off. For the BCM I warm it up and "freeze" the barrel to get a good shrink fit, then they tighten up together.

[Mustangfreek]

[FLshooter]

Wheeler 220 grit lapping compound . After I lap the upper, I clean it out well with brake cleaner , then I wash it out w/soap and water. I put black Molly grease on the barrel extension and on the reciever threads,screw on the barrel nut and tork to 40 ft lbs. I never found it necessary to loctite barrels.
But I have friends who use loctite .One in particular who came over and we took apart 6 of his AR15 uppers and put red loctite on all the barrel extensions.All his rifles were already accurate.I have seen nothing to convince me that I should be putting loctite on my AR15 barrels.

[Hansel&Grendel]

Interesting. I enjoy research and am finding that a non-embedding lapping compound such as Garnet (Brownells) in 600-grit or finer is ideal for upper receiver lapping because it does what it is supposed to do and washes away rather easily. Not sure what composition Garnet is but a silicon carbide lapping compound, and to a degree an aluminum oxide lapping compound, embeds and tends to keep irritating the lapped surface. Not sure how that works with an upper receiver with a compliant torqued barrel because one assumes that the upper receiver mating surface is no longer under friction unless at a micro-abrasion level that may or may not be consequential.

Also, I reached out to a longtime firearms industry "Yoda" who does a lot of precision machining and he uses Loctite 242 all the time to bed a barrel extension to an upper receiver. The 242 is purposed to fill the small voids between the barrel extension and the upper receiver; and is not meant to "glue" them together. He cautioned against using any surface cleaning primer to prep either the barrel extension surface or the mating area inside the upper for the 242 in order to avoid creating an undesired adhesion issue.

He also tipped that if you want to remove a stubborn/tight barrel, remove the barrel nut of course and in a swift motion, bring the area of the upper receiver that the gas-tube enters, down onto the edge of your bench. The inertia of the barrel will pop it right out of the receiver.

[biodsl]

Better idea than loktite is to get a BCM or Aero upper, those are tight fits to start off. For the BCM I warm it up and "freeze" the barrel to get a good shrink fit, then they tighten up together.

Apparently JP intentionally manufactures their uppers to require a thermofit and eliminates the need for bedding. I've done the heat gun/freezer routine to fit a barrel into a Precision Firearms upper, but I've never taken a torch to it!

https://www.youtube.com/watch?v=-cOlNGxUhsk&t=4s

[Mesa1978]

I have 2 Grendels built on Vltor uppers. The unlapped and unbedded upper with the Stoner barrel shoots as well as lapped and bedded upper with the Shilen barrel, both came with bolts supplied and I have Geissele triggers, PRS and JP captured buffer springs installed for both. The Shilen has the better scope.

[Hansel&Grendel]

I found a December 2015 post by LRRPF52 on page 1 of a 3 page thread that address the questions raised in this thread. Once again, fascinating reading:
https://www.ar15.com/forums/general/...per/5-1822691/
Additionally, Mark at Precision Firearms says that he uses Blue Loctite as well on barrel extensions (see the last comment in the thread):
http://www.65grendel.com/forum/showt...bed-with/page2

[Old Bob]

I had to use the thermofit method to install a 6.5 Grendel Wilson Combat barrel into a Precision Firearms side-charge upper. I was gonna try lapping the receiver face but the lapping tool wouldn't fit. Under 10X magnification, I couldn't see any gaps in the receiver/barrel extension connection. The mating looked perfect.

I've tried just about most methods to join uppers & barrels from just throwing them together, to lapping to lapping & bedding to thermofit. Just been lucky I guess that I haven't had any of my builds turn out to be horrendous shooters. I haven't tried shim stock & probably won't. I don't have any plans to build another AR. But then, who knows what lies ahead in the future.

[Hansel&Grendel]

For those using Loctite 242 "Blue" for bedding the barrel extension to the upper receiver, be aware that Loctite 242 has a shelf life of 24 months unopened and 12 months after opening:
https://www.fourwheeler.com/how-to/t...octite-go-bad/
Also, be aware from where you buy your Loctite. Some online vendors state that their Loctite is manufactured in the United States. Yet, Henkel KgaA (the corporation behind Loctite) "has been operating for three decades in China, and has about half a dozen factories in the country, including one in Shanghai, which is the largest adhesives plant in the world. Sealants, epoxy, silicone and other adhesives for industrial and hobbyist use are sold under the Loctite brand":
https://www.telegraph.co.uk/news/201...ing-seizes-55/
In other words, be careful where you buy your Loctite (counterfeits exist) and that it does have a shelf life.
Lastly, here is a fascinating discussion on upper receiver lapping, headspacing and also barrel extension bedding to the upper (note the bad advice and faulty "facts" that some of the Yoda's dispel/address in their corrective posts):
https://forum.snipershide.com/thread...pping.6929270/