Friend says KAC M4 RAS is a free floating rail because top half clamps to barrel nut?

[JoeSixPack]

I was thinking this was not a free floating rail but a friend says it is.

The KAC M4 RAS is a two part rail, a top half and a bottom half. The top half contains the top rail and the side rails. The bottom half has the lower rail.

The top half clamps to the barrel nut so in effect I guess the top half is as much of a free floating rail as any of the other free floating rails that clamp to the barrel nut.

But the bottom half of the KAC M4 RAS does not clamp to the barrel nut (it's removable so a grenade launcher can be attached).

Well my friend says that the bottom half of the KAC M4 RAS can not put upward pressure on the barrel because when you put upward pressure on the bottom half of the KAC rail it presses against the top half of the rail which is locked to the barrel nut.



I'm thinking he may have a point.

[Failure2Stop]

Your friend is wrong.
If the rail contacts the barrel at any point other than at the barrel nut, it isn't free floating.

[Dave L.]

Your friend is wrong.
If the rail contacts the barrel at any point other than at the barrel nut, it isn't free floating.

Yup.

[a0cake]

Yep, F2S is obviously right. It's not a FF rail. While the design may mitigate some of the POI shift issues with non free floating rails, it still does not qualify as being free floating. Another example is the Surefire M73. Somewhere along the line Surefire marketed it as a FF rail even though it was not so (they have since stopped). It does do an outstanding job of avoiding problems with non FF's, but it still is what it is. Same for the KAC.

[Robb Jensen]

It's rigid but not freefloating. With freefloating there is no foward of the barrel nut contact with the barrel/handguard cap.

Sometimes with freefloat tubes you still have to be careful that there isn't anything contacting the gas block if the gas block is covered by the freefloat tube/rail. If there's contact there it'll cause all kinds of accuracy problems.

[JoeSixPack]

It's rigid but not freefloating. With freefloating there is no foward of the barrel nut contact with the barrel/handguard cap.

Sometimes with freefloat tubes you still have to be careful that there isn't anything contacting the gas block if the gas block is covered by the freefloat tube/rail. If there's contact there it'll cause all kinds of accuracy problems.

What's really confusing is how non-free floating rails or handguards could have that much of an impact on accuracy. The military has tens of thousands of KAC M4 RAS equiped M4's not to mention M4 carbines with the factory plastic handguards. This can't be causing all kinds of accuracy problems otherwise our troops wouldn't be able to hit anything. How are Marine recruits managing to qualify at 500 yards with iron sights without free floating rails?

I just can't help but wonder if this free floating rail business isn't all snake oil.

I would really like to see a study that shows any significant real world POI difference between a non-free floating and free floating rail on a carbine.

[yhmspecter]

What's really confusing is how non-free floating rails or handguards could have that much of an impact on accuracy. The military has tens of thousands of KAC M4 RAS equiped M4's not to mention M4 carbines with the factory plastic handguards. This can't be causing all kinds of accuracy problems otherwise our troops wouldn't be able to hit anything. How are Marine recruits managing to qualify at 500 yards with iron sights without free floating rails?

I just can't help but wonder if this free floating rail business isn't all snake oil.

I would really like to see a study that shows any significant real world POI difference between a non-free floating and free floating rail on a carbine.

Watch a rifle that was filmed with a hi-speed camera firing a round, watch what the barrel does. Then think about something contacting the barrel during this process, then rethink your last post

[QuietShootr]

What's really confusing is how non-free floating rails or handguards could have that much of an impact on accuracy. The military has tens of thousands of KAC M4 RAS equiped M4's not to mention M4 carbines with the factory plastic handguards. This can't be causing all kinds of accuracy problems otherwise our troops wouldn't be able to hit anything. How are Marine recruits managing to qualify at 500 yards with iron sights without free floating rails?

I just can't help but wonder if this free floating rail business isn't all snake oil.

I would really like to see a study that shows any significant real world POI difference between a non-free floating and free floating rail on a carbine.

hehe... anyone who uses a VFG can make a non-FF carbine walk all over the place, particularly if they're a really hard holder. I can crank an M4's POI down several inches at 100 just by varying the tension I'm pulling back on the VFG with. The difference is, my FF gun will shoot to the same POI whether I'm locked up on it tight putting rounds downrange offhand while moving, or holding it much less firmly while sitting/prone/whatever.

This has been done to death. There are many advantages to FF rails, not the least of which is minimizing POI change as accessories are mounted and dismounted, and there is even some evidence to show that non-FF guns' bolts break sooner because of uneven loading on the bolt lugs caused by pressure from a VFG or heavy accessories mounted on the handguard.

[a0cake]

Your intuitions are misleading you. Accomplished shooters on both the military / civilian side nearly unanimously advocate the use of FF rails.

Most of us who have used both don't need to "see a study." Our experience is the study.

Here are a few things to think about / areas where FF rails shine:

- When a bipod is rail mounted.
- When shooting off a rigid improvised rest.
- When putting extensive pressure on a rail mounted sling.
- When bearing down on the forearm to control recoil.
- When the barrel / rail gets hot and the metal starts to "move."

Think about barrel harmonics and its effect. With a non-FF rail, the barrel will "vibrate" differently based on your grip on the rail. With a FF rail your grip pressure and placement has less of an effect.

This may sound all academic but I can assure you it is also observable in practice.

What's really confusing is how non-free floating rails or handguards could have that much of an impact on accuracy. The military has tens of thousands of KAC M4 RAS equiped M4's not to mention M4 carbines with the factory plastic handguards. This can't be causing all kinds of accuracy problems otherwise our troops wouldn't be able to hit anything. How are Marine recruits managing to qualify at 500 yards with iron sights without free floating rails?

I just can't help but wonder if this free floating rail business isn't all snake oil.

I would really like to see a study that shows any significant real world POI difference between a non-free floating and free floating rail on a carbine.

[sinlessorrow]

Its not a FF rail because both the top and bottom contact the handguard cap which contact the barrel, thus putting stress on it

Theres a reason why the sopmod block II has the DD RIS II in it, even the .mil know the need for a FF rail if you want an easy to install one get the DD omega rail, its one of the best investments you can make.

Not trying to say the M4/M5 RAS is bad but alot has changed since it came out