AR-15 Barrel Mounting - Help Me Decide What to Use

[skeeljc]

I have a 24" White Oak SS Heavy Varmint barrel (.920" at muzzle) with a 1.000" diameter tenon. I want to mount it in a DPMS Lo-Pro upper receiver which has a 1.0020" diameter where the barrel tenon fits.

My question is what do I use to fill the gap between the barrel tenon and the receiver?

I am familiar with LocTite 638 Retaining Compound and Loctite 60 minute Extra Time Epoxy.

Looking for something that will hold up with heat and would permit the barrel to be removed.

 

[Sniper338]

I have a 24" White Oak SS Heavy Varmint barrel (.920" at muzzle) with a 1.000" diameter tenon. I want to mount it in a DPMS Lo-Pro upper receiver which has a 1.0020" diameter where the barrel tenon fits.

My question is what do I use to fill the gap between the barrel tenon and the receiver?

I am familiar with LocTite 638 Retaining Compound and Loctite 60 minute Extra Time Epoxy.

Looking for something that will hold up with heat and would permit the barrel to be removed.

So Ive been here. I run a heavy 24 inch bull barrel too in 6.5 grendel. Ive build my share of ARs, and I have experience. (What Im saying is listen)... Im about to bring to light something thats going to throw you for a loop.

Heavy barrels do not work well with **Most ALL** upper receivers. Upper receivers are thin, and the tennon receiver part is thin. When you introduce a heavy long barrel like we have, it creates flex in the upper receiver. Put your upper in a receiver vice, tighten it up in the vice to where it doesnt move. Slip in that heavy barrel and pay close attention to the front of the receiver when you let the weight of the barrel onto it, it flexes. Slightly put pressure/move around the muzzle end of the barrel and watch the receiver bend everywhere.... done it, seen it, ive corrected it easily.

So here we just identified the weak point in our current set up that we see creates a stress point and we know will move alot when shot. We dont like this...

Theres one company that makes an upper receiver speacilty built for these types of applications, youve heard of it, seekins precision. Look at the IRMT receivers, theres a triangle one, and a round one to choose from.

This youtube video explains the situation very well. Its a sit down with snipers hide owner SHLowlight and Mr. Seekins himself.

The tennon on these uppers are beefed up alot. They are billet uppers machined from one piece of material and the tennon is integrated into the receiver itself, all one piece... the handguard mounts to the receiver vs the barrel nut unlike all other receivers. This reduces even more flex/stress points.

I currently have everything in one gun case so here comes more info and pictures.

Heres a pic of mine for "authenticity" for this post.

I forget the brand that makes them but this spacer goes in the rear of the receiver to tighten any gap/play between the upper and lower. You sand it a bit at a time until the right width is found where it closes tightly/very snug, but still closes as the pins can be pushed through to secure the receivers together. This is for any back and forth slop/play/vertical movement. (The U shaped spacer below.


The picture below shows trigger shims.com spacers for the take down pins. You have to configure it, there are multiple sizes, but you find what works and they go on your take down pins and take out slop/play/wiggle between the upper and lower with any side to side movement.


I cant upload more pics so I will continue in a second post here below.

 

[Sniper338]

The picture below is the U shaped spacer in place in the upper. You can also see the JP silent capure buffer, which is a very nice addition too which quiets down the cycling of the gun.

This is a picture of the front pivot take down pin. The spacer is inside the lower, but outside the upper receiver take down pin holes. You can see the silver on the right side.


These spacers are cheap, easy, and sure make a big difference tightening up all the fit between the upper and lower.

Below shows what I use on the barrel tennon and threads. Havent had an issue yet. Just make sure to use a q tip and clean out excess once you slip in the barrel and crank down the barrel nut. Let it dry over night before continuing to assemble the gun. (Side note.. I do check for square tennon to barrel lug fit and correct if not sqaure).


Hard to really see much without disassembling my gun... but I was trying to show how thin a regular receiver is compared to the seekins thats built for this application.


I went with the triangle receriver/handguard set up. Handguards are specific for each receiver. I wanted the flat bottom to mount a bipod rail to for use with my joypod bipod. I also like straight sides on the handguard for bracing against something, and the flat bottom for shooting through a window or on a flat horizontal brace.

Honestly, these uppers are the only uppers Id buy even if I wasnt using a heavy barrel. The aspects, pro's, etc, all still remain in effect even with a lighter contour barrel.

 

[Sniper338]

Other suggestions, parts I have if anyone is curious:

Les baer internal upper parts bolt carrier, bolt etc.

Seekins rails
Ergo palm swell grip
Magpul PRS butt stock
Phase 5 bolt release to make it right handed and ease of use
Bob sled mag
Satern custom machine cut rifling barrel custom order, not their cheap barrels.
Dropped/drooped trigger guard because it looks better imo.
Trigger is a 2 stage Geissele set VERY VERY light for an AR15. Theres no 1st stage creep, I took that out, its just a very crisp 8-10 oz brake.

SLR adjustable gas block, non set screw, its the one with bolts that clamp to tighten. Theres debates between a clamp vs set screw type gas block, ive never found a difference on paper, but I use the clamp type just to factor that issue out.


Another side note: without using a good analogy, run ARs wet. I use hobo oil in mine. Put oil on very internal part that contacts metal and moves when cycling. Clean often, and re-oil. (Which I need to do myself again ha!)

Any questions about any of this or other parts of a build pm me directly, I dont keep up with threads and other post.

 

[GSS]

Sniper338
Interesting Info...

Plain to see where the PRS shooters could gain from these receivers.

These receivers wouldn't be legal for the NRA / CMP matches I shoot in the Service Rifle category but they would be for people that shoot Match Rifle Categories.

Thanks for posting.
George

 

[Sniper338]

Sniper338
Interesting Info, Thanks for posting

I guess I should add this gun shoots, or has shot... 2 inch 5 shot groups at 750 yards. For an AR I consider this outstanding. I dont have the pictures on my phone though.

Hope my post helps people out. Im not affiliated with any company listed*** Im not even in this industry. Ive just put in the time and research, I got some time in with all aspects of AR platforms.

 

[Sniper338]

My post are all updated now as Ive gone back several times to add more or edit.***

 

[GSS]

I guess I should add this gun shoots, or has shot... 2 inch 5 shot groups at 750 yards. For an AR I consider this outstanding. I dont have the pictures on my phone though.

Hope my post helps people out. Im not affiliated with any company listed*** Im not even in this industry. Ive just put in the time and research, and carry a full auto colt M4 for work, I got some time in with all aspects of AR platforms.

outstanding

How many shots typically in those 750 yards 2" groups?

Were they shot off a benchrest on the ground with a bipod and rear bag, or other setup?

Thanks again for posting all the great info !

 

[Sniper338]

outstanding

How many shots typically in those 750 yards 2" groups?

Were they shot off a benchrest on the ground with a bipod and rear bag, or other setup?

Thanks again for posting all the great info !

5 shot groups, its said above. Bipod and rear bag. Im only bragging a little, but the intended purpose is to show these things I brought to light do make a big difference in the overall function and building of an AR.

 

[skeeljc]

Here is the rifle I am working on.


It currently has an Aero Precision upper. I suspect that upper receiver flex is holding it back from peak precision. It will shoot 1/2 MOA groups about 80% of the time. I removed the movement between the upper and lower by reaming the takedown pin bores oversize and installing oversize pins. The front lin is a .250" shoulder bolt. The Aero Precision lower has a set screw which pushes up against the rear pin boss on the upper to further limit movement.

Here is the DPMS Lo-Pro upper compared to the Aero Precision upper.



Note the additional material in the walls of the DPMS upper. The wall thickness of the AP upper is .095" thick where the DPMS wall thickness is .250".

 

[skeeljc]

Here is another comparison of the two receivers.



The DPMS receiver is 7073 T6 extruded aluminum and the Aero Precision is 7075 T6 forged aluminum.

 

[skeeljc]

Back to my original question. What do I use to fill the gap between the barrel tenon and the upper?

I am running a JP Silent Capture spring/buffer, JP Low Mass carrier, JP Enhanced bolt and a Geissele High Speed National Match trigger.

 

[skeeljc]

Here is what the rifle will do most of the time. these are 100 yard groups.

 

[jkohler]

You can use 620 or 609 loctite. Or just use some shim stock around the barrel extension.

 

[Sniper338]

Im bowing out. I dont have anything nice to say at this point. I answered your original question 3 times, in text In a picture, and in a video if you pay close attention. Jkohler answered it twice. Thats 5 times now your question got answered, plus a fk ton of more info. Blue locktite, blue locktite is the answer. Attention to detail goes a long ways.

Shims around the barrel extension and inside the receiver tennon is very bad advice though. If its that bad, you need a new receiver or barrel extension.

Ill post this too. This is what a 5 shot group should look like consistantly. Picture is cropped to not show the reloading data or fowler shot as thats all irrelevant.



Im done now lol.

 

[skeeljc]

Im bowing out. I dont have anything nice to say at this point. I answered your original question 3 times, in text In a picture, and in a video if you pay close attention. Jkohler answered it twice. Thats 5 times now your question got answered, plus a fk ton of more info. Blue locktite, blue locktite is the answer. Attention to detail goes a long ways.

Shims around the barrel extension and inside the receiver tennon is very bad advice though. If its that bad, you need a new receiver or barrel extension.

Ill post this too. This is what a 5 shot group should look like consistantly. Picture is cropped to not show the reloading data or fowler shot as thats all irrelevant.
View attachment 1143891


Im done now lol.

No doubt you have experience with AR's. I am just looking for OPINIONS. Opinions often vary.

Your comment on upper flex was eye opening. Thanks for that.

 

[skeeljc]

I think the recommendation on Loctite 620 will be good. I reviewed the 620 data sheet and it does well up to 250 degrees C retaining 75% of its strength.

Blue thread locker 243 has low compressive strength and poor heat resistance.

Does anyone have experience removing a barrel that was installed with Loctite 620?

 

[Fast14riot]

I used blue loctite on both of my heavy barrel ARs. My #1 upper has a 1.050 diameter barrel under the hand guard (heavier than normal and required a modified barrel nut), that upper was built 5 years ago and it still going strong.

Truthfully, you will see far more flex from shooting in a sling than from the weight of a heavy barrel.

 

[PhilKeil]

I used blue on mine as well

 

[Walt Krafft]

Blue Locktite