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Posted: 6/3/2015 7:39:17 PM EDT
| I recently purchased a new AR-15 upper on GunBroker. After receiving, inspecting and mounting the upper, I took it to the range for zeroing. I was unable to zero the upper because I could not get better than a 2-3 inch grouping at 25 yards. The seller seems to be giving me the run around telling me to try a different scope, iron sights, different ammo...etc. Well, I have tried a different scope and iron sights with the same results. I even switched back to my old upper (also a 1:9 twist) and was able to shoot better than 1/2 inch groupings at 25 yards (with iron sights). Am I completely off base expecting that a AR-15 upper should be able to shoot better than 2-3 inch grouping at 25 yards with just about any ammo? |
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For a little more information...all shots were taken using bench rest with front and rear supports.
This 5-round group was shot with my original AR-15 upper using iron sights because scope had been removed. This 10-round group was shot with the new AR-15 upper using 20x scope. |
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Quoted:
I recently purchased a new AR-15 upper on GunBroker. After receiving, inspecting and mounting the upper, I took it to the range for zeroing. I was unable to zero the upper because I could not get better than a 2-3 inch grouping at 25 yards. The seller seems to be giving me the run around telling me to try a different scope, iron sights, different ammo...etc. Well, I have tried a different scope and iron sights with the same results. I even switched back to my old upper (also a 1:9 twist) and was able to shoot better than 1/2 inch groupings at 25 yards (with iron sights). Am I completely off base expecting that a AR-15 upper should be able to shoot better than 2-3 inch grouping at 25 yards with just about any ammo? Perkmeister, When this happened to my upper I realized that my bore was heavily coated with copper residue!!! I had to soak that bore down well with a solvent like Birchwood Casey's Bore Scrubber and brush it out well. Those wet patches came out greener than my lawn. Anyway after almost a full weekend of soaking and cleaning my groupings was fantastic. Of course in your case this is assuming your bore is not chrome bored. Impala |
| Is the barrel free-floated? If so where is the muzzle end in reference to the center of the end of the FFHG? If it's off center the face of the upper may have high spots, or the barrel nut is loose. Both can allow the barrel to move slightly. High spots have to be lapped down with a lapping tool. Loose barrel nut simply needs to be tightened to the next notch for the gas tube to pass through. These conditions can exist without the barrel being centered in the FFHG, but that can be a strong indicator of where the problem is. |
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Ditto, start with a good barrel and chamber cleaning. Here, I would start with Hoppes to get the fouling and assembly grease out of the barrel, then switch over to Sweets for the bore to get any copper fouling out, and CLP with a chamber brush to get the chamber clean. Hoppes is good for getting out the asssembly/storage grease, but since it leaves behind a protective coating that does not mix well with CLP, The hoppes needs to be totaly flushed out in the end.
Hence when the barrels are assembled, lot of assembly grease, and even storage grease to keep the bore from rusting (all which needed to be removed before the barrel is shot. Once the barrel is clean, then next items that comes to mind is maybe a loose barrel nut, or even a problem with the FS/Brake. Barrel nut is easy to check, as well as pulling the FS to run the barrel without it (to weed it out as the problem). With these items weeded out, then if the barrel will still not group, its the barrel that is the problem child for the ammo being run. |
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Quoted:
Is the barrel free-floated? If so where is the muzzle end in reference to the center of the end of the FFHG? If it's off center the face of the upper may have high spots, or the barrel nut is loose. Both can allow the barrel to move slightly. High spots have to be lapped down with a lapping tool. Loose barrel nut simply needs to be tightened to the next notch for the gas tube to pass through. These conditions can exist without the barrel being centered in the FFHG, but that can be a strong indicator of where the problem is. The barrel is free-floated...see attached picture. (The scope has already been moved back so that it is only making contact with the upper receiver - same results) 
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Huge problem right from the start,
You have the scope front mount on the hard guard and the back scope mount to the upper receiver rail. Both scope mounts need to be to the upper receiver rail only!!!!!!!!!! Maybe with a monolithic upper can you get away with mounting the scope mount slightly forward of the barrel nut on it, but all float tubes will have slight movement in then, so you don't mount magnification optics to the hand guard rails since any slight movement in the hand guard will change the POA of the scope. |
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Quoted:
Huge problem right from the start, You have the scope front mount on the hard guard and the back scope mount to the upper receiver rail. Both scope mounts need to be to the upper receiver rail only!!!!!!!!!! Maybe with a monolithic upper can you get away with mounting the scope mount slightly forward of the barrel nut on it, but all float tubes will have slight movement in then, so you don't mount magnification optics to the hand guard rails since any slight movement in the hand guard will change the POA of the scope. I mentioned that in the comment...I had already moved the scope. 
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So now, clean the barrel and chamber, make sure that the barrel nut is tight so the barrel is not moving around in the receiver, and run the upper without the brake on it.
Hence the Brake may be the problem child causing the problems to begin with, and with the brake removed before the barrel is cleaned, will allow you to look at the muzzle crown for any problems there as well. |
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Quoted:
So now, clean the barrel and chamber, make sure that the barrel nut is tight so the barrel is not moving around in the receiver, and run the upper without the brake on it. Hence the Brake may be the problem child causing the problems to begin with, and with the brake removed before the barrel is cleaned, will allow you to look at the muzzle crown for any problems there as well. Pictures with Muzzle Brake removed... 
Thanks again for all of your help... |
First of all, please tell me that you can wipe your arse better than you can clean a muzzle crown.
Tear off a piece of green scotch pad with Hoppes or CLP, and with the pad on your thumb with a little Hoppes or CLP, pressure flush against the surface of the muzzle in a circular motion with the pad on your thumb to get all the fouling off the muzzle surface. The green scotch pad is not going to harm the crown!!!! Once clean, I need to be able to see the edge of the muzzle to inside of the barrel rifling in a photo, so make sure to light in the the bore as well when you take another photo of the crown to post after cleaning, so I can see what the last 1" of bore rifling look like as well (want to make sure that is not gouged to shite with someone using a pilot crown tool that I just despise). Next, I need you to clean the inside of the brake, and post a photo of the threaded section inside of the break back wall as well (need to see the wall where the bullet passes through the brake after leaving the muzzle to see if there is problem there too). From what I am seeing now, the muzzle was not crowned for shite, and guessing that the inside of the brake at the entry channel to the brake section has not been produced worth a shite either. Bluntly, with the amount of fouling I am seeing on the crown for less than 200 rounds, looks like the barrel has seen thousands of rounds instead. Half of the problem looks to be the shite crown, while the other half of the problem I'm guessing, is the back section of the brake that is not directing gases cleanly instead (both of which are easily corrected if you have a lathe). To add, not a huge fan of deep taper/chamfered crowns, since they are reserved for someone that does not know how to clean a barrel correctly; wants to over scrub the barrel but pushing a brush from chamber to muzzle and out the muzzle, then keep dragging the brush back into the muzzle back to the chamber over and over again . Hence if you use something like Sweets to clean the bore (which does not take a lot of scrubbing to remove copper, and pull the brush off the cleaning rod instead of dragging the mess back down the bore instead, you don't need the deep chamfer crown to begin with. 
Note, the crown is the the edge of the muzzle to the edge of rifled bore as noted in the 60* surface in yellow marked in the below photo. Instead, I will just kiss off with a brass crowning lap plug with lapping compound to just slightly radius the bore to muzzle face instead. This leave a perfect crown edge, and if needed down the road, not much metal needed to be removed to refresh the muzzle/crown at a later time. 
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| After purchasing some "American-made" (seller requested) match/target ammo (Federal Gold Medal Match Sierra Matchking 69 Grain) and firing at 25 yards and only achieving a 1.5" group, the seller finally agree to allow me to return for repair/replacement. Thanks for all of your help. |
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FYI, ideal bullet weights for barrel rifling twists rates..
223/5.56 nato 1 in 12 twist, up to 55 grain bullets, but will happier/group better with a lighter 45- 50 gr bullet instead 1in 9 twist, up to 62gr 1in 8, 62gr to 69gr. 1 in 7 twist, 65gr to 75gr bullet So unless the barrel is a 1/8 twist or faster, 69gr match ammo is not the best choice for testing it. |
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A 1/9 will easily handle 68 grain Hornady bullets and Nosler or Sierra's 69's. Even a 1/10 twist will stabilize these bullets. Some people claim they can shoot 75/77 bullets from their 1/9's but that's a crap shoot. More times than not they keyhole, especially from shorter barrels or at longer ranges. My bolt action 1/9 twist .223 with a 26" barrel loves Hornady 75 grain hpbt match bullets driven around 2725 fps. My 1/9 AR's not so much.
A 1/8 will stabilize every bullet that will fit and feed from a magazine. 75 or 77 grain match bullets from any source, 75 A-Max or 80 SMK's when seated close to the lands. 1/7 works with everything too. 90 grain SMK's need 1/6.5 for reliable stability. |
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Quoted:
A 1/9 will easily handle 68 grain Hornady bullets and Nosler or Sierra's 69's. Even a 1/10 twist will stabilize these bullets. Some people claim they can shoot 75/77 bullets from their 1/9's but that's a crap shoot. More times than not they keyhole, especially from shorter barrels or at longer ranges. My bolt action 1/9 twist .223 with a 26" barrel loves Hornady 75 grain hpbt match bullets driven around 2725 fps. My 1/9 AR's not so much. A 1/8 will stabilize every bullet that will fit and feed from a magazine. 75 or 77 grain match bullets from any source, 75 A-Max or 80 SMK's when seated close to the lands. 1/7 works with everything too. 90 grain SMK's need 1/6.5 for reliable stability. Agree with you if all we are talking if the bullet will key hole or not, but when we start talking heaver ammo, and pushing them out to the 600 meter range, this is when barrel twist places a huge roll. Also the elephant in the room, bullet sleep distances, and if you back off the speeds so the bullet sleeps fast for say a 100 range target, the reduction is speed will create a huger monster trying to buck wind at the 600 meters instead (not even including the greater bullet drop, since there round is covering less distance per second as gravity pulls the bullet down at a constant rate. Lastly, Boat tail design bullets do not like to jump long distances to lands. Hence if you are jumping a BT bullet say .080" and a flat tail bullet to say the 100 yard mark, the flat tail bullet is going to group tighter every time. |
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Just an FYI...I have not received the upper back yet (probably today) so havent had a chance to take it out to the range, but this is what I got back from the seller...
"The gas block was leaking badly which causes inconsistent velocities and that results in inconsistent P.O.I. I checked and cleaned the barrel just in case and it is spotless with perfect rifling. Its headed back to you today. Sorry about that. I've never had any trouble out those gas blocks before. I installed a new melonite/QPQ treated gas block and gas tube that is much tighter around the gas tube and barrel." -seller |
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