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Posted: 6/22/2012 6:19:37 AM EDT
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I've got one of the really long 13 inch Troy TRX Extreme rails on my AR-15 and an LDI DBAL laser mounted to the top rail about 8 inches from the receiver. I took off and reassembled my handguard a couple days ago, but when zeroing the rifle I could look through my ACOG scope and see the red laser through the scope jump an inch upward every shot. I'm not sure where to start to fix this problem. My ACOG was still totally zeroed from before the handgaurd movement, but now the groups were much smaller, so I apparently fixed the harmonics/accuracy problem, but now my laser wont sit still. I'm assuming this has to be the handguard moving around on me. I cranked the screws down pretty tight, any tighter and I envision the threads ripping right out. Its got me questioning if way forward on the hand guard is really a good place to trust one of these lasers. Anyone else have this kind of problem?
In my mind the only good option seems like spending a bunch of money on an MRP or mounting the laser directly to the receiver and using a mini red dot on it or behind it. I know the military guys mount their lasers pretty far forward, and some of those issue rails are pieces of crap, so I'm wondering if this is something most people just deal with and rezero lasers a lot or what. Luckily its not a lot of work to rezero since I've got the ACOG and visible laser to cheat off of , but still a PITA. Heres a couple pics of the rifle I took before I had the laser system: 
You can't really see it, but I built the rail a little bit like a ship in a bottle. I chopped a small section off the top of the front of the rail so I could put in a noveske switchblock for silencers and still get the 2 extra inches of sight radius. Those gas blocks are not designed to fit inside any rail and its a pain in the ass to put the gas block in there. I noticed a year after building the rifle that the handguard was bumping the screws on the gas block. In retrospect I suppose this was also from the rail creeping its way into the screws. This is what prompted the take it all apart and put it back together. I had to grind a couple little grooves in the rail to leave room for the screws and still maintain the free floating feature. |
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I put my laser just in front of my optic and I use a pressure pad on a VFG. I use a segment of bicycle inner tube material to secure the pressure pad to the VFG.
Further back on the rail is likely the most stable point on the rail to attach the laser. It might also help to keep the weight balanced too. I don't have any experience with that hand guard so I couldn't comment on its rigidity or stability with a laser mounted to it. |
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Because the handguard is free floating, it's naturally going to flex independently of the barrel, so the farther away from the receiver you mount something, the less stable (relative to the barrel) it will be. I don't think tightening the mounting will help; it's like standing on a diving board. Unless you make the board itself more rigid, it will still bend no matter how rigid the mounting point is.
The military issue rails are supported at both ends, so it will apply some pressure to the barrel when you rest it on something, but the handguard itself won't bend as much. |
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Quoted: I've got one of the really long 13 inch Troy TRX Extreme rails on my AR-15 and an LDI DBAL laser mounted to the top rail about 8 inches from the receiver. I took off and reassembled my handguard a couple days ago, but when zeroing the rifle I could look through my ACOG scope and see the red laser through the scope jump an inch upward every shot. I'm not sure where to start to fix this problem. My ACOG was still totally zeroed from before the handgaurd movement, but now the groups were much smaller, so I apparently fixed the harmonics/accuracy problem, but now my laser wont sit still. I'm assuming this has to be the handguard moving around on me. I cranked the screws down pretty tight, any tighter and I envision the threads ripping right out. Its got me questioning if way forward on the hand guard is really a good place to trust one of these lasers. Anyone else have this kind of problem? In my mind the only good option seems like spending a bunch of money on an MRP or mounting the laser directly to the receiver and using a mini red dot on it or behind it. I know the military guys mount their lasers pretty far forward, and some of those issue rails are pieces of crap, so I'm wondering if this is something most people just deal with and rezero lasers a lot or what. Luckily its not a lot of work to rezero since I've got the ACOG and visible laser to cheat off of , but still a PITA. Heres a couple pics of the rifle I took before I had the laser system: http://i120.photobucket.com/albums/o189/UTCenturion/DSC_0456.jpg http://i120.photobucket.com/albums/o189/UTCenturion/DSC_0449.jpg You can't really see it, but I built the rail a little bit like a ship in a bottle. I chopped a small section off the top of the front of the rail so I could put in a noveske switchblock for silencers and still get the 2 extra inches of sight radius. Those gas blocks are not designed to fit inside any rail and its a pain in the ass to put the gas block in there. I noticed a year after building the rifle that the handguard was bumping the screws on the gas block. In retrospect I suppose this was also from the rail creeping its way into the screws. This is what prompted the take it all apart and put it back together. I had to grind a couple little grooves in the rail to leave room for the screws and still maintain the free floating feature. Recoil? You see the laser jump an inch on the target, but it only has to move a fraction of that at the gun. Just a thought. |
| I've had a laser mounted at the furthest 9 o'clock position on my RRA w/ MI midlength freefloat handguard for a couple years now. After at least 1500 rounds the laser has not shifted one bit. POA should not change, you may want to also check the zero w/ your irons, as it appears your front sight is mounted on the tube. If the tube is moving, your irons will be off also. If your irons aren't off, take a good hard look at the laser. Make an index mark on the laser's elevation dial and a matching one on the housing and see if they move relative to one another after shooting a few times. Good luck. |
| when I tightened up the rail i actually had measured around the barrel with a caliper and added shims to make sure it was perfectly parallel to the barrel. I think that may be my problem. Either the barrel nut isn't spec or my receiver is a tiny bit out of spec and over 13 inches the problem is exageratted. Now the free float is trying to wiggle its way back into the its proper place on the barrel nut. I so don't want to buy another receiver. I've got a .22 ar with a real receiver. i may break both down and swap receivers to see if the problem goes away. |
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You mounted the laser on the tube, then removed the tube, and re-installed it. I wouldn't have expected anything BUT a loss of your laser's zero from this action. Those type of rails would be very hard to re-install to the exact same spot. It would take a very very very small difference in where the tube was re-mounted to throw that laser off by an inch.
Also, the Troy doesn't seem like the most robust FF tube out there, and likely locks up and indexes like the tubes of many "gaming" rifles...not a hard use rail like the DD RIS II or Larue which prevent any rotational movement. But even still, if you mounted a laser to a Larue or Daniel Defense RIS II, and then removed and re-installed the rail...you'd be dealing with the same issue. My advice: Make sure your rail is installed solidly, and don't remove it any more...or get a rail that is designed with these types of accessories in mind to prevent loss of zero once everything is mounted. |
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Quoted:
You mounted the laser on the tube, then removed the tube, and re-installed it. I wouldn't have expected anything BUT a loss of your laser's zero from this action. Those type of rails would be very hard to re-install to the exact same spot. It would take a very very very small difference in where the tube was re-mounted to throw that laser off by an inch. Also, the Troy doesn't seem like the most robust FF tube out there, and likely locks up and indexes like the tubes of many "gaming" rifles...not a hard use rail like the DD RIS II or Larue which prevent any rotational movement. But even still, if you mounted a laser to a Larue or Daniel Defense RIS II, and then removed and re-installed the rail...you'd be dealing with the same issue. My advice: Make sure your rail is installed solidly, and don't remove it any more...or get a rail that is designed with these types of accessories in mind to prevent loss of zero once everything is mounted. You totally missed the point dude. I rezeroed the laser after putting it all back together and the laser moved an inch every shot. BOOM 1 inch high. BOOM 2 inches high. Boom.. 3 inches high. etc 7 times. Before I when I had it installed it didn't lose zero with the laser, but that time It was obvious the rail was off of center. I think I've got one of the three important parts out of spec. And after forcing them to be aligned correctly, it is bouncing back to where it wants to be: out of alignement. . |
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I gotcha...Sorry for my misunderstanding.
Hmm...Yeah, I'm guessing it's probably the rail flexing at the receiver junction, but as was stated by someone else...it would be a good idea to ensure that the laser and front BUIS stay zeroed together since they're both on the rail. That way you would at least know that there isn't anything funky going on with the laser itself. I like the idea of these light minimalist rails, I've just never really trusted their method of mounting...which is why I like the MRP platform...but I would also be comfortable using Larue or Daniel Defense rails...Though I understand a lot of folks don't want full quad-rails. I could be wrong, but with the military issued RAS non-FF rail, if pressure is applied to the barrel or hand-guard, then they both flex/shift/move as one...along with any sighting system attached to them. This means that the laser dot moves, but the bullet's POI does so also...in the same direction. With a FF rail, these two things can move interdependently causing the laser dot to move, but not the bullet's POI. |
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Quoted:
I gotcha...Sorry for my misunderstanding. Hmm...Yeah, I'm guessing it's probably the rail flexing at the receiver junction, but as was stated by someone else...it would be a good idea to ensure that the laser and front BUIS stay zeroed together since they're both on the rail. That way you would at least know that there isn't anything funky going on with the laser itself. I like the idea of these light minimalist rails, I've just never really trusted their method of mounting...which is why I like the MRP platform...but I would also be comfortable using Larue or Daniel Defense rails...Though I understand a lot of folks don't want full quad-rails. I could be wrong, but with the military issued RAS non-FF rail, if pressure is applied to the barrel or hand-guard, then they both flex/shift/move as one...along with any sighting system attached to them. This means that the laser dot moves, but the bullet's POI does so also...in the same direction. With a FF rail, these two things can move interdependently causing the laser dot to move, but not the bullet's POI. Yeah I've thought about this. Considering my main function is the laser sighting system and not 600 yard accuracy, I've definitely considered milling out a small sleeve to fix the barrel and rail together. I also shoot with a big heavy suppressor on there as well which could exacerbate the misalignment. |
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