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Posted: 12/7/2015 8:32:48 PM EDT
| Ok got a odd thing going on with my ar. Recently did a lot of upgrades to it putting a carbon fiber free float fore grip, adjustable gas block, precision armament M4-72 muzzle brake that's awesome, trinity force redcon 3x9 scope, and hiperfire 24c trigger that's really awesome! But here's the problem. I've got it sighted in on a bipod and have great grouping. Can pretty much put each shot through the same hole at 100 yards on the bipod. But when I pick it up to shoot off hand the groups move about 4 inches lower than group I had while shooting on the bipod. It's really weird and really frustrating! I know I'm not jerking the gun anticipating recoil since there's practically none with that muzzle brake and the trigger being so smooth. Anyone have any input on why the shot placement would change when I'm not on the bipod? Thanks guys for any ideas why this is happening! |
| Barrel is torqued on there pretty good. Don't know exactly what it's torqued to but when I installed the fore grip had to put the barrel nut on there pretty tight to line up the gas tube. And using the bipod puttin upward pressure on there is the only thing I could think of myself but being a free float tube didnt think it would effect it. But I guess it's possible. At least I have offset sights that don't seem to be effected like the scope is |
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If you are sure that the tube is a free float tube and not putting any pressure on the barrel, then double check the room between the gas block to inside of the float tube.
Hence on the bipod with the upward pressure on the end of the tube, the gas block may be making contact to the inside bottom of the float tube to change the POI. As for barrel nut, should be installed at more than 35ftlbs, but no greater than 80flbs instead. Also, make sure that the float tube has been installed tight enough to the barrel nut that the float tube is not shifting/bending and causing a contact of the inside of it to the gas block (or any other items inside the tube). Lastly, with the bolt off the carrier, use the carrier with key dry fit into the upper receiver to make sure that the gas tube is correctly aligned with the carrier key. Note, if you have the barrel nut gas block tube channel indexed correctly with the upper receiver gas tube channel, then the gas tube should be loose through both channels (and not binding from the two channels being off alignment instead). |
| Float tube isn't close to gas block or touching anything and gas tube doesn't contact bolt. As far as tourqe on the the barrel nut im sure its on the high side close or maybe over 80. Had to do it that tight just to get the gas tube in there. Had it in the looser hole but could turn it by hand back and forth against the gas tube. So pretty confident that everything is installed correctly. So it any of that would effect point of aim on a bipod and shooting offhand it's all good. Just frustrating how it changes |
| I had an issue with indexing a barrel nut while staying within proper torque spec and what solved my problem was using a lapping tool on the receiver. The lapping tool took off only as much material as needed to true the front of the receiver which also created enough tolerance to index the barrel nut and torque within spec. I'm not sure what the source of your problem is, but if you want to obtain proper torque and eliminate this as potentially the problem then lapping your receiver might be something to consider. Since I bought that lapping tool I stripped down all of my ARs, lapped them all and probably will lap all future builds too. For that reason, to me, it's worth having in my tool kit. |
| Never thought it might be a parallax issue. Just slipped my mind. But this poi shift happens no matter what position I use when I change from bipod to offhand. It's a consistent poi change. So now I'm starting to think that I'm just going to have to make notes and figure out what changes I need to adjust my scope and remember those when I change for bipod to offhand and dial that in when I change position. Sound like a reasonable solution? |
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Quoted:
Never thought it might be a parallax issue. Just slipped my mind. But this poi shift happens no matter what position I use when I change from bipod to offhand. It's a consistent poi change. So now I'm starting to think that I'm just going to have to make notes and figure out what changes I need to adjust my scope and remember those when I change for bipod to offhand and dial that in when I change position. Sound like a reasonable solution? Not really, since it doesn't matter if I'm prone and sling up so tight that my arm is falling asleep on a DCM build, or shooting rapids without sling pressure isntead, the rifle should not change POI between the two with a hand guard float system at the same yardages shot. Hence the reason for the float hand gaurd is so the barrel is always free floating instead. As for a parallax issue, maybe, but not 4 MOA isntead. So it all coming back to either the barrel not being tight to the upper receiver (hell may have cracked the upper receiver barrel socket to the receiver from over tightening it), or the maybe even the brake causing the shift from bench to free hand instead. Since pulling the brake to shoot the rifle without it for testing is easy, I would start there to begin with. If the brake is not the problem, then it time to pull the barrel to check it's install next. |
| Thanks Dano for all the input. I'll pull the brake off and see if it changes anything and if it doesn't I'll pull the barrel nut off and check to see if the reciever got cracked from over tightening. I'm pretty sure the barrel isn't lose. Not sure what's the best way to check if it's lose but I can't feel any movement when I try to move the barrel up and down or side to side. |
| Yeah the scope is mounted to the receiver not touching the handguard and its tight in its mount. I went ahead a tore down the handguard and took the barrel nut off to check to see if I cracked the reciever while tightening the barrel nut and it's not cracked. But the barrel was a little loose in the reciever without the barrel nut holding it in so what I did was put some blue locktight around the barrel extention and reinstalled the barrel nut and handguard. I don't have a torque wrench but pretty sure it's under 80 pounds tight. I ordered a torque wrench so I'll be able to make sure in the future. Hopefully the locktight on the barrel extention will tighten it up. I read in a few places about people doing that if the extention had a little play in it so figured it was worth a shot. Other than that everything looks good. If it keeps up changing its poi when changing the type of shooting I do I'm just gonna build another upper! |
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