Free float rails and non free float rails? also some talk about barrel flex

[R.Miksits]

It was my understanding that we went to free float rails to improve accuracy.
Did non free float rails really affect accuracy that much?

So does that mean that if i took a 10 pound weight and tied it to a barrel it would cause a shift in accuarcy?

[rob_s]

This has been discussed here repeatedly, but.....

FF rails aren't just for accuracy. They are also better insulators/heatsinks, better platforms for optics/lasers/irons, generally more secure, etc.

[Army Chief]

Rob is absolutely correct, though I suspect that the measureable difference in performance between the two rail types is actually quite negligible for the overwhelming majority of users.

Unless you plan to mount very precise aiming systems well forward on the carbine, it probably makes almost no difference whatsoever -- provided we're comparing a quality free-float rail to a quality hanguard replacement rail. (True, there is a lot of "snap-in" junk out there on the market, but I'm assuming that the average M4CN member has done his research, and is not simply a best price shopper.)

A lot of guys go with the free float system, not because they need the capability, but because people they respect have had good things to say about it. That is not a bad thing, but real range credibility is still measured by hits downrange, and not by how tricked-out your carbine is. Five minutes on TOS will give you some idea how far afield some guys get -- though I've always found it instructive that the most serious SOCOM operators I know almost universally prefer very basic carbine configurations. Less is often more.

Rail systems add capability. There can be no argument about that. They also, however, tend to add complexity and weight. The trick is to find a balanced configuration that provides the necessary advantage without adding too much unnecessary/superfluous crap. That will almost certainly vary from mission to mission, but if you're routinely hauling an 11 pound carbine around you have very likely missed the point.

While the free-float rail assemblies remain the highest evolution of the technology, they are not readily serviceable in the field, not easily removed/replaced and are not necessarily designed for the kind of hard use that they would be likely to encounter in military applications.

Two-piece systems such as the RIS, RAS II and Omega offer most of the advantages of a free-float rail system with few of these disadvantages, and they are more easily replaced when things do go wrong. That said, they can affect accuracy somewhat at longer distances, they can loosen up and allow component zeroes to shift, and they can be broken.

Which is better suited to your purposes is largely dependent upon where you are, what you're doing with the carbine in the first place, and what sort of maintenance and repair support you can expect to have access to. I see strong arguments for both types, quite honestly, but if there is one overriding point to be made here, it is this: only you know where, when, and how you intend to employ your weapon, so only you can make an informed decision about which -- if any -- rail system best meets your needs.

Chief

[Ellery Holt]

Five minutes on TOS will give you some idea how far afield some guys get...
Chief

Ah, is it time to bash the people at TOS again? Sigh.

Actually, five minutes at ar15.com would provide an excellent quantitative comparison between free-float and non-free-float uppers. The post is by member Molon and the bottom line of his investigation is this:

http://ar15barrels.com/tech/freefloat.jpg

[Army Chief]

Ah, is it time to bash the people at TOS again? Sigh.


No, not at all, Ellery. I don't see that there is anything to be gained by making derisive holier-than-thou observations about TOS or any other online resource. The remark wasn't borne of arrogance or elitism, but rather the fact that the mean demographic at TOS, when coupled with their much larger membership base and admittedly broader focus, makes the wheat more difficult to separate from the chaff at times.

In examining this particular topic, I just wanted to focus on real, operational concerns, and steer clear of any semblance of "my gun is cooler than yours because I use this rail system and you use that one." To me, the free-float versus non-free-float question is less about the carbine itself, and more about how it is likely to be employed. What we're essentially doing here is dividing the question along the lines of military versus, well, just about everyone else (i.e. LEO, practical rifle shooters, gamers, enthusiasts).

The accuracy question provides an excellent line of demarcation. Assuming these are reliable measurements using like equipment (obviously the key factor) at 100m, then clearly a free-floated barrel possesses some inherent advantage. This ought not come as news to anyone, as the competition shooters who brought the AR into prominence at Camp Perry and elsewhere have known about this for a long, long time. The question is, does such a system belong on a carbine (which is not really intended to engage distant targets), and by extension, how well would it hold up against the rigors of combat?

Rather than debate minutiae, I would simply contend that real-world hit probability at the most common engagement distances (say, 200 meters or less) is not profoundly affected by one's choice of rail system. If you're looking to routinely reach out to 400m or 500m with a carbine, then there may be an argument here, but I would also suggest that you've probably selected the wrong weapon. Is the inherent capability there? Yes. Is this the carbine's strong suit? Certainly not.

That leaves us at something of a fork in the road. If you're a military-oriented user that puts a premium upon being able to install, service and repair the system on your own in an austere location, then you've probably got a slightly different set of requirements than the LEO or practical shooter who is looking for the best possible accuracy, and who is never too far from a competent support chain.

All of this notwithstanding, Ellery was correct to bring the focus back towards accuracy, as that was the original question. Obviously, I don't feel that there is enough of a difference between one rail system or another to really make this a real-world consideration, but others will surely disagree. I take no issue with dissenting opinions -- I just prefer to "keep it real," and I think we need to agree on what constitutes acceptable accuracy before we get too preoccupied with looking to the M4 as we might a 20" match-prepared AR in the hands of a competitive Distinguished Rifleman. The M4 is foremost a tool for the close to medium-distance fight, and sub-MOA accuracy -- with or without a rail -- strikes me as something of a tertiary concern.

Chief

Disclaimer: most of my shooting impressions here are based upon heavy barrel profiles in military trim. It would stand to reason that the M4A1/LE 6920HB might be somewhat less affected by a RAS or Omega type system.

[Rmplstlskn]

Ah, is it time to bash the people at TOS again? Sigh.

Actually, five minutes at ar15.com would provide an excellent quantitative comparison between free-float and non-free-float uppers. The post is by member Molon and the bottom line of his investigation is this:

http://ar15barrels.com/tech/freefloat.jpg

In the CARBINE world with REALISTIC "possible" engagements, BOTH of those platforms would equal "TANGO DOWN..."

Once you get into longer barreled, accurate platforms, the free float is a must for BEST accuracy. I have always been amazed at how little pressure it takes on a non-free float barrel to alter point of impact.

My worst offender, of which I still lack an AFFORDABLE solution for, is my DSA SA58 FAL Carbine. It has an excellent Badger barrel, capable of excellent accuracy, but pressure on the FAL standard handguards can alter POI noticeably. Even resting in a bench rest gives a DIFFERENT POI than when cradled in the hand (most FAL's seem to respond better to hand rather than bench rests). Then there is the sling attachment on the barrel as well, which can throw it all off....

So, it is clear that a FF is best for MOST, but it comes with a premium...

BTW, Molon is one of TOS's jewels...

Rmpl

[Stickman]

It was my understanding that we went to free float rails to improve accuracy.
Did non free float rails really affect accuracy that much?

So does that mean that if i took a 10 pound weight and tied it to a barrel it would cause a shift in accuarcy?

FF rails are more about stability for most of my work, though I have them on weapons that are dedicated for precision work as well. I've run a duty carbine with non-FF rails, and they loosen up pretty quick, which is something that I wasn't overly impressed with.

FF rails do allow for greater accuracy, but I doubt that most people are shooting to a degree where it matters. For people who want that extra level of accuracy, the FF rail may not ever make the difference that a shooting school would, but thats up to the individual shooter to determine.


ETA- Yes, roping a 10lb weight off the front of your barrel would make a difference, though it would make much less of a difference for some people than others.

[Harv]

Rmplskns


In the CARBINE world with REALISTIC "possible" engagements, BOTH of those platforms would equal "TANGO DOWN..."


and THAT right there is why I never get wrapped around the axle over the FF vs Non free float.

I run a KAC RAS which I have had for many years... it along with the carbine it's attached to.. can still shoot way better then me any day..

Unless you are running a precision rig with optics and a good trigger and good ammo.. your will rarely see much difference in your performance.

It's not something I lose sleep over... With a lot of the newer FF rails systems that no longer require removing the Delta ring or FSB.. and for the price.. you can go FF and let yourself sleep easy.

If I guy wanted a rail.. you could get a good FF that YOU can mount yourself..
I stuck with KAC because it was the standard for many years.. I was issued one and was familiar with it.. and I got a good deal on it.

I still like them and find them to be an excellent piece of kit if you just want a light and a VFG. And Good deals can be had on used ones all the time (saw one with rail covers for $100 the other day....)

And yes.. Molon is one of "good guys on TOS and I enjoy reading his posts.. he puts a lot of effort into them...

[BushmasterFanBoy]

I personally use my FF rail to mount an Eotech on. I know that my gun can out shoot my skills any day, so I just really mount it there for a stable platform to get a reliable zero with. On my other carbine with an aimpoint, the non FF rail is there simply to hold a VFG / conduct less heat.

[III]

There are a lot of misconceptions about heat transfer. The more heat that conducts to the rail the better. This allows the barrel/gun to cool faster and can help prevent cook-offs and increase barrel life. We just got done doing a huge study on heat transfer for the IAR program and learned some stuff that we never knew.The more contact the rail has near the chamber the more heat can be transfered. Contact at the front is not as critical. The original RAS has a leaf spring in the front and allows for expansion of the barrel and rail to occur (they are different materials and expand differently) . I believe the spec was 2.5 in over a 6 or 8 thousand round cycle. The RIS could not meet the spec due to clamping on the front.The alignment problem was not number of rounds but temperature related. Most rail systems due to the material and shape make excellent heat sinks and dissipate the heat very well.I think people would be surprised which rail designs have a negative effect on rifle performance the way they clamp and where they clamp is important. The RIS and RAS perform very differently and most people would have a hard time telling the differences between the two.As I recall the M4 with standard double heat shield handguards can not pass the ccok-off acceptance requirement of 210 rounds.With a RAS installed it will pass.

[jmart]

I always thought the big area where they came into play was isolating the barrel from sling pressure. They've proved their worth in HP competition, when those guys sling up they put a lot of pressure on the forend, so they went witht he internal steel tubes. I have no idea how much this comes into play with carbine slings or bipods, I've never seen anyone sling up in a VTAC or Vickers like a competitior does with a Les Tam or Turner, those are two completely different animals.

[III]

I think most free float rails will flex a certain amount when that kind of pressure is applied.

[jmart]

Even if they flex, that's not a bad thing in and of itself, unless along the way a handguard cap contacts the barrel affects barrel harmonics. The whole idea is to let the barrel float and that's what the FF systems do.

[rhino]

When you're talking about hanging a weight on the end of the barrel, you're getting more into a shift of point of impact more so than altering inherent accuracy of the barrel (harmonics notwithstanding). The same thing is true with using a shooting sling. This is where the freefloats make the bigger, more useful difference for most of us. If you use a sling or a bipod, you probably want to freefloat.

As an example, when I first started shooting 3-gun back in the mid 1990s, I used borrowed rifles and didn't know much about them other than how to make it go boom and other manipulations safely. I once used one of those plastic clip-on bipods that clip near the end of the barrel on a skinny-barrel Colt and it changed the point of impact about an inch (high) at 50 yards vs. using the magazine for a monopod.

Applying a little mechanics of materials, the farther toward the end of the barrel you apply the force, the greater the deflection for a given load. So even a small load toward the front end of the barrel can make a significant difference. Longer lever = bigger bending moment

[R.Miksits]

Thank you all for the info.My question was answerd to a certien degree. I was did not mean to imply I was doing non free float VS free float each serves there purpose.. I was more trying to understand barrel flex and how light amounts of weight would cause something made of steel flex enough to shift POI, and how screwing non free float rails to a barrel and or clamping them on could affect POI or MOA.

R.Miksits

Re-reading my inital post I apologize for being very misleading.

[Rmplstlskn]

I once used one of those plastic clip-on bipods that clip near the end of the barrel on a skinny-barrel Colt and it changed the point of impact about an inch (high) at 50 yards vs. using the magazine for a monopod.


My awakening to barrel deflection and the small amount of weight needed to deflect was on a ACOG-mounted 20" gov't-profile barrel with a YHM non-free float rail system and a Harris bipod near the FSB... I was frustrated at the RANDOM poor accuracy I was getting at 100 yards until I realized that I was placing my hand on top of the rail near the receiver to keep the rifle from jumping after the shot and changing my bench position. It didn't seem like much pressure at all but it made a difference of INCHES at 100 yards... That 20" gov't profile was very easy to deflect... Not a problem with a SOCOM M4 barrel... :D

I then and there knew that if I was going to play around with 77g handloads I needed a different configuration, a FREE FLOAT config, which lead to my present SPR...

I love FF rails, I just don't like the expense (duh!)... :rolleyes:

Rmpl

[jmart]

Thank you all for the info.My question was answerd to a certien degree. I was did not mean to imply I was doing non free float VS free float each serves there purpose.. I was more trying to understand barrel flex and how light amounts of weight would cause something made of steel flex enough to shift POI, and how screwing non free float rails to a barrel and or clamping them on could affect POI or MOA.

R.Miksits

Re-reading my inital post I apologize for being very misleading.

Even though barrels are amde of steel, they vibrate. Actually as the bullet goes down the barrel it sets up an oscillation cycle that will move the muzzle ever so slightly.

The idea behind free floating is to let that oscillation cycle occur without any interference. In the best cases the cycle is approaching the end of an oscillation node (muzzle at at end of a cycle and reversing direction), it is at this point where the muzzle movement is "quiet". if we can tune a load so it exits the muzzle at that point, we can achieve pretty good accuracy, but even if we can't get it right at that node, if we can at least let the barrel oscillate consistently, we can achieve decent accuracy.

When you don't free float, you lose that consistency. In some instances we're putting a bunch of sling pressure on teh barrel, so that affects oscillation frequency, in other instances it's light pressure, then resting on a bipod or up against a barrier. each of these instances affects how the barrel oscillates, and w/o that consistency, acccuracy suffers.

It all boils down to degrees and the ranges you shoot at. All things being equal, a pencil weight barrel will oscillate more than a heavy weight barrel, and a long barrel will oscillate more than a short barrel. these issue show up more when shooting at longer ranges. When shooting up close, it doesn't really matter, at that point free float rails serve more as the heat sink and something that allows easy mounting of lights and other add-ons.