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Posted: 1/14/2010 7:31:47 AM EDT
| This question may have been asked a thousand times but I didn't find it in search. I just bought a BCM 20" upper. My goal is to achieve 1 moa accuracy. I know ammo is a good part of it. However, I'm considering getting free float rails. How much can I expect to gain in accuracy using free float rails |
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This question may have been asked a thousand times but I didn't find it in search. I just bought a BCM 20" upper. My goal is to achieve 1 moa accuracy. I know ammo is a good part of it. However, I'm considering getting free float rails. How much can I expect to gain in accuracy using free float rails if your goal is to achieve the most accuracy from that upper; you'll have to freefloat the barrel. I can't remember the article, but an author tested freefloated vs. non freefloated using the same barrel and same ammunition. The results were quantifiable and what I (and the author) considered significant. Heavy sling tension, IIRC, resulted in an inch+ greater group dispersion in the non-freefloated barrel. (at 100yds) |
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Basically think of the barrel as a guide for the bullet you fire. If you attach something to it, especially something that applies a lateral force to it (like your hand, tension on a sling, etc), you are going to deflect the barrel. This deflection of the barrel since it is not consistent and varies will affect where that barrel will guide your bullets to. Of course, this translates to accuracy.
My understanding is that even things that you do not apply force to that is attach to the barrel i.e. gas block may also affect accuracy, this is because it affects the harmonics of the barrel, but this is something that I don’t have the background to explain. |
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Kind of off topic: Is the KAC M5 a free float tube? no Thanks. Thinking about an A4/SAM-R type upper. If I went with a BCM factory 20" A4 upper and say a larue FF rifle tube and kept the factory front sight tower, that wouldnt be considered to be "free floated" either correct? |
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barrels flex when fired, causing the hand guard to move because its attached to the barrel
free floats are the barrel nut and hand guard as complete unit that does not touch the barrel or they can be attached to the barrel nut only, allowing the barrel to flex and not move the hand guard,when fired. the more solid a free float,the better IMO. some question if there is much to gain,because of the direct impingement system on AR platform. never shoot with your barrel rested on a sand bag or anything else either. |
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I know this is a newb question, but why does the free float system increase accuracy? How does the clam shell hand guards cause inaccuracy? Because they result in external force on the barrel. Resting the standard forearm on a rest/sandbag still results in some pressure being applied to the underside of the barrel. Add a sling to the mix and the external force is significantly more. As another poster mentioned, anything touching the barrel also effects barrel harmonics. This topic is above my pay-grade, but again it does effect a rifle's accuracy. Free floating the barrel simply removes variables from the equation. You can sling up/mount the forearm in a rest without exerting pressure on the barrel. Since we are talking about a chromelined std. barrel the OP isn't going to magically shoot .25" groups, but he will give the barrel the best possible chance of being as accurate as it can be. I really wish I could find the article I read. It was in a gunrag and I'm sure I've still got it. The measured differences were significant. It surprised the author and it surprised me. |
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Kind of off topic: Is the KAC M5 a free float tube? no Thanks. Thinking about an A4/SAM-R type upper. If I went with a BCM factory 20" A4 upper and say a larue FF rifle tube and kept the factory front sight tower, that wouldnt be considered to be "free floated" either correct? Actually it would. |
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Kind of off topic: Is the KAC M5 a free float tube? no Thanks. Thinking about an A4/SAM-R type upper. If I went with a BCM factory 20" A4 upper and say a larue FF rifle tube and kept the factory front sight tower, that wouldnt be considered to be "free floated" either correct? Actually it would. No shit? Huh. I would have thought that the front sight tower clamped to the barrel would have some negative effect on accuracy and therefore not "free-floated". Little did i know 
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Kind of off topic: Is the KAC M5 a free float tube? no Thanks. Thinking about an A4/SAM-R type upper. If I went with a BCM factory 20" A4 upper and say a larue FF rifle tube and kept the factory front sight tower, that wouldnt be considered to be "free floated" either correct? Actually it would. No shit? Huh. I would have thought that the front sight tower clamped to the barrel would have some negative effect on accuracy and therefore not "free-floated". Little did i know
Actually its the consistancy that matters. The front sight post effects the barrel, yes. but it effect the barrel exactly the same way every time you pull the trigger. Your handguards on the other hand effect the barrel differently depending on how you are holding them or what they are resting on. So if one time you get a solid grip or use a sling it will effects the harmonics of the barrel differently than if you hold it lighttly. With a free float barrel, no matter how you hold the handguard this force is not tranmitted to the barrel. Thus no change in barrel harmonics from shot to shot. With a free float barrel you gain consistancy and thus accuracy. Yankee Dogg |
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Basically think of the barrel as a guide for the bullet you fire. If you attach something to it, especially something that applies a lateral force to it (like your hand, tension on a sling, etc), you are going to deflect the barrel. This deflection of the barrel since it is not consistent and varies will affect where that barrel will guide your bullets to. Of course, this translates to accuracy. My understanding is that even things that you do not apply force to that is attach to the barrel i.e. gas block may also affect accuracy, this is because it affects the harmonics of the barrel, but this is something that I don’t have the background to explain. Keep reading, and stop passing incorrect information. Has nothing to do with the force applied to the barrel. It has to do with the harmonics of the barrel...freefloating the barrel allows for consistent oscillation. |
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The forces applied to the hand guard or forces applied to the barrel changes the POI and can have a detrimental effect on accuracy due to harmonics. It is a force applied by a sling or a hand or bag. You do not change harmonics of the barrel. The harmonics stay the same unless you increase the diameter or use a tuning device changing the nodes on the barrel. When you search for accuracy, you are looking for the nodes of vibration that match your load. In a sense you are finding a load that is consistent with the harmonics of the barrel. Harmonics are way over board for this discussion and Jlows description is spot on for floating a barrel. You remove the known forces being applied to the barrel so that you now have consistant harmonics just as ckay says. Free floating bolt action rifles has always improved accuracy. It was an improvement in consistency that resulted in a rifle that now showed more accuracy on paper. What I'm saying is a tube may or may not decrease the group sizes. Accuracy is about consistency in shooter, rifle and ammunition. Consistent wind helps also. And hold hour lips the same way each time also.  |
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Basically think of the barrel as a guide for the bullet you fire. If you attach something to it, especially something that applies a lateral force to it (like your hand, tension on a sling, etc), you are going to deflect the barrel. This deflection of the barrel since it is not consistent and varies will affect where that barrel will guide your bullets to. Of course, this translates to accuracy. My understanding is that even things that you do not apply force to that is attach to the barrel i.e. gas block may also affect accuracy, this is because it affects the harmonics of the barrel, but this is something that I don’t have the background to explain. Keep reading, and stop passing incorrect information. Has nothing to do with the force applied to the barrel. It has to do with the harmonics of the barrel...freefloating the barrel allows for consistent oscillation. Next time you go shooting lay that barrel on a horizontal or vertical support. Or use a TIGHT sling. Try all three. Then follow your own advice 
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Basically think of the barrel as a guide for the bullet you fire. If you attach something to it, especially something that applies a lateral force to it (like your hand, tension on a sling, etc), you are going to deflect the barrel. This deflection of the barrel since it is not consistent and varies will affect where that barrel will guide your bullets to. Of course, this translates to accuracy. My understanding is that even things that you do not apply force to that is attach to the barrel i.e. gas block may also affect accuracy, this is because it affects the harmonics of the barrel, but this is something that I don’t have the background to explain. Keep reading, and stop passing incorrect information. Has nothing to do with the force applied to the barrel. It has to do with the harmonics of the barrel...freefloating the barrel allows for consistent oscillation. Your both right!! It does both!! Free floating prevents outside forces ( like your hand on the handguard, or all the aftermarket tacticrap stuff people add) from touching the barrel and causing it to bend ever so slightly. Thicker "HBAR" barrels are less affected. If one pulled on a sling, or a front pistol grip hard enough you could actually bend your barrel to point toward a different direction your optics are pointing at. It also keeps the harmonics consistant. Pressure on your barrel, like your hand on the handguard, or a bipod or sling are not consistant. Each time you fire, the lack of consistant harmonics causes the tip of your barrel to be pointing in a slightly different directen each time the bullet exits - resulting in larger groups. Before I had a free float tube, cold shots would be about 1 inch groups at 100 yards with my Bushmaster. After free floating, my cold shot groups would be touching. I've since decided this was to be a light weight (duty) gun. I had the HBAR turned to an ultra light, and ditched the float tube for a Surefire light intigrated handguard. Now my groups are about 2 inches off sandbags at 100 yards, but its very light to carry and easy to swing for clearing corners. |
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Before I had a free float tube, cold shots would be about 1 inch groups at 100 yards with my Bushmaster. After free floating, my cold shot groups would be touching. I've since decided this was to be a light weight (duty) gun. I had the HBAR turned to an ultra light, and ditched the float tube for a Surefire light intigrated handguard. Now my groups are about 2 inches off sandbags at 100 yards, but its very light to carry and easy to swing for clearing corners. 1 moa groups before ff - touching groups after ff. Unless you're competition shooting, it seems like a lot of work/money for little gain. So you shoot a guy in the nose when you were aiming for his eye...meh. Am I missing something here? |
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Wouldnt changes in barrel profile (like a govt profile) or the gas block on an AR also cause this "harmonic imbalance"? sure, it may shift point of impact, but the shift will be consistent from shot to shot=adjust scope/sight elevation and windage to compensate unlike inconsistent sling tension or forearm rest pressure. |
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