Wondering about AR "bedding" quirks

[Harter66]

Ive been working with a Blackthorne 6.8 SPCII. It's assembled to an Aero lower from what is described as milspec parts (I don't really know what that means in modern terms beyond it should fit with all the other MIL-spec parts). It is of 16" M2 mid length gas ,so shark fin and a bird cage and a M203 groove and 1-11 twist . The 1st hurdle was to get the upper and lower rigid there was a little more wiggle than made me happy like getting 2 minimum parts together. With that resolved I shot 40 factory's for brass and proceeded to order out a 279-124 NOE while discovering that the 27-130 fp takes up so much case space that I can't reach my target speed . The NOE boolit seats out to magazine length and adds lots of case space . Well that's great . So far I have worked loads from reasonable to I may have lost my mind but the primers aren't flat and the heads are so it must be less not ok. I used 2 primers ,2 boolits ,4 powders and run 120 rd in 40 loads from 1900- 2300 fps . What do I have ? 40 loads with no leading, no feed issues and no groups .
I've shot ,
IMR 4350 from a suggested 105% of 25.1 from quick load to 28.5 not compressed from 1750 to 2050 fps . Compressed I could probably get 30.5 in the case . 4 equalatoral triangles at 4-8 " high and right 8x10.
H4198 20 to 24 gr from a quick load suggestion 2080 dropping off to 1900 then back up to 2300. This 1 produced variations of horizontal stringing from the 6" right of POH with a 3" drop diagonally to an 8" flat line to 12" low and back up to a 3x6 triangle with its apex at POH .
H322 random placement of hits 2020-2180 fps high and right of POH 6 loads however stayed inside an 8" circle.
10x POH hits but 4" at 50 yd and unable to reach desired velocities 6+ loads inside 8".

The FC 115 FMJ factory loads went high of POH at nearly 2500 FPS and put 38 rd into an 8" circle from a new function checked only upper on a platform I've never shot with a bore sighted optic .
Being what it is , a cheap seats kit with a 5lb single stage trigger , I'm happy with a consistent 3" group at POH . But I would like to see it the same shape every time also I have 1 more optic I can try on it but the post K4 isn't well suited to load development.
Could there be a burr , lump ,bump ,something touching or (reaching a little)lube issue maybe even walk between the upper and lower ?

[country gent]

A couple things to check and look at are with the gas tube itself. 1St is it binding in the reciever barrel nut junction? tight here affects barrel harmonics as the tube is restricted. 2ND is the key on the bolt and the gas tube centered and properly alighned with each other. Again misalighnment here affects barrel harmonich and bolt alighnment. Also check for binding caused by handgaurds. When the recievers are installed and the pins seated how much movement is there? are the pins snug in the recievers holes? There used to be a "cammed front pin that could be installed and then rotated to take out play also. I never cared much for them though. There are rubber wedges to remove play also. I have seen the rear lug bedded into silicone rubber with a .005-.008 pin in place of the originial. Wax the boss on the upper heavy and let cure for several days before disassembly. binding or tight spots can cause changes that affect accuracy.

[Boolit_Head]

There are several schools of though ion this. Most agree the upper to lower fit only addresses the accuracy potential in your mind. Rattling just makes you think it is less accurate in other words. Most agree accuracy lies in the quality and fit of the upper's components. With that said I still like mine to fit reasonably well and not be a rattle trap.

Some get into a bunch over how the barrel nut is torqued down and prefer to shim to align the gas tube. Most agree torquing and unscrewing the barrel nut 3 times with anti seize to seat the threads helps. Along with that the torque range is 30 to 80 pounds. Some prefer it in the lower range thinking the upper puts undue stresses in the upper which contribute to inaccuracy. Some don't care and crank the torque up. I'm in the camp that likes the lower torque values. Barrel extension fit to upper receiver will contribute. Most of the mil spec uppers can vary widely in quality. I just built one on a BCM upper and could not fit a .001 feeler gauge between the barrel flange and upper when tapped in prior to seating the barrel nut. The BCM upper was snug enough it took some light taps to seat it. Some other brands require the upper's to be trued. Otherwise it can do odd things and put undue stresses and harmonics into the barrel when shot. The companies buy forgings and finish them themselves to different degrees so some are better than others. I went with a BCM m4 since it was tighter tolerances and usually trued up and square.

In some ways the general rifle accuracy tricks still apply such as is the muzzle properly formed and no nicks. A good trigger will help same as any other rifle. Along with that check the muzzle device if any for proper alignment and no signs of strikes.

Is there any generalities to the groups such as vertical stringing?

I am not real familiar with the blackthorn upper you described, was it a kit? If so who assembled it? What barrel brand, length, profile? Is the fore grip a free floated tube or mil spec? What optic and mount are you using?

I just finished my upper last night and am deciding which optic and mount so I have not had time to get out and test this combination of parts. It should be interesting though, I wanted a short barrel that would be spot on for out to 200 yards. I used a stainless 16 inch medium quality fluted barrel in 223 Wylde with a good reputation, a BCM upper with a free floated fore grip. since it is mainly for paper punching and critter control I went with a intermediate length gas system and a adjustable low profile gas block under a rifle length tube and a comp instead of flash hider. It should be interesting and punch groups well under 1 moa if I do my part. I am leaning toward a 4x12 scope for it right now.

[Harter66]

Boolit
It is a zero nothing M2 kit though the rifle appears to be an M4 with the fin sight/gas block . The trigger is probably about 5# world's better than a M14 Remington and single stage but much like an 03A3.

I've wondered about the gas tube and intend to change it out with a quad rail I have on hand . I don't expect match grade from this rifle but it would be nice to have it shoot either to 1 place with several loads or under 3" that I can move to where I'm aiming.
The rings are a blocky high profile preferred AR that the manufacturer of escapes me at the moment hooked to the integrated flat top rail. The scope is a Bushnell sport view 3-9x32 it has had several homes but was eventually bumped for 3-9 x 40s . I have a Weaver K4 post I can try , not a great choice for precision work .

Gent
This thing was about as loose ,I was going to say as a hot dog in a hallway but it was closer than that . I have it shimmed solid waiting on my allowance to recover so I can make an order of stuff again . The offset pin or wedge pin may be essential here. I think an accu-wedge is in order as well . The pin in the gas tube is intriguing more research into why as much as how I guess .

The groups run from 3@4" the 8" lay a rule horizontal . They seem to have 4" between each shot ...... maybe 5s instead of 3s are called for .

[country gent]

The gas tube needs to pass thru the reciever barrel nut and handgaurd into the bolt key freely no binds or rubs. You may end up bending the tube to fit into place freely its fine work but when done it shows. Sometimes the gas nut is of time and binds on the tube or the gas block is of time throwing it off. Try tapping it with your finger tip lightly from the 4 directions close to reciever. you should hear tink tink every time as the tube springs to that side do it with bolt open and closed. a thud in one direction or more means an issue with rub or bind.

[Boolit_Head]

Ok, Harter you are saying some things that really don't make much sense to me. Ignore the gas tube in the terms of the free float tube. The free float tube only removes any possible funky harmonics or pressures that could throw off the barrel. Remember all sorts of discussions on bedding bolt action rifles and some prefer a pressure point near the front of the barrel. In the AR that is a bad thing and can throw stuff off mainly because there is not much way to make the pressure uniform. The typical mil spec hand guard system is clamped in by the delta ring and can impose odd pressures on the barrel and throw off the harmonics sort of like a rifle that did not pike a pressure point. That is why AR barrels are free floated. It has nothing to do with the gas system other than the two should not rub.

Yes as Country mentioned the gas tube has to be aligned and not rub the gas key when the bolt is closed. If that rubs it can cause issues with the bolt that could possibly effect accuracy. Ignore the pin in the gas tube at the front gas block/front sight, it secures the gas tube and is not going to effect accuracy.

There have been whole discourses written about the effects of carbine versus mid length versus rifle length gas systems and how the harmonics effect things. Let's leave that for another day and work with what you have.

What you have described is the typical M4 profile carbine kit scraped together from cheap parts to market a kit for 400 bucks. The thin m4 barrels will never really shoot good as they were not intended for small groups. The best I got out of a heavy profile m4 barrel was just over 1 moa. Now not to offend when I say cheap parts it is only to reference what i takes to get a accurate AR. I've got about 2 to 3 times that in just my upper, the lower I went cheap with a Anderson stripped for 39 bucks and the typical parts kit and Magpul carbine stock.

My latest build for reference. This is my version of a do all rifle. Short enough do fill most defense needs. Built with the right parts to reach out to medium distances and spank pesky critters. Swap out the optics from a scope to red dot and go three gunning. If you notice my hand guard does not touch the barrel at the front. The gas tube and block is completely under the hand guard so nothing touches or throws off the barrel. It's a thicker fluted design so it's stiffer than the M4 profiles. I always hated the decrease in the profile for the m203 launchers and figured that had to throw off a barrels accuracy.

Attachment 149468

Post a picture of your rifle so I can confirm my assumptions about the configuration of the parts. Once I do that I can help you sort out the build and maybe shrink the groups some.

[Beef15]

ARs are pretty simple cheap poorly made ones can shoot 4" groups with bulk ammo. At this point you are not fine tuning, you have a gross problem. I'd venture you either have ammo issues, a bad barrel, bad optic, loose barrel, loose optic, or a combination thereof.

Take the handguard off, check for barrel movement and/or tightness of the barrel nut. If that checks out get a variety of factory match ammo, mount a known good optic properly and securely and head to the range. You might even consider having someone else more familiar with ARs shoot it.

[Harter66]

Attachment 149484

It's on the left .

I've been thinking about the overall minutia of that which is observed .
I think I will try some more jacketed and see if it is a failure mechanically or if it's something I need to rework it was also suggested that my steps may be too large and be jumping over the load . It's also possible that it simply won't do what I need it too.

[Boolit_Head]

That is pretty much what I expected. I'd start by taking off the fore grip and trying some of the same ammo to see if it made a difference. Shoot it without anything touching the barrel to see. I'd also consider free floating the barrel and lapping the receiver. If you do lap the receiver take only what is necessary off or you will have issues with m4 ramps if your upper has them.

https://www.youtube.com/watch?v=BHJ5sEhSm1g
https://www.youtube.com/watch?v=JBeIAjE1BhI

If you take the barrel nut off seat it three times when you reinstall it.

[xacex]

Before you make any changes go find some match ammo, and shoot that out of it. You may have some tuning to do, but if it wont shoot with match ammo there is something mechanically wrong with the components of the rifle. Add in the fact you have a scope mounted to that rifle and you have two things that could be wrong. The rifle, or the optic. Out of dozens of AR's I have assembled in different calibers over the years I have only had one that was like this. The final determination was that it was a bad barrel. Once in a while you get a dud I guess. You wont know till you shoot factory ammo, and preferably match ammo.

[Harter66]

Xacex ,
It's 140 miles to a known box of 6.8 ,so I don't fore see that happening right away .

A Navy armoror (sp) tells me that the A2/3 is only good ,in GI dress, for about 4" @100 . Since that is essentially what I have to work with I'll take what I can get.

After a morning spent shopping I found a wrench and upper block along with a free float fore end kit with a new gas block ,tube and 12" of add on rail under $100 on the door step.

The gas tube may be touching/loading at a point in the receiver or may have crud touching. I did a fairly complete wash out of the upper and spot lubed .
I will pursue a replacement for the optic over the weekend and possibly be able to run a check Sunday .
I learned that I will be making the 140 mile trek this weekend anyway so maybe I will have "good" ammo to check the accuracy with and create a new base line.

[xacex]

With today's components an A2, or M4 build such as yours should have no problem doing 1" regularly. The 4" quoted by your local armorer is military standard before they say that the rifle does not meet specification. But, they get shot a ton more than a civilian rifle, and that is expected with iron sights. I have an A2 dissipator build I did in 300 blackout, and it does MOA so the idea that not free-floating an AR will only net you 4" has not been my experience. A 6.8 is a reasonably accurate cartridge to begin with, and you should be doing 1.5" without problem if you are an accomplished shooter. I say that because I shoot better than family in the military, and they are happy with getting 4" so it depends on skill set as well.
Using factory match ammo with a known accuracy standard across several rifles gives you a starting point on where your rifle should be shooting with you behind the trigger, and should be used as a baseline. I know, I have been reluctant to buy ammo in the past as well, and usually only do it if there is something that is not shooting well no matter what load I used. I consider myself a good reloader, and shooter but you have to get past the idea that buying ammo is never required. For me it is more of a pride thing.
I prefer to start with the least expensive option, and work my way up when building, or working on a problem. I have found just assuming it is the most expensive part from the get go has netted me a loss in the past. Ar's are no different. A $20 box of ammo is cheap. Between you and I the free float rifles look nicer, and seem lighter, but switching between free float, and standard hand-guards has not got me much of an increase to be worthwhile. The best money spent in an AR is the barrel. That being said the most accurate 6.8 barrels I have seen have been ar15 performance (ARP) barrels. I have one of there .264 LBC barrels that shoots in the teens for 5 shots at 100 yards(It is one hole, and you have to use a program to discern the holes) with reloads not match ammo. Sub MOA is easy.
While you have the rifle apart I would true up the upper receiver. That has been done on the last 15 rifles of so I have assembled, and make timing the nut easy to a specific foot pound of tension without shims, and makes sure the upper, and barrel are aligned. This helps with iron sight picture, as well as accuracy. That alone has netted me .3" of an increase of accuracy on some rifles. The tool was not to expensive, but do not overdue it. You can ruin an upper if you take off to much. Just use the tool till the barrel contact point is flat, and shiny, then test tension of the nut, and adjust accordingly.

[popper]

I like the one piece scope mount - more rigid upper. Looseness with the lower will affect recoil, take the slack out by 'loading' it fore, aft, up, down, anyway you can. Make sure the barrel socket is clean and true (trueing kit), use light oil when assembling - NO dirt allowed. Barrel pin must slide in the receiver slot a tad loose - the receiver threads clamp the barrel tight - the reason for wide torque specs. Recoil is through the barrel/receiver joint - if it's loose - bad shots. Also check your shooting style. Round handguard will tend to 'walk' on the front stand from firing torque, usually up and left. I have to bear hug mine to get good groups. 4# trig is better than 7# standard. Do NOT use locktite on upper parts! I shoot 308 & BO, both do well, thought the 6.8 was good BR caliber.

[Harter66]

The upper was pre- assembled and function fired before shipping. I'm sure it is all lowest bidder. It is at times like this that I rediscover that I am simply both a cheap skate and a poor rifleman,1 aggrevates the other . Probably nothing wrong with the rifle just the nut behind the trigger.

Fwiw I have this 06' that won't shoot anything factory for several reasons I won't elaborate on as it doesn't exist except as a special order or horrible mistake .

[xacex]

It probably cost more for that assembled upper than what you could have built one for. That is why I got into these things. It was a favor that ended up going much further. Popper makes a great point about the trigger, and cantilever scope mount. A gritty, creepy 7 pound trigger is not going to help with groups. All of mine get a trigger job, but it is something I would pay someone for, or get a drop in trigger if you are not totally familiar how these things work. Real easy to make one accidentally full auto, or make the safety drop the hammer. I have a neat trick for taking up over travel, but you do it wrong the disconector will let go of the hammer if you pull the trigger before taking it off safe. Once you take it off safe, it will discharge. Never had it happen, but found out doing a function test. One turn to many and it gets dangerous quick. Shooting AR's is a little different, but intuitive once you get to know them. Nothing beats time behind the trigger. There are a bunch of tweaks you can do to the rifle for not much money to get them to shoot better, but like I mentioned shoot a box of match ammo in it to get a good base of where you are at before you go further. There are a bunch of us AR nerds in here, you are in good company.

[popper]

Try some loads single loaded to see if accuracy improves. I mean drop the case in the chamber, then drop the bolt. I'm using a similar boolit in BO and found the nose hit the ramp/something and accuracy was terrible. Lots of horiz. stringing, bad groups, etc. Except for the m4 barrel, it doesn't look like a bad upper.

[Harter66]

It probably cost more for that assembled upper than what you could have built one for.

Maybe so but I would have had to have caught every special and every close out. It's on the to do list but not this year . It's pretty easy to make the jump from 600 to 1500 ,it makes me sad to look at my well worn Savage and know that if it could be replaced directly would be north of 800 now and I nearly broke the bank a 200 when I bought the left hand high grade 110 LH .......

More to the point single fed mag out is the only manor that it hasn't been tried . The 6.8 is intended for my Ms to hunt hogs, she has a fused wrist hince the choice of the AR . She's 5'4" and needs a 10 inch length of pull and a pistol grip . Otherwise the only way for her to shoot is with her middle finger on the trigger or to support the fore ungripped across the back of her thumb and pointer finger . The 5 rd mag well length mags are about as common as wisdom accompanying wisdom teeth . It is possible that the Caldwell bag was no help but with the sling ring forward and the grip planted on my bench that should have removed about 80% of the shooter induced errors . The trigger break is clean and no more than 5# with little to no creep or over travel. When I shoot both open right handed the brass is a distraction ,left handed (dominant) 1 eye I never see it . I have adjusted holds from my normal 3 point sitting GI 03 sling wrap sans the sling to mag well face control nothing touching the fore stock.

I feel like after looking at my hit map that velocities may be part of the key for this rifle . It may need a particular speed at a particular pressure and get there in some particular way . (I can see the 27-130 @138 gr being run wide open with Superformance and FC small mag pistol primers in Win brass with a fender washer under the Ford truck lug nut that is a flash hider thread protector. ...... comically as an exaggerated example of oddity).

If available I will try at least 2 more factory loads 3 if possible. I recall 1 of my pets that refuses any load in a particular brand of brass so I suppose I should try some other brass in the AR ....... Ironically I have Rem and S&B for the LRP brass in the Carcano 7x6.8 mildcat wouldn't it figure to have to swap to make them both just happy happy ......

[popper]

More to the point single fed mag out is the only manor that it hasn't been tried
Just to verify the FP nose isn'tgetting dented.I tried free recoil from bags, didn't work. Have to set my shoulder square against the stock, hold the magwellwith the left hand, handle off thebench. What is your alloy?Normal process for an unlapped barrel(factory) is about 50-100 jacketed. I'm running 45-50K on my loads, about the same as yours.

[Harter66]

I think we're in the same pressure range .

WCWW /1-20 75/25 plus tin . Hornady checks .
It has 39 jackets and 126 GC cast down range .
I still have 80 SSTs I hate wasting them that way .....
Maybe I'll luck into some Serrias while Im killing time in the city or something cheap to throw away . Of course now I be forced to buy that Lee sizer that may or may not be .279 .

[popper]

WCWW /1-20 75/25 plus tin

WCWW = water dropped? I think you need 4-5% Sb for that pressure, yours appears like 3%. I've done it but by adding Cu, no tin, heat treated (PC) for 308W. In my 300BO I found the case/boolit dia. ratio was causing nose dents and accuracy problems. FP boolit didn't like the feed ramp.