I posted this elsewhere with only one response.  Maybe it is more interesting here.  

What I would like is an 18" barrel. I realize that RRA offers a stainless varmint barrel in that length, but what I want is a service grade 18" upper. Do they exist and if not, why not?

It seems to me that 18" is the best compromise. Extra velocity over the 14.5/16" and a little space savings compared to the 20".

If they do not really exist, can I just have a 20" barrel cut and recrowned? Would the rifle function correctly? I do not know too much about gas issues (except when I eat beans!) blocks, and tubes. Maybe this is why it is not common.

While I am on the subject, why are the gas ports for the Dissipator in the carbine position? Wouldn't I want the supposed advantage of a std. length gas tube? (wait, I think I might know this answer....to make a diss with rifle ports a manufacturer must cut down a 20"...on the other hand they have lots of 16" carbine ported barrels laying around....more economical....am I right?)

Again, sorry if this is a newbie question but I need some guidance form you gurus.

Thanks

I'm not sure eithr with the 18" question.  BUT the dissipator is a 16" barrel, so a 16" barrel needs a carbine position gas port.   It doesn't matter what kind of handguards you cover the barrel with, or what kind of sight you atach to the barrel past the gas port, on the gas port, etc.  

The position of the gas port and the diameter of the hole is based on a mathematical calculation to optimize the function of the gun.  You want just enough gas to flow back to operate the action fully, without overdoing it.  I'm sure someone else can offer a clearer explanation than me...  But that's what I have figured out on my own.      

WaterSniper is correct.

The key element in placing the gas port is the distance between the port and the muzzle.  If this distance is too short, then there will not be enough pressure to push the bolt carrier back far enough to cycle the next round.  Too long, and you are wasting pressure.

As to the 18" barrel, I have been looking and have found the same results as you.  Either I have a smith like Kurt fix one up for me, or go with the RRA.  The RRA isn't a bad price, but I have decided my self to just go with a 16"

A regular A2 barrel cut to 18" and crowned will function reliably.

Cutting a 2o inch barrel is exactly what i did for my SPR.

i got a 20 inch 1:9 twist stainless steel match barrel from Georgia Precision and had it cut and crown to 18 inch SPR length.  It did not have any adverse effect on cycling.  The gun is reliable and now serve as my 3gun match weapon.

20" bbls are standard and were a convenient length when the original AR15/XM16 was developed.

16" barrels exist only because shorter barrels aren't US-Legal unless they're registered NFA weapons.

14.5" bbls are the result of using standard carbine handguards, first used in the XM177-series, with the requirement to fit a bayonet.  They're the same length in front of the front sight as 20" bbls.

I wish there was some [i]scientific[/i] for any of these barrel lengths, but there isn't.

-- Chuck

Chuck, I know what you mean but we all do what we can.  I 'scientifically' decided on 18" by cutting an A2 barrel (that I picked up cheap and needed to neuter for post-ban use) a little at a time until the length and balance felt right, then took it to the range to make sure it would still run reliably.  

Edited because I can't tpye nor splel.

Well this is timely, I've ben kicking this around too.  I have a generic 20" pre-ban upper that I've been trying to figure out what to do with & was thinking of cutting it down as short as possible (17-18" ??) & just have it crowned or maybe a muzzle brake but how viable is this.
 No functioning issues ??

It does seem like a 18" barrel would be a great compromise.  
The RRA unit looks nice but a little heavy.  
I wonder if Kurt can turn down one of those stainless barrels ?

So you did this & it workd fine ??
No issues with gas port size etc. ?

Thanks!

The info. on the Dissi is correct.

I wonder is the gas port location any different on the Armalite or RRA "mid-length" handguard carbines ???

[url]www.armalite.com[/url]

Yes it is.  That is why they have a mid-length system to begin with.  To locate the port the same distance from the muzzle on a 16" barrel as it is from a 20" barrel.

I have a dream config of a 18 inch barrell with mid length handgaurds and a custom gas port location.

I don't think I'll EVER see that built.

Works fine.  Accuracy from a barrel that was crowned with a hand drill isn't going to win me any awards at Camp Perry, [sniper] but it works just fine as a blasting rifle [uzi]

18 inchers? Dont hold a drive by bayonet well. We wouldnt have 16 inchers but a law forces us to have em. Id love to have the Val Kilmer style Ar15 that he used in HEAT.
GG

Yeah, but neither do 20" or 14.5" barrels if they're on a post-ban rifle.  [rolleyes]

[url]www.metalcraft-ar15.com[/url]

" The newest in a line of custom rifles, the Tactical Responder is not your ordinary rifle or carbine.
The T/R fills a niche for a very maneuverable rifle, yet having more velocity than a carbine and
the kind of accuracy that is expected from a Krieger barreled upper.

It features:
A-3 Upper receiver
18" (carbine + 2") Medium Bull contour, stainless steel, cut rifled Krieger barrel
Armalite intermediate gas system and handguard
Available with 1-8 or 1-9 twist "

I think KKF has been making dissy styled rifles from 20"ers, cutting them down to 16" and opening up the gas port.

I'm pretty sure that Wilson combat offers an 18 in barrel on one of their rifles.  You should check out their website.

a long time ago in a far away galaxy,,,  i heard that cutting a chrome lined barrel can cause the chrome to flake or peal off. is this true?

i would love to cut a bushy A2 profile to 17.5 or 18. KKF are you ready? i have an Colt 20" skiny 1 x 12  thats like new and would make a great canidate too. picked it up for 70 bucks at the SAR show in phoenix last year

A properly done hard chroming will neither flake nor peel even when cut. A botched job will flake or peel even without cutting into it.