ocd
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Post by ocd on Apr 13, 2015 5:19:50 GMT -8
#1. Will a loose upper/lower affect accuracy enough to cause a 3" group at 100yds? #2. Will a loose gas key on the BCG affect accuracy?
22" bull barrel. It was shooting less that 1/4" groups consistently for the first 20 rds. Now they are all over the place. I noticed the receivers were a little sloppy and the gas key came loose. This is a brand new build. First 20 rds out of the barrel were spot on. Next 40 were all over. All I did after the first 20 was clean the barrel-Wet patch Hoppes9, wet nylon brush 5 strokes, patch dry. That's it! Barrel did not come loose as far as I can tell. The only thing I noticed was the receiver fit was sloppy. And the gas key of course.
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Deleted
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Post by Deleted on Apr 13, 2015 6:00:53 GMT -8
#1. Will a loose upper/lower affect accuracy enough to cause a 3" group at 100yds? #2. Will a loose gas key on the BCG affect accuracy?
22" bull barrel. It was shooting less that 1/4" groups consistently for the first 20 rds. Now they are all over the place. I noticed the receivers were a little sloppy and the gas key came loose. This is a brand new build. First 20 rds out of the barrel were spot on. Next 40 were all over. All I did after the first 20 was clean the barrel-Wet patch Hoppes9, wet nylon brush 5 strokes, patch dry. That's it! Barrel did not come loose as far as I can tell. The only thing I noticed was the receiver fit was sloppy. And the gas key of course. The gas key is most likely your problem. I would tighten it and reshoot for groups. Your upper to lower fit: Some say it makes no difference. Some say it does. I'm on the side that say a tight fit is better. The slop can't help accuracy.
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ocd
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Post by ocd on Apr 13, 2015 8:18:02 GMT -8
Thanks for the reply. I am going to try it after getting the gas key redone before I start getting all kinds of accurizing voodoo parts. Can you explain how it affects it? I'm curious
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Post by GLSHOOTER on Apr 13, 2015 9:58:43 GMT -8
Thanks for the reply. I am going to try it after getting the gas key redone before I start getting all kinds of accurizing voodoo parts. Can you explain how it affects it? I'm curious It's kind of mish-mash effect in engineering interaction. One causes the other problem and that causes another. Separating the issues is like trying to un-mix a Bloody Mary. I'll give it a shot and, yes, that is a pun. Starting with the loose key issue you will experience as a given gas leakage. This will impede reliable functioning and once it starts getting loose it tends to continue to increase. It doesn’t just “wear in” like mating parts via direct usage or lapping them together. As it changes the gas leakage changes resulting in inconsistent speed of rearward travel and inconsistent bolt seating along with the cartridge. Imagine having a slightly oversized case and letting the bolt resize the case with the chamber. Variation from one to the next with more or less seating thrust while miniscule is still coming into play. The gas tube in a well set up AR platform will slide up inside the key without interference. AS the key loosens it will tend to pivot up and down or left and right on the two bolts. If it hits one side or the other then torque is applied to the tube transmitted to the barrel as stress. Stress variation causes an alteration in harmonics. Changes mean down range it just won’t put them all in the same place. It will as a side effect also wear out the little button area on the end of the gas tube and this will cause lessened seal of the gas going back in the key. Once again this can induce changes on bolt carrier acceleration speed and degree. . Do you see a trend here? The parts of a DI system are much more interrelated than many realize. An interference fit here can make a big difference over there. Not only do we deal with the reciprocation bolt but we must address the uppers. As an illustration of this we talk of “squaring” or lapping” an AR receiver. This basically allows for the barrel extension, hoping it’s straight, to mount up on the upper with the center of the bore being centered with the long axis of the upper. Not up, left, right, down or drifting up or down angularly. In other words we want it straight down range. Given everything is square and mounting our glass or irons we should need less adjustment or deviation from the center of the adjustment available. If we fail or if something else is causing a mis-match then it is going to require a lathe, files or shims to get everything looking in the same direction. If we are “out of square: not only will the groups be off but a potential exists of the bolt, almost certain BTW, not locking up squarely and in a worst scene scenario lug breakage due to unequal stress may occur. This last is more common than many realize. So in summation: Loose key: inconsistent gassing and bolt movement and impingement on the gas tube causing degradation in accuracy due to interference between the key and the gas tube. This subject is covered in a series of books I have by Zediker and I highly recommend everyone buy his missives on building, shooting and reloading for the AR. To me they are while not always the be all to end all a great jumping off point just like the PO Ackley, Elmer Keith and Townsend Whelen's works provide a perspective of the interactions in all aspects of the platform. Greg
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ocd
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Post by ocd on Apr 13, 2015 10:13:17 GMT -8
That is by far, the most precise, in-depth, detailed, to the point, and on topic reply I have ever seen on a forum! Thanks! I wouldn't have been able to come close to guessing that!
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Post by GLSHOOTER on Apr 13, 2015 10:29:31 GMT -8
Thanks. My Ghost Writer, Casper, works hard for me. Critical thinking, as in examination not negative, is always fun.
The good thing is the board doesn't encourage scum sucking pig arugemnts and EVERYONE can contribute their ides/experiences
Greg
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ocd
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Post by ocd on Apr 13, 2015 15:15:19 GMT -8
Lol thank god!
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ocd
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Post by ocd on Apr 16, 2015 4:26:02 GMT -8
Well the gas key wasn't the problem. Still shooting wild groups. Weird. I have rounds flying all over the target.
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Deleted
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Post by Deleted on Apr 16, 2015 5:06:13 GMT -8
Something has come loose. Check the upper over one part at a time. Also check your scope and mount setup. If it shot .250 groups and just quit, something is causing it.
Is it the same ammo? How tight did you torque your barrel nut when you built it? Are your rings on your scope tight?
Or has the scope gone South? ...Greg
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ocd
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Post by ocd on Apr 16, 2015 6:38:09 GMT -8
Something has come loose. Check the upper over one part at a time. Also check your scope and mount setup. If it shot .250 groups and just quit, something is causing it. Is it the same ammo? How tight did you torque your barrel nut when you built it? Are your rings on your scope tight? Or has the scope gone South? ...Greg Yes same ammo. Black hills 175gn match. I took the handguard off last night. Barrel seems tight. Gas block was tight. I torqued the barrel nut (greased) to 32ftlbs then loosened then worked my way up to about 38 getting the gas tube hole to line up. Scope rings are Burris tac something 6 screw rings, tight, torqued, and locktited. They were lapped and fitted to the scope before I put it on the 308. I put 332 rounds through my .338 Lapua with that scope with no problems. I'm going to check the gap around the barrel extension and the receiver with some feeler gauges. I do plan to try my upper and bolt on my buddy's lower, then back to my lower with his BCG. I am going to try one of my other optics on it first and foremost. The scope I'm using is an SWFA SS fixed 10x. Says its rated up to 50BMG. Although I somehow end up breaking every scope I've owned. Not by abuse but just from shooting.
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ocd
Junior Member
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Post by ocd on Apr 16, 2015 11:55:31 GMT -8
Got a .002" feeler gauge in around the left side of the barrel extension between it and the receiver. Went in snug
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ocd
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2 issues
Apr 18, 2015 12:55:58 GMT -8
via mobile
Post by ocd on Apr 18, 2015 12:55:58 GMT -8
Problem solved! But not remedied. I cut and stuffed 2- .001" shims between the inside of the receiver and barrel extension on the left side and .0015" worth toward the bottom. Almost half the diameter of the extension wasn't in contact with the receiver. Shot a 1/4" 3-shot group at 100 with 4 other rounds that wandered about a half inch.
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Post by GLSHOOTER on Apr 18, 2015 13:44:01 GMT -8
Problem solved! But not remedied. I cut and stuffed 2- .001" shims between the inside of the receiver and barrel extension on the left side and .0015" worth toward the bottom. Almost half the diameter of the extension wasn't in contact with the receiver. Shot a 1/4" 3-shot group at 100 with 4 other rounds that wandered about a half inch. Did you square the receiver? Greg
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ocd
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Post by ocd on Apr 18, 2015 14:10:46 GMT -8
No. I searched high and low for a receiver lapping tool for a 308. Nobody makes one. I contacted PTG a while ago and they offered to make me one but I didn't want to wait months for it. I think what happened was I had put the barrel in and out of the receiver a few times to measure for a handguard. It did get looser and looser. I'm going to ditch that dpms and get a 7075 billet upper. Might even epoxy it in there
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ocd
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Post by ocd on Apr 18, 2015 14:17:07 GMT -8
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