I understand that the free floating barrel is a good thing, but why the $400 ARMS, when fulton armory sells a float tube with all kinds of rails on it for $125, just for one example?  I want to build a multipurpose rifle with two uppers, and trying to maintain some kind of budget.  Thanks!

Looks, Integrated in to the top rail for a very steady mount.

Other than that, it prob functions as well as any other high quality free float rail system.

I started thinking about it and the fact that it is mounted to the flattop probably makes a big difference in how steady it is.  But I would not mount optics to it, just the forward grip, and flashlight, and bipod on the precision upper.  Optics would be mounted directly to the flattop.  So a less expensive float tube is cool then huh?  Oh and another question, the described swan/weaver rails on fultons tubes are the same as 1913 picatinny rails, right?

Another advantage to the ARMS SIR is the cooling capability that its design incorporates.

Also, you are correct that the ARMS Swan Sleeves function the same as any 1913 rail system.  They are, in essence, merely "risers" for optics.

Hope this helps!

I have never seen anything showing that the SIR has a "cooling capability" better than any other well ventilated FF rail system.

Since I use the EoTech Sight, I like to mount it as far forward as possible. Thats where the #50 comes in very handy.

I must admit, $359 is a lot of money for a friggin handguard, and I'm positive that it doesn't cost nearly as much for ARMS to produce one.
However I LOOOOVVVVEEEE my SIR!!! And I've NEVER EVER looked back!

A lot of R&D was done, testing is continual, that's a lot of what gets factored into the final production costs of a product.

Love my SIRs, too though.

Makes for a more solid platform?  Sure.  Built in capabilities for future electronics/power systems?  Sure.  Capability for selective rail attachment and polymer lower for less heat transfer? Yup.

Alters optic mount heights - yep.  That's why they are not my cup of tea - to much vertical bulk for me, don't like altered optic heights.

What's your mission?  What are your requirements?  It's different for different people.  Other FF systems that attach forward of the receiver are enough for me, sturdy enough for me.  YMMV.  One of my units is a compromise - the KAC RAS II.  I'm sure 3rdtk will chime in and tell you why SIRS are supposed to be superior to all others, however, it doesn't address my personal requirements and won't change my opinion, but I don't use mine for work like some others...  I'm still waiting on the URX-2.

Agreed, but in drew's original posts he did not specify that his other option included a "well ventilated FF rail system", only a free float tube forearm.  And in that case I believe that the SIR might have an advantage over a regular FF tube.

Also, a while back there was a post that someone did comparing the ARMS SIR to the Firsh Handguard system.  If I remember correctly, the SIR was better at dissipating heat than the Firsh, but that example is only a comparison between the ARMS and the Firsh (which is ventilated), nothing else.

The SIR is also one of the HEAVIEST rail systems.

I am amazed at how little (if any) consideration weight is usually given when it comes to choosing rail systems and accessories on this site?

Requirements:  Free float the barrel for better accuracy, ventilate the barrel for better cooling, have lots of rails to attach accessories to.  Honestly the only accessories I am sure I would attach are a forward grip and flashlight.  I don't want the weapon to get bulky up front.  I will be attaching this tube to a Bushy M4gery, the one that comes with the fixed telestock and mini ycomp brake.  I will get the flattop upper for it.  Any comments as far as a BUIS system is concerned?  Looking at the Arms 40 and the GG&G MAD.  Might someday put an Aimpoint on it, but I would mount it to the flattop.  Thanks for all the feedback!

The SIR system certainly does not make optics mount any higher, that is if you don't use high mounts on something that allows med and low mounts to be used. A one piece continuous rail is going to be more expensive to make and cost more. That one piece certainly provides a more precise means to keep the rail in perfect alignment to the receiver, that you don't get with two piece not even connected to the receiver, close is not good enough for all occasions, just depends what the needs are. The SIR's are made to allow the grenade launchers, M203, etc. to be mounted without disruption of any of the aiming devices.
It's a military system that is not designed for just great accuracy at a range, it's made to accomodate all types of current and future plus integrated type devices for the rigors of combat of the future. Removal of the lower hang guard for cleaning or exchange to a grenade launcher, shot gun, and other devices without disrupting the alignment to the receiver is also very important. Not having to cover alum rails with panels to temporarily protect the hands from getting burned in rapid fire is certainly a good feature.
Jack