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Posted: 8/26/2014 7:04:38 AM EDT
[#1]
No problems, thats what happens when the barrel heats up when your shooting. Very typical. Even your highest end target barrels will exhibit this when heated up.
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Posted: 8/26/2014 8:33:19 AM EDT
[#2]
The sustained rate of fire for an AR-15 type weapon is 12-15 rounds per minute.
Even at that relatively slow rate, the barrel is going to heat up and you're going to see what you are seeing: bigger groups. |
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Posted: 8/26/2014 8:54:32 AM EDT
[#3]
I've never noticed any of my ARs opening up like that, not a 10-15" increase. This is shooting bench rest with a rate of fire of maybe 30 rounds over 30 minutes.
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Posted: 8/26/2014 9:26:34 AM EDT
[#4]
Check to see if barrel is tight to receiver.
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Posted: 8/26/2014 10:07:39 AM EDT
[#5]
Aside from the heat issues, you also get fatigued from muzzle blast and recoil.
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Posted: 8/26/2014 11:10:57 AM EDT
[#6]
No way does ANY of my 16 plus AR's group that large from that little sustained fire. Not a chance. They do open up somewhat but not like that at all.
Here is one mag 20 shots relatively rapid fire at 100 yards from my noveske afghan There is something wrong with that bushmaster...I would check and see if barrel is tight 
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Posted: 8/26/2014 11:34:29 AM EDT
[#7]
What kind of sights is he using? I would put forth eye fatigue as a possible cause. That, or flinching.
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Posted: 8/26/2014 11:39:40 AM EDT
[#8]
Drail; are the results you posted the other guy's or yours? You should fire it yourself with known accurate ammo before trying to figure out what may be wrong with it. The shooter is a huge factor. When shooting for accuracy, you must maintain discipline throughout the session, not give in to more rapid fire and looser sighting. Otherwise, something is changing as the barrel heats up. The cold groups would be bad if the barrel were bad as far as chamber and bore are concerned. The Bushmaster probably has a 'government profile' barrel. If the bore were off-center, and the barrel warped a little from uneven heating, it seems like you would get 'stringing' as the shots followed the direction of the very slight bend, but that should stabilize after a uniform temperature is reached.
However, if you have a known good barrel, that certainly could tell the tale if it holds groups as it heats. If you swap it, you might consider squaring-off the nose of the upper if you have access to the mill for that purpose. I still think the problem is more installation than wear or defect. This would be even more likely if the rifle were bought used by the owner because there's no telling what may have been done to it in the past. |
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Posted: 8/26/2014 2:25:20 PM EDT
[#9]
Quote HistoryQuoted:
Drail; are the results you posted the other guy's or yours? You should fire it yourself with known accurate ammo before trying to figure out what may be wrong with it. The shooter is a huge factor. When shooting for accuracy, you must maintain discipline throughout the session, not give in to more rapid fire and looser sighting. Otherwise, something is changing as the barrel heats up. The cold groups would be bad if the barrel were bad as far as chamber and bore are concerned. The Bushmaster probably has a 'government profile' barrel. If the bore were off-center, and the barrel warped a little from uneven heating, it seems like you would get 'stringing' as the shots followed the direction of the very slight bend, but that should stabilize after a uniform temperature is reached. However, if you have a known good barrel, that certainly could tell the tale if it holds groups as it heats. If you swap it, you might consider squaring-off the nose of the upper if you have access to the mill for that purpose. I still think the problem is more installation than wear or defect. This would be even more likely if the rifle were bought used by the owner because there's no telling what may have been done to it in the past. Thanks for the note saxman. The barrel nut was slightly loose. Loos enough it was getting some gas leaking when shooting with a can. I tightened it and he shot again and got the same results. The shooting is him and his brother. I've got the rifle back and am going to test it myself when the weather cools a bit. He did buy the rifle used from an individual so no telling what kind of round count it had. It is a Govt Profile barrel. He's got good glass on the rifle, a Zeiss 4-12x42. I'll see what results I get and post again. Thanks! |
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Posted: 8/27/2014 3:25:17 AM EDT
[#10]
Scope mount loose? I know highly unlikely
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Posted: 8/27/2014 11:25:23 AM EDT
[#11]
The can is going to change the POI of the zero, plan and simple, and could be the difference of the spreed (some shot suppressed, and some shot with the can removed).
As for the barrel nut, Plus 35 ft lbs of torque, but do not exceed 80 ft lbs to get the top spine void/barrel nut gas tube channel on a float tube indexed with the upper receiver gas tube channel. Also, with the barrel nut loose now, would be a good idea to pull the barrel and clean both the upper receiver barrel socket, and the barrel extension so they do mate up correctly when you go to reinstall the barrel (don't forget to grease the mating areas between the nut and extension so you don't get false torque readings). If you find that you have to go past 80ftlbs to get the top spine void indexed correctly, don't, but instead back the barrel nut off and start your torque again. It may take a few tighting and loosing, but you will get the barrel nut correctly indexed without going over 80ft lbs of torque on the barrel nut (read going past 80 ftlbs, and you just bending/destroying the upper receiver barrel socket). As for the scope, should be good, but do go back and recheck the scope mount. Here to, you may need to loosen and tighten the bolts a few times to seat the mount correctly to the rifle and scope. |
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Posted: 8/28/2014 10:37:14 PM EDT
[#12]
Some of you guys must have some janky ass rifles if groups open up 10moa after 20 rounds.
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Posted: 8/29/2014 6:48:51 AM EDT
[#13]
I got out and shot the rifle myself today. Shoots like $#!^ !!!!!! This was shot with no can at 100 yards after removing the barrel and reinstalling and tightening the nut. The Zeiss Scope on the rifle has been checked by Zeiss. He first thought it was the scope and sent it in to Zeiss to be checked. It checked good. Mount is very solid.
I shot a box of 20 rounds of Winchester USA 55gr FMJ. Not the best ammo but I've had good results from it in the past. The scope was way off with only 1 of the first 3 shots on paper. I made two quick adjustments to get it close to center. After that, I fired the remaining 14 rounds with about 15 seconds between shots. The two adjusting groups were much bigger than what I normally shoot with my own rifles. As you can see, the 14 shot group got bigger and bigger as I shot. I'm going to suggest he get a new barrel and a free-float rail. Any other thoughts or suggestions? 
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Posted: 8/29/2014 11:16:52 AM EDT
[#14]
If you know that the barrel is correctly mounted, the bore of the barrel is correctly cleaned (sweets copper solvent does such in short time) and even the FS pulled to check the crown and clean that if fouled, then may be time for a new barrel if this is not a duty/service type rifle, or one that the owner is going to burn the new barrel to the ground shooting is rapidly like a 22lr to over heat the barrel (cause rapid plasma erosion).
55 gr white box through a 1-7 barrel (never said what twist the barrel was), groups look acceptable for the first few strings, and on the last long string, looks like the barre was heating up/ starting to flex more, and the last of the shots starting to spread out. Note, with the barrel cleaned up, I would try some M-193, M-855, and maybe even some Mk-262 ammo through it to see if the barrel is just not digging the white box ammo instead. Depending the barrel twist, the 55 and 62 grain should be somewhere in the 3MOA range, and if the barrel is 1/7, then should be tighter with the Mk-262. |
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Posted: 8/30/2014 6:13:35 PM EDT
[#15]
The upper needs to be disassembled and the front sight post or gas block removed along with the gas tube in order to torque the barrel nut properly. The threads should be cleaned then lubed with high temperature moly fortified grease. The nut must be tightened to 35" lbs. three times and then tightened past 35 inch lbs. until the gas tube aligns and can pass through the upper receiver without binding against the barrel nut.
Simply snugging it a little with the gas tube still in place will not work. If the rifle is relatively new I'd send it back to the manufacturer under warranty. Quality ammo is mandatory when trying to figure problems out. Ball ammo is insufficiently accurate to let you know if you've got it fixed, 52/53 grain match ammo, 69 or 77 grain Sierra Match Kings will let you really see how your rifle can perform. It will also spoil you into becoming a reloader. |
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