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Posted: 10/20/2003 4:07:19 AM EDT
I am organizing a short lightweight precision AR w/ 16" SS match barrel, midlength FFRAS (probably DD), and small gasblock under the forend. 1-8 or faster twist to work with 77Nosler/nato. I was planning M4 diameter (0.71"?) but am also considering 0.75".

Is an M4 0.71" diameter and what would such a 16" barrel weight?
How much additional weight if I went to a 0.75 barrel?
Would I gain much in strength with the 0.75"?
Anyone make a tiny lightweight alum M4 diameter gas block? most seem 0.75"

Does anyone stock anything close to this or, do I go custom only?

thanks
Link Posted: 10/20/2003 12:08:24 PM EDT
In terms of weight, I am not sure what the actual numbers are, but the .75" is going to be about 11.6% heavier than the .71". In terms of strength, I doubt you would be able to tell through casual use, however, that cut out for the 203 on the M4 barrel always looks like a weak spot (probably doesn't make a difference). If you are looking for an actual midlength gas system, then I would think the closest you could get to this is a RRA SS HBAR 16" w/ midlength port. Armalite also might have this, but I couldn't find it and it may not come in 1/8 or 1/7. If you plan on just a regular carbine gasport with the mid-ffras, then I think you might be able to get DPMS or Olympic barrels. They would come in the right twists. However, with all of those, the barrels are usually HBAR or Bull. You might also try [url=http://www.compasslake.com/Price%20List.htm]Compass Lake Engineering[/url] As far as I know, Bushmaster only has SS on their 20". If you can't find exactly what you want, you could always check with Kurt (KKF on industry boards). HTH
Link Posted: 10/20/2003 12:35:43 PM EDT
[Last Edit: 10/20/2003 12:40:43 PM EDT by blikbok]
The M4 profile should be either .67" or .61" under the handguards. BM doesn't quote it, but says it's similar to the Government model A2 profile: "Diameter tapers from .980 inches to .670 at the chamber end to .610, then steps up to .100% at the sight base, and tapers to .720 from the sight base up to the last 5/8" which is .700 at the muzzle for reduced weight." http://www.bushmaster.com/shopping/barrel-assemblies/pbbl20a2a.asp And I got different numbers than Vector Joe, but I ran them with 7184.159 kg/m^3, which I think is for 4150 steel. I also didn't subtract for the chamber, properly, just assumed a straight .225" bore. If you have a density for the SS steel you want, I'd be glad to re-run my numbers.
Link Posted: 10/20/2003 12:48:35 PM EDT
Thanks. I have an M4 barrel with ff forend and it seems to work fine so I was considering that diameter without the 203 cut. I was planning the standard 16" gasport placement. However, after your response I did what I should have done first, which was to visit the MarylandAR15's weight list (thanks Forest) and find part of the answer which is as follows: HBAR 0.75" entire 16" generates a barrel weight of 2.7 lb M4 0.675" to 0.605" under handguards and 0.715" in front generates 2.2 lbs. Bushmaster is 0.75" under sight to muzzle. Given I have a FF forend, there is no strength advantage to increasing the diameter in front of the handguard. Would there be accuracy or strength problems if I tapered from 0.675 to 0.605 under handguards and kept that diameter to the muzzle?
Link Posted: 10/20/2003 8:51:37 PM EDT
I went with a .750 fluted but I didnt weigh it before I put it on and it is a nice package. lightweight and super accurate.
Link Posted: 10/21/2003 6:55:21 AM EDT
I don't think there would be strength issues, since there is still plenty of steel near the chamber and throat, but rigidity might be compromised, especially under long strings of fire. What is a typical course of fire for your competition? .605" -.225" = .380" /2 = .190" barrel walls. .710 - .225 = .485 /2 = .2435 .750 - .225 = .525 /2 = .2625 going from .2435" walls at .710" OD to .190" walls at .605" OD is a loss of 28% thickness. I don't have my modulus of elasticity formulae handy.
Link Posted: 10/21/2003 12:13:29 PM EDT
blikbok:... What is a typical course of fire for your competition? thanks for all the great numbers. I am interested in a lightweight "social" general purpose rifle than can manage both CQB and very long range stuff (e.g., 4" at 400m). Thus the problem would either involve lots of rounds inside 10m or one cold shot at 500m I think I can get a 16" preban with DD FFRAS, CAR stock and 0.75 fluted to be under 6.75lb - that probably is the way to go as per K9dpd's approach. In thinking about it a bit more, I worried about the accuracy of a match barrel turned down to 0.6" and worse yet bending it around a tree by mistake :-). thanks again for all the help - at least I now have the specific gravity of 4150 steel.
Link Posted: 10/21/2003 12:27:57 PM EDT
[Last Edit: 10/22/2003 10:12:33 AM EDT by blikbok]
I agree you will likely be happier with a fluted .75" than a solid one of smaller diameter. Ar15barrels.com will make a .550" "featherwieght" barrel in .17" or .224" bore. Their "lightwieght" .625" profile is available up to .308" bore, "due to barrel wall thickness safety restrictions." However, the .750" "heavy" appears to be available from .17" to .416" (I mention ar15barrels.com as an information source, nothing more: I've never dealt with them aside from reading the website.) Please don't rely on that density figure-- I can't attribute it yet. Also, and it gives wieghts a little lighter than Bushmaster's figures, so while it may be correct for the steel, I'm not sure it takes into account the coatings and such.
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