wobblinwheel Posted August 21, 2011 Report Share Posted August 21, 2011 Not too long ago, I installed a UTG bipod on my Colt/Umarex m4 carbine. At first, I put the rail-mounted bipod on the bottom rail of my Umarex free-floating quad-rail foregrip. I immediately noticed I had to re-zero my scope at 40 yds. (the distance of my "range" in my back yard.) I said something about this to my brother, who has been an avid target shooter for MUCH longer than me, and he told me that using a bipod, as opposed to soft bags, will ALWAYS change the point of impact. Well, I told him he was fullashit. It just didn't make sense to me that WHAT you were resting you rifle on would actually CHANGE where the bullet hits! Well guess what? I recently removed the front sight, and installed a gas-block quad-rail in it's place (the front sight was visible through the scope). This is when I decided to re-mount the bipod to the bottom rail of the gas-block. BAM-BAM-BAM, shooting 2 inches high! My brother made me look like an idiot again! Have many of you shooters out there experienced this also? It seems the closer to the barrel the bipod is mounted, the HIGHER it shoots. Is this a common result, or does this vary from gun-to-gun? I also found out the rigidity of the SURFACE that the bipod rests on also makes a difference on POA. This is mind-boggling! Quote Link to comment Share on other sites More sharing options...
imschur Posted August 21, 2011 Report Share Posted August 21, 2011 This is one of the reasons centerfire and target rifles free float barrels. Nothing to influence the barrel. A sling mounted to a small diameter barrel can easilt change point of impact.This is why free float hanguards are so popular with AR-15 shooters. The bipod would mount to the handguard with zero influence on the barrel Quote Link to comment Share on other sites More sharing options...
techmike Posted August 22, 2011 Report Share Posted August 22, 2011 Let me see if I have this correct, Colt/Umarex M4, with the OEM quad rail system attached to the upper receiver, and the barrel floats. Bipod attached to the quad rail. Everything works fine at this point. You removed the front sight, installed a gas block quad rail to cover up the holes in the barrel shroud. You then mounted the bipod to the new gas block quad rail, (the barrel is not free floating anymore), and point of impact went up 2 inches. Mounting the bipod directly to the barrel, (shroud in this case), is causing the barrel to flex.Put the bipod back on the M4's rail, and that should put your POI back close to where it was. Probably won't be exactly the same because the sight was changed out for the gas block.Edited to add:If you are interested in the technical stuff, search "barrel harmonics".TM Quote Link to comment Share on other sites More sharing options...
wobblinwheel Posted August 22, 2011 Author Report Share Posted August 22, 2011 Actually, attaching the bipod to the handguard affected POI LESS than on the gas block, but did raise POI about an inch. Brother says rigid bipod on rigid surface can cause barrel to "bounce", throwing off accuracy. I wonder if the barrel-within-a-barrel setup on the Colt is more, or less, succeptable to this? I wonder if extra tension on the flash suppressor would change this affect? Quote Link to comment Share on other sites More sharing options...
wobblinwheel Posted August 25, 2011 Author Report Share Posted August 25, 2011 I seem to have stabilized the accuracy a little by moving the gas block a little further away from the foregrip. I believe they may have actually been touching. I slid the block forward enough to slide a credit card between the two. Should replacing the sight with the lightweight aluminum gas block have changed the "floating" characteristics of the barrel? Quote Link to comment Share on other sites More sharing options...
asmurff Posted August 25, 2011 Report Share Posted August 25, 2011 I'm enjoying reading this thread :thumb: Quote Link to comment Share on other sites More sharing options...
techmike Posted August 25, 2011 Report Share Posted August 25, 2011 I'm enjoying reading this thread :thumb:Really??? :o Quote Link to comment Share on other sites More sharing options...
asmurff Posted August 26, 2011 Report Share Posted August 26, 2011 Really??? :oHey I'm learning something. Quote Link to comment Share on other sites More sharing options...
wobblinwheel Posted August 30, 2011 Author Report Share Posted August 30, 2011 Since some of us are actually interested in learning something, my "genius" brother hooked me up with another idea that improved my POI. I have the Colt/Umarex "Compensator" on my rifle, which I like the looks of, and it reduces the report considerably (with a little "help"). This compensator partially slides OVER the barrel, and has a threaded "bushing"? that fits rather loosely over the barrel at the back-end of the can. When I described this loose "rattly" fit at the end of the compensator to brother-man, he said: "YOU GOTTA GET RID OF THAT SLOP! PUT A SET SCREW IN THERE TO STABILIZE THAT THING!!" So I did. Drilled and tapped a hole in the threaded end-cap/bushing and installed a 8-32 allen set screw, and snugged that sucker up. Guess what, noticeably tighter 20-round groups, with NO fliers (so far). Apparently, something rattling around on your barrel screws things up also. Yep, I'm still learnin'! Those of you who have the Umarex "Compensator" might want to consider this simple upgrade. Just tap on the rear-end of it when it's installed, and you will hear it rattle against the barrel. Quote Link to comment Share on other sites More sharing options...
techmike Posted August 30, 2011 Report Share Posted August 30, 2011 If you are interested in the technical stuff, search "barrel harmonics".TMMaybe you didn't read that part. In the spirit of learning something, Especially from 7:35 to 7:50 - an excellent slo-mo of a barrel whipping during firing. If you hang something on your barrel, it will change POI. If you have something on your barrel and it is loose, you will never shoot accurate groups. (Except by accident) This I know, my Garand had a slightly loose gas block, and shot 8 ring groups @ 100 yds. Now on to another subject on the Tac22 learning channel. ;D I have the Colt/Umarex "Compensator" on my rifle, which I like the looks of, and it reduces the report considerably (with a little "help"). I am thinking that ATF would frown at your homemade suppressor. They are also illegal in some states. Last I heard, it was up to 10 years/$250,000 - and that is just the federal rap. It probably isn't wise to post that info on the net - it never goes away. C:-)BTW, I also have an (unmodified) comp from Umarex on my M4. It is not loose, and doesn't rattle on the barrel. There is probably an other issue if yours rattles - could be missing the o-ring - if you overheated it to get it apart, the o-ring was probably vaporized. TM :thumb: Quote Link to comment Share on other sites More sharing options...
wobblinwheel Posted August 30, 2011 Author Report Share Posted August 30, 2011 Apparently the hole in the end-cap on mine must be slightly oversized on mine, as there is play where it fits over the barrel. I replaced the oring, and can feel it engage the barrel shroud when screwing it on. Not worried much about "modifications", as it sure ain't no suppressor. Using subsonic ammo has made all the difference in the world as far as noise goes, and the set screw has seemed to provide a tighter group at 38 yds. after further testing since last post. Quote Link to comment Share on other sites More sharing options...
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