View Full Version : 10.5 SBR upper: accuracy
PalmettoPrecision
10-05-14, 21:14
I'd like to see some opinions on this, and also get you guys to check up and make sure I'm not overlooking anything that may be contributing. I can't seem to get this upper to produce what I consider acceptable groups. Barrel is from a very reputable manufacturer.
10.5 in. 1:7 twist 5.56
VLTOR MUR receiver
LMT enhanced carrier
Wilson bolt
SLR sentry adjustable GB
Wilson TRIM hand guard
I used a vortex PST 4-16 w/ Bobro mount for testing.
2887928880
This is from 100 yds. Various factory match loads in 69,75,77 grain. All produced similar results. I have checked the barrel nut torque twice and it is perfect. Rifle functions well. I shoot from a solid bench with bags. Two shooters today produced the same results, and the three previous range trips have all produced the same results. 180 rounds through the rifle.
Am I missing anything? Open to all questions and suggestions. Thanks!
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PalmettoPrecision
10-05-14, 21:42
Wilson TTU 2-stage trigger.
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I saw this happen lately with another barrel. I eliminated every variable and then noticed leakage at the gas block (also adjustable). Replaced it with a standard block and it started shooting around 1 MOA. YMMV
PalmettoPrecision
10-06-14, 00:05
I saw this happen lately with another barrel. I eliminated every variable and then noticed leakage at the gas block (also adjustable). Replaced it with a standard block and it started shooting around 1 MOA. YMMV
That's interesting. Why would the leakage affect accuracy?
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That's interesting. Why would the leakage affect accuracy?
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Accuracy is mostly a consistency issue. The bullet needs to leave the barrel at as close to the same speed as possible everytime.
If the gas block is leaking it will effect the pressure in the pressure vessel. If it leaks the same amount of pressure every time or a very minute amount then it's not likely to be an issue but if the bleed off varies from shot to shot then the projectile will not be leaving the muzzle at the same speed each time. Different velocities change drop, spin drift, wind drift....ext which in turn changes p.o.i.
I saw this happen lately with another barrel. I eliminated every variable and then noticed leakage at the gas block (also adjustable). Replaced it with a standard block and it started shooting around 1 MOA. YMMV
That's interesting...
What muzzle device?
I have a Northtech Defense 10.5 on my Sigbrace equipped AR and it will put down under MOA groups rather easily. The length shouldn't be an issue accuracy wise.
Rick
I doubt it is the length as that really has no bearing here.
On the assumption it is not the barrel then you must eliminate all other potential accuracy robbing issues:
Is the gas tube binding in the carrier key?
Muzzle device over torqued?
Tried a different optic?
Good luck.
PalmettoPrecision
10-06-14, 14:03
Gas tube and carrier key mate perfectly. I removed the muzzle device for testing and there was no appreciable difference. I have used two different scopes, with the same results.
Iraqgunz, what exactly did you attribute the poor accuracy to in your situation? My gas block is leaking, and will try a new one soon. I wonder how a leaking gas block could turn a 1MOA upper in to a 4MOA upper...
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PalmettoPrecision
10-06-14, 15:51
He explained how gas leaking at the same amount of pressure every time wouldn't be an issue. I should have rephrased, how could it leak irregularly? The block is tight, doesn't move at all. I would think it would need to shift or its placement change somehow to cause irregular pressure changes.
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I highly doubt that gas would be escaping regularly and I would venture to guess it isn't. The key here, is when you say that you have checked everything and can not find the obvious, you need to look at what's not obvious.
Failing that, contact the barrel manufacturer and tell them about the issue. Maybe you have a bad barrel. It happens to the best of companies.
He explained how gas leaking at the same amount of pressure every time wouldn't be an issue. I should have rephrased, how could it leak irregularly? The block is tight, doesn't move at all. I would think it would need to shift or its placement change somehow to cause irregular pressure changes.
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Renegade04
10-07-14, 07:58
I have a question. Why are you concerned about getting MOA accuracy from a 10.5" SBR at 100 yards? These are CQB type platforms. Their primary purpose is to engage at short range targets. At 100 yards, getting MOH (Minute of Head) is what I am concerned with on my 10.5" LMT SBR. I could probably get about 2MOA if I really tried, but that is not the purpose. If I want 1-2 MOA or less on a 100 yard target, consistently, I will use one of my SPRs or maybe even one of my M4geries. Another question is, have you bothered contacting the barrel manufacturer yet? If at all possible, I would recommend shooting it at 25 yards and 50 yards to see what kind of accuracy you get from these distances. Also, you may want to shoot it from a good solid vise as well.
I have a question. Why are you concerned about getting MOA accuracy from a 10.5" SBR at 100 yards? These are CQB type platforms. Their primary purpose is to engage at short range targets. At 100 yards, getting MOH (Minute of Head) is what I am concerned with on my 10.5" LMT SBR. I could probably get about 2MOA if I really tried, but that is not the purpose. If I want 1-2 MOA or less on a 100 yard target, consistently, I will use one of my SPRs or maybe even one of my M4geries. Another question is, have you bothered contacting the barrel manufacturer yet? If at all possible, I would recommend shooting it at 25 yards and 50 yards to see what kind of accuracy you get from these distances. Also, you may want to shoot it from a good solid vise as well.
He wants MOA accuracy out of his SBR, you don't require it for yours. It's just a matter of personal preference really.
He explained how gas leaking at the same amount of pressure every time wouldn't be an issue. I should have rephrased, how could it leak irregularly? The block is tight, doesn't move at all. I would think it would need to shift or its placement change somehow to cause irregular pressure changes.
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One would have to assume that the leakage is aberrant. Loose fittings of any kind seldom settle in the same place consistently.
go outside and loosen your water hose until it begins to leak at the spigot. then repeatedly kink the hose and abruptly release, the water will stream out at differing angles and volumes. this is caused by the fittings shifting with the impulse of water pressure.
Velocity can be effected by as little as .01 grains of powder, slightly more or less neck tension, longer duration of neck tension, non uniform flash hoses, hell loose chambered or long throated barrels typically have less accuracy potential because of varying amount of gas that escapes AROUND the case before the brass expands to fill the chamber.
We cannot say for sure what the issue is with your firearm without inspection, but i can say that you will have to address every possible issue one at a time. personally i would replace the gas block first, re-crown the barrel second. assuming of coarse that your barrel torque is within low spec, scope and rings are secure, .......etc.
Good luck, just work the problem. it can be fun.
PalmettoPrecision
10-07-14, 11:38
One would have to assume that the leakage is aberrant. Loose fittings of any kind seldom settle in the same place consistently.
go outside and loosen your water hose until it begins to leak at the spigot. then repeatedly kink the hose and abruptly release, the water will stream out at differing angles and volumes. this is caused by the fittings shifting with the impulse of water pressure.
Velocity can be effected by as little as .01 grains of powder, slightly more or less neck tension, longer duration of neck tension, non uniform flash hoses, hell loose chambered or long throated barrels typically have less accuracy potential because of varying amount of gas that escapes AROUND the case before the brass expands to fill the chamber.
We cannot say for sure what the issue is with your firearm without inspection, but i can say that you will have to address every possible issue one at a time. personally i would replace the gas block first, re-crown the barrel second. assuming of coarse that your barrel torque is within low spec, scope and rings are secure, .......etc.
Good luck, just work the problem. it can be fun.
Greatly appreciate the insight. I replaced the gas block last night and will test more this weekend.
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