Chasing accuracy problems in AR build

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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Chasing accuracy problems in AR build

Postby carpenterminator » Sun May 10, 2020 7:38 pm

I've been lurking here for some time, and love the great information that I find. I built a 450 Bushmaster based on a Saturn barrel/bolt from Brownells and Seekins upper and lower. It's a non-adjustable gas block, and the buffer is an H2-weight JP silent captured spring. Since it was put together, it feeds perfectly, but I've been troubleshooting bizarre inaccuracy (~5 MOA). I've had three different sight/scope setups on it, tried Remington, Winchester, and Hornady ammo, and the last thing that I tried was lapping and truing the upper receiver face, and bedding the barrel with green Loctite. None of these steps changed the group size. I'm almost out of ideas (headspacing?), what about you people?
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Re: Chasing accuracy problems in AR build

Postby Hoot » Sun May 10, 2020 8:11 pm

Tell us more about the setup on the bench that you're using for accuracy testing. A steady rest setup or a Lead Sled is pretty important for cutting down the MOA number, not to mention a steady shooting table. At the local club, ours are precast concrete sitting on cinder blocks. IE "Rocks of Gibraltar". Also, besides switching out the scope, did you also try different mounts? They're not all created equal. If you were local to the Twin Cities, we could go to my local range and try a few things. I don't think the headspace, if awry, would affect groups that much. Are you using a muzzle brake or flash hider. If so, have you tried shooting with them off? Muzzles sometimes can be set to just the worst possible spot. There are certain lengths of barrel/muzzle device combination that can leave the barrel whipping up and down. Speaking of which, are your groups opened up just vertically or just horizontally or both?? I use a lot of muzzle devices and personally have never seen a muzzle device turn a 1 MOA rifle at 100 yds into a 5 MOA rifle. Is this your only AR rig? They don't handle like any bolt action I've ever shot, especially in this thumper caliber. It took a little while when I first got my 450b upper, to get used to the necessary bench presentation to milk all the accuracy out of it. Not at all like shooting a bolt action. Not to insult you but I assume you have shot other caliber rifles for accuracy with success?

Lot of questions I realize. Break each one out with its own answer.

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Re: Chasing accuracy problems in AR build

Postby Al in Mi » Mon May 11, 2020 4:45 am

Not saying a bad one didn't get out, but I've played with a few of those barrels and all have been accurate with about anything we put thru them.

My first guess would be if you have a muzzle brake and bullet rub, second guess would be the crown, third guess would be copper fouling. But as Hoot mentioned, give us a run down of your set up and then we'll chip away.
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Re: Chasing accuracy problems in AR build

Postby carpenterminator » Mon May 11, 2020 8:08 pm

Hoot, Al,

Thanks for the fast response! I'm in Texas, mostly shooting in rural west Texas, with a very solid folding table and a Lead Sled for dialing in. It was clean. I'm also shooting harder-bucking calibers, e.g., 300 WSM on the last trip. 'Next trip will be to a city range with fancy shooting tables, but I've shot there before in the various attempts with this rifle. The thumper was the 5th of 6 AR builds, which have all come out very nice. My home-built 6.5 Grendel is as accurate as most of my bolt action hunting rifles, at about half an inch at 100 yards.

Tell us more about the setup on the bench that you're using for accuracy testing. A steady rest setup or a Lead Sled is pretty important for cutting down the MOA number, not to mention a steady shooting table- At the local club, ours are precast concrete sitting on cinder blocks. IE "Rocks of Gibraltar".
* For this last exercise, Lead Sled with 3 lead bags on a strong folding table, on firm sandy soil. Other rifles, including harder recoiling calibers, are predictable, <1 MOA performers with the sled.

Also, besides switching out the scope, did you also try different mounts? They're not all created equal.
* I've been using American Defense Manufacturing "Recon" mounts for 2 out of three, the third is an EOTech XPS2 / doubler with integrated quick release mounts. I'll see if I can find something simpler in the drawer.

If you were local to the Twin Cities, we could go to my local range and try a few things.
* Thanks! I'm getting desperate, and with the price of fuel being low...

I don't think the headspace, if awry, would affect groups that much.
* Great, I don't want to buy 450 gauges :)

Are you using a muzzle brake or flash hider. If so, have you tried shooting with them off?
Muzzles sometimes can be set to just the worst possible spot. There are certain lengths of barrel/muzzle device combination that can leave the barrel whipping up and down.
* Great idea! It's a flash hider thing, I haven't tried with it off, but I'll do that next. It has plenty of clearance to the bore. The crown looks OK, but other than trying with the flash hider off, having the crown redone might the the cheapest thing to try at this point...

Speaking of which, are your groups opened up just vertically or just horizontally or both??
* Both windage and elevation.

I use a lot of muzzle devices and personally have never seen a muzzle device turn a 1 MOA rifle at 100 yds into a 5 MOA rifle.
Is this your only AR rig? They don't handle like any bolt action I've ever shot, especially in this thumper caliber.
* I got my first AR in Marine Corps boot camp in 1980, I have 5 ARs in 4 calibers these days.

It took a little while when I first got my 450b upper, to get used to the necessary bench presentation to milk all the accuracy out of it.
Not at all like shooting a bolt action. Not to insult you but I assume you have shot other caliber rifles for accuracy with success?
* I've wrung great performance out of every rifle I have, except one of those famous mid 80s Mini 14s that I wish I didn't buy :) Looking forward to getting this one figured out!

Thanks Again,
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Re: Chasing accuracy problems in AR build

Postby Hoot » Tue May 12, 2020 6:42 am

That summary is indeed worrisome. Given what you have said, to me it would seem like your experience, coupled with that setup, would shoot the eyes out of a rattle snake at 100 yds. :|

Grasping at straws

Having now looked up the details about your barrel and upper/lower components, its apparent that you didn't skimp on quality. That's the perplexing part.

Did you have any trouble torquing the barrel nut to spec while getting a gas tube hole to line up properly? IE you didn't have to reef on it too hard, to the point where you may have distorted the receiver? I haven't seen one torqued to the point where that would happen myself, but the all knowing they say it is possible.

Has it ever shot even one decent group since you put it together? IE is the problem intermittent?

There's several experienced 450b members in the Lone Star state if you want an in person, second opinion. Ahem, c'mon guys, find your keyboards!

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Re: Chasing accuracy problems in AR build

Postby carpenterminator » Tue May 12, 2020 4:02 pm

Hoot,

Thanks, I had been wondering if perhaps the barrel nut is too tight- it could happen. The Seekins hand guard and barrel nut don't require shims and torquing to the next gas tube hole, but I use anti-seize dope on the threads, which could cause it to be over-torqued, even if the wrench says 40. Since the barrel is glued to the upper anyway, I should stop worrying about disassembly, clean it to dry threads and re-torque it to 40 or so. I also need to try it without the flash hider, and put some simple rings on a scope to mount the optic.

Best Regards,
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Re: Chasing accuracy problems in AR build

Postby Hoot » Tue May 12, 2020 9:28 pm

carpenterminator wrote:Hoot,

Thanks, I had been wondering if perhaps the barrel nut is too tight- it could happen. The Seekins hand guard and barrel nut don't require shims and torquing to the next gas tube hole, but I use anti-seize dope on the threads, which could cause it to be over-torqued, even if the wrench says 40. Since the barrel is glued to the upper anyway, I should stop worrying about disassembly, clean it to dry threads and re-torque it to 40 or so. I also need to try it without the flash hider, and put some simple rings on a scope to mount the optic.

Best Regards,


No No! I put copper bearing antiseize paste on all my barrel nuts. Think of it this way, Now you're really seeing the torque reading accurately, not vise versa. Galling (unlubricated) barrel nut threads can give a false higher torque reading and god help yu when it comes time to take them off! I must admit though, I've never loctited a barrel in, regardless of color, but that's just me. The only thing on my rifles that sees loctite are the scope mounting holes in a tapped receiver. They get degreased and a pin prick of blue carefully applied to the screw threads. Never had anything unwantedly loosen up yet, once I torque screws to their proper spec.

Your issue is indeed a conundrum and I hate conundrums. Everything has a rhyme or reason. Finding it can be a real put off after having turned over all the stones with no luck. I have a Remington 700 with a Criterion RemAge 6mm XC barrel on it. Shoots amazing bug holes but with even modest loads I get heavy bolt lift and top of lift click using Peterson brass. Drove me crazy chasing gremlins with it. It wasn't until I went back in time 18 years and made some brass out of resized 22-250 cases. That's how the first 6XC shooters got their brass as no one made it. With the resized 22-250 brass, the problem vanished. I love Peterson brass and I was hell bent on being able to use it. Especially since most of the heavy hitters using this caliber are having no such problems with their custom rigs with custom cut chambers, yatta yatta, are using Peterson. After doing a lot of research and with the help of Dave Manson, I discovered that Criterion used a reamer designed back in 2003, when resized 22-250 brass was all that was available. This despite the caliber having gone through a couple of revisions to which my Peterson brass was compliant with. I went in with another 6XC shooter who also has a Criterion barrel with the same problem, on a modern reamer to kiss a few thousandths off certain parts of the chamber. Waiting for PTG to get it done has been agonizing (8 weeks and counting) but got an email yesterday that it would ship this coming Friday. Up here, our comfortable shooting season is pretty short. I can hardly wait to get this project done and behind me. The point being, I know how maddening it can be chasing gremlins in what should be a "done deal".

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Re: Chasing accuracy problems in AR build

Postby Texas Sheepdawg » Tue May 12, 2020 10:09 pm

Well. I’m in Texas but if you’re in West Texas, I may as well be in Missouri. But after reading your description of your setup, I can only suggest one thing. A Lemon barrel. Your experimenting with optics pretty much rules out issues there. Satern is a damn good barrel company. You should call them and tell them what’s going on. I wouldn’t waste anymore ammo.
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Re: Chasing accuracy problems in AR build

Postby carpenterminator » Wed May 13, 2020 8:46 pm

Thanks guys, it's not the barrel nut then, I can't believe that this beefy Seekins upper is cracked, I'll try a last time on a rock of Gibraltar table at a city range (I'm in the DFW area, I just like shooting in west Texas), with some basic scope rings... and then call Brownells. They say that they have a great, lifetime warranty. I imagine Saturn and Brownells get all kinds of unhappy idiots wanting replacements, refunds, etc.

Again, thanks for all the help, I'll post what happens after I can get to a city shooting range (are those even open?)
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Re: Chasing accuracy problems in AR build

Postby Hoot » Wed May 13, 2020 8:57 pm

carpenterminator wrote:Thanks guys, it's not the barrel nut then, I can't believe that this beefy Seekins upper is cracked, I'll try a last time on a rock of Gibraltar table at a city range (I'm in the DFW area, I just like shooting in west Texas), with some basic scope rings... and then call Brownells. They say that they have a great, lifetime warranty. I imagine Saturn and Brownells get all kinds of unhappy idiots wanting replacements, refunds, etc.

Again, thanks for all the help, I'll post what happens after I can get to a city shooting range (are those even open?)


You may wind up sending the complete upper to them (minus the scope) to see if they can suss it out. I would if they offered that. BTW, how much did you have to face off to get the receiver trued? I'm guessing not much, though I can't imagine and I have a great imagination, how that could be a factor if the nut cinched up before it ran out of threads. now I'm really grasping at straws! ;)

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