Some time ago, I was approached by someone I work with who wanted to convert from an existing DPMS carbine into a non-competition, long-distance, precision marksmanship rifle.  We talked about it for a little while with the result that a work order was drawn up and I agreed to spearhead the project.

For finishing consistency, a DPMS upper was specified as a complete replacement for the existing one, although I expect about any upper could be coated with Teflon.  The new upper was to be a 20" length barrel, 1:7 twist, chrome lined, threaded muzzel (read: GemTech), free floating barrel with KAC Long RAS and the handguard retention cap removed, flattop gas block with bayonet lug (don't get hung up on this), on an A3 (NATO chambered) receiver.

When I contacted DPMS, things started to go offline.  It seems they don't have a 1:7 barrel available.  They could build the upper with a 1:8 barrel if they went to (Teflon-finished) DCM stainless steel.  The buyer would have to provide the gas block and the KAC FF RAS.  Due to some errors on their website, I was able to arrive at a price of $522.  (Come to think of it, I should check on the chamber of the SS barrel.)

On my shelf rests the RAS, new in its box.  Continual contact with YHM first pushed back the release of the gas block in question four weeks and, as of today, three more weeks - estimated.  A KAC two-stage trigger, Harris bipod, KAC adapter, grip, ambi-safety, tactical bolt catch, Match scope, everything else is on the shelf ... waiting.

Here's the question.  I know this individual has/plans to shoot Mk262Mod1.  I've read the Ammo Oracle report, but came away unclear as to the necessity, versus advisability, of the 1:7 twist barrel for 77gr rounds.  I don't recall where, just now, but I also recollect reading an engineering reference that indicated a twist of 1:8-running decimal was ideal for this round, although not being able to find it again hinders my ability to be confident in that number.  Plus, who can say if that will be the only weight Match round fired.

While I'm awaiting the release of the gas block (and contemplating stocks), I would appreciate some feedback.  While I'll certainly arrive at my own recommendation and discuss it with the user, there are folks on this board with considerably more experience building "tack driver" rigs.  Not being a marksman, I'd like to benefit from the collective knowledge, here.

Also, is there a point to chrome lining a SS barrel?  While it might help fight rust in the humid, saline environment of FL, won't filling the grooves even this slightly have a detrimental impact upon accuracy?

Am I missing a better upper source, concerned about nothing, or about to make a huge mistake?  Ok, blast away.

I changed the subject line to "Precision AR15 Upper" with the hope of attracting the desired feedback.

why a rail gas block if you alreay have a long RAS?? go with a low profile LaRue or PRI block, you will be a lot happier. and get your parts faster.

1:8 would be your ticket for those MK262...

With a telescopic optic mounted to the rail and an NVD mounted in front of that, there's no room for a folding forward iron sight.  I suggested a flip-up front sight gas block, but the user doesn't like the look of any of the current crop.  The railed gas block allows a sight tower or flip-up to be added later.  Does the LaRue or PRI block have a bayonet lug?

The only source I've found, to date, for the gas block is from YHM.  They just keep pushing back the release date.

If departing from DPMS, the rifle would be in need of an entire upper, not just a barrel,  It would also have to color match the lower.  Plus, I'd wonder about price comparison, including removal of the handguard retention cap and installation of both the RAS and the gas block.  In either case, the specification would be for a NATO chamber, as opposed to a Match chamber, assuming this was available from either source.

With respect to chrome lining, I've read next to nothing about the process in SS barrels, leaving me a bit short on both fact and opinion.  To what extent does it impact accuracy?  Is there an appreciable advantage in high humidty or saline environments, like exist in FL?

what made you go with a NATO chamber instead of a match chamber?

SS barrels never come chrome lined that I have seen plus chrome lined is viewed as not as accurate as non chrome barrels.

Ability to fire the widest available selection of ammunition determined the chamber.  This is to be a non-competition rig.  It won't be examined to comply with some set of contest rules.

I've also noted that chrome lining is held (at least on these boards) as being to the detriment of accuracy.  What I haven't seen is supporting evidence.  Personally, I have no opinion on it one way or the other, but apocryphal or anecdotal data really isn't all that helpful in arriving at an informed one.  My hope is that with the sum of knowledge and experience on this board in long range accurcy with the 5.56mm rifle, I'll be able to obtain a more rational and scientific position.

Clearly, there is some advantage to it (else noone would pay to do it), most likely in cleaning and resistance to rust.  The outside would be Teflon finished (unless there's a much better way to color match the Teflon lower) to resist rust, although there have been comments even on this board about keeping up with internal rust in a bare SS barrel.  It would stand to reason that filling the grooves in the barrel reduces the purchase on projectiles, thereby potentially slowing rotation and lessening accuracy.

BUT, is it actually true and to what degree is the effect?  Is the trade off worth it for the longevity gained (the rifle will be in humid, saline FL) for the accuracy lost if that loss turns out to be marginal?  I can simply find no data.

For instance, a bull barrel would be best, but the intention is to set up for a sound suppressor. This requires threading.  If done "properly" and carefully, the impact on accuracy should remain fairly marginal.  Perhaps the addition of a Vortex flash suppressor would gain back what was lost, thereby returning to a net of nearly zero(?).  This determination would be based on, say, results of the Vortex tests conducted recently (so it's said) at the APG (which I've also not yet seen).

Another possibility held to improve accuracy would be fluting, although it would only be desired beneath the hand guard.  Fluting is said to help reinforce barrel strength and, thereby, accuracy.  But, would the effect still exist if fluting is not taken to the muzzle?  Can one even flute a SS barrel?  Next, would it be advantageous to ensure the DCM barrel DPMS offered was button cut (instead of broach)?  You see, if it's preferred but not offered, it would seem we'd quickly be going into custom manufacture, unless these are actually not that uncommon precision features someone already manufactures.

In the realm of accuracy, my initial impression is that there is alot of balancing which goes on, although again, this is my first precision AR15.  AND TRULY, it's probably more of an excercise in how to obtain the best hybrid between a precision distance rifle and a field gun, with emphasis on accuracy.  I'd really enjoy input from those who build and shoot this way.

Once I arrive at a set of finished specs, I'd love to send them out to the industry partners on this board for evaluation and feedback. I'd have to determine who could do the best job building it and what the best price would be.

At the moment, the specs below are what has evolved.  Some of these items may prove incompatible and, if so, will be modified as responses and evidence supports.

DPMS 20" HBAR A3 (flattop) 5.56mm complete upper
• Teflon color match exterior finish
• 1:8 1:7 twist button cut 416 stainless steel barrel
• M4 feed ramps with associated upper receiver cuts
• free-floating barrel
• fluted/dimpling barrel beneath the hand guard only
• chrome lined barrel
• hand guard retention cap removed
• front sight base replaced by customer-provided YHM-9836A gas block
• threaded muzzle
• SEI G6A2 Vortex flash hider
• 5.56m NATO/Wylde chamber (SAAMI not acceptable)
• no carry handle
• no hand guards
• install customer provided KAC free-floating long RAS
• standard bolt and charging handle

Watcher, I have been away from the game for awhile but, I never saw or heard of a chrome lined barrel being used by high power competitors. I used, as well as most of the top shooters, stainless barrels. The maintenance was easier and they shot well. Any of the big name suppliers can make a very accurate barrel. As for fluting, I had mine done because I liked the look. It is supposed to increase the surface area of the barrel which in turn helps cooling. I am not an engineer so I can't effectively argue as to the merits of enhancing accuracy or stiffness. Hope this is of some help.  

Living in Miami, I've never had rust problems with any of my stainless barrels (Douglas, Lilja, Krieger)... Just take a little extra care with them and you're good to go... IMHO, trying to chrome line one is a waste of time and money.

My browser keeps crashing and I keep losing my fully-detailed reply.  So, pardon me if I've decided on something MUCH briefer.

I've struck out the chrome lining requirement.

Specific differences are listed, hereafter.  What are the practical difference between Wylde and NATO chambers -- rounds that won't load, places Wylde fails?

"Derrick" chamber (Derrick Martin of Accuracy Speaks) = 2.442"
"Wylde" chamber (from AR15 competition pioneer Bill Wylde) = 2.445"
"AMU" chamber (U.S. Army competition team) = 2.500"
SAAMI chamber normally = 2.410"
NATO chamber normally = 2.550"

Are WOA barrels button cut or broach cut?  What steel is used?  The answers were not found on their website.

Does WOA or someone equal make an upper to these specs using DPMS receiver and 1:8 twist?  I had a whole explanation of how I've been coming to see the 1:8 as an attempt to create a happy medium, but that's gone now.  Suffice that it doesn't seem to overspin lighter rounds and still does well with the likes of the 77gr range of rounds.

How much accuracy loss are we really talking about between 1:8 and 1:7 twist using an average of 77gr ammunition?

Do we have test results yet to know if a Vortex flash hider will substantially make up the difference?  If it were so, that combo would seem to yield a 1:8 barrel able to use a wider selection of ammunition than the 1:7 with similar accuracy results.  For other than competition shooters, this would seem desirable.

For the purposes of a non-competition precision rifle (should I just be calling this an "accurized" rifle, instead?), my  feeling is the Wylde chamber, with its reduced bullet "jump" might turn out to be more accurate for the same overall usability as the NATO.

I had to go back and re-read the projectile spin data, but you're right.  It seems overspin is an issue mainly with the very lightest rounds, which wouldn't ordinarily be considered for this rifle.  Thus, it would seem that the additional velocity of the 1:7 twist might improve accuracy at the greater ranges possible for 77gr.+ projectiles.  At least this is something I can leave alone from the original specs!

I'm still awaiting the release of the YHM-9836A gas block, so ETA isn't a huge consideration, at the moment.  What I'd really need to know is if someone can provide an assembled DPMS receiver upper with this barrel along with all the other specs.  Price would be after that.

As side notes, I would be interested in finally discovering the truth behind the Vortext flash hider's accuracy claims.  Also, what's the truth behind the useage of "tuned" and/or chrome "lined" bolts?  Some websites suggest they are some kind of proprietary secret to success, except you'd think if that were true more would be heard about them.  Last, should I be considering a special buffer and spring for the A2 stock?  I have one on the shelf, but I think it was from a BM carbine.  Reducing/eliminating perceived recoil is, as you might expect, important.

One area not discussed was feed ramps.  Are M4 cuts in the barrel extension now pretty much standard?  Even on SS barrels?  Or, should they be included in the specs?

Quoted: It's not going to be an ultra lightweight gun if you go with a service rifle contour anyway :)  That weight should soak up recoil just fine.

I realize that.  I'm almost afraid to estimate the weight, which is, in part, why fluting under the handguard is desirable.  Doubtful there's any more reduction in weight that can be achieved within the specs....  Should I be striking out or changing the "HBAR" spec?

Knight's and DPMS said the same thing about the bolt.

We can build on the DPMS upper, and can just order that from DPMS or you could :)

You don't say?  I'd probably be interested in getting a quote, with the understanding that actual build might be more than a month away, awaiting the gas block.  Also, as a dealer, you're more likely to get a good price out of DPMS than I am, I would think.

Interesting thread, tag

E-mail on the way.  There was a glich in my e-mail client, so please confirm receipt, if you would.

Just so I can feel my molars grind, would anyone care to estimate the overall weight of this rifle, unloaded, less the accessories I haven't mentioned here?

Does LaRue Tactical produce a product line catalog?  I've been to their website, which is rather sparse, and did not find one.  I have an ARMS cataog in transit and wouldn't mind comparing available mounts for the scope.

Just for the sake of clarity, the Vortex flash hider included in your quote, is it the SEI Vortex G6A2, P/N 1001V?

Also, can you provide the SEI blank firing adapter, P/N 1015?

Stainless barrels cannot be chromed lined.  PATRIOT ARMORY.

After reading another topic, I've become curious as to the effective difference between fluting a barrel and "dimpling", such as is supposedly available in Knight's SR-25.  I understand that both processes save approximately one pound in weight.

How common is the ability to perform dimpling amongst machinists?
Are there unique (dis-)advantages to either process?

One philosophy, ascribed to by Bushmaster, for example, suggests that in addition to lightening the weight of a barrel, fluting creates ribs which reinforce the barrel with respect to deformity in high-heat conditions.  Increasing the surfce area in the creation of flutes, also aids in cooling.  The claim is that this improved rigidty and cooling have a by-product which is increased accuracy under such conditions.

The counter-argument goes something like -- unless one is firing high volumes or extremely hot loads of ammunition at a rapid pace, the chances of reaching a heat level where deformity occurs is rather low.  In that case, the object of any metal removal is simply weight reduction.  The increased surface area created by the dimples also aids in cooling and does not draw undo attention to the operator as a priority target.

For the intended application, I figure the existance of a riflescope is sufficient evidence of a priority target, making that point null.  After discussing the matter with the user, who doesn't overly appreciate the look of fluting (hence the specification for being under the handguard only), dimpling the full barrel length might result in a comparative weight savings.

Personally, I've never weighed barrels with either type of machine work.  Thus, I am unable to offer an opinion regarding which process would be more advantageous for this upper.  No doubt comparing the respective costs would also effect the final determination.

Facts?  Opinions?

It's a fairly interesting topic, not least because of the misconceptions involved.  Indeed, removing material has never had the result of increasing stiffness (moment of inertia, modulous of elasticity).  That notion has always bothered me.

That leads us to cooling.  Yes, the fluted barrel will cool faster, but it's not because there's an increase in surface area.  It's because cutting the flutes brings the surface of the barrel closer to the heat source (found at the bottom of the flute).  The process of conduction transfers the heat (a form of energy) through a conductive material and once it reaches the surface, it can be radiated into the environment beyond.  Cutting the flutes speeds that process, allowing the release of energy faster than a uniformly thick barrel.

The balance to this is one must remember that the barrel is actually thinner in this location.  There's a point at which the barrel becomes dangerously thin.  Done improperly, fluting could lead to barrel swelling or even bursting.  My thought would be one would never be safe fluting a lightweight barrel, but I'm not a gunsmith or a machinist.  How can you know if a machinist did the job on your heavy barrel correctly?  Look at the flutes.

From Score High Gunsmithing: Like most things, it seems like there is an easy way and a right way.  Setting up a barrel in the mill for fluting is a very exacting, very time consuming task.  Then there is the actual cutting.  The tool must be very sharp.  Cuts need to be small and go-slow.  The convex cutters designed to be used on a horizontal mill set up have a very large cutting surface that creates a lot of heat and friction that will dull a cutter in a hurry. Dull cutters will bend metal out of their way.  A burr running along side the edge of a flute is the metal that was bent out of the way.  Stress has just been introduced into the metallurgy of the barrel.  It takes lots of small cuts and all day to flute a barrel.  If the time wasn’t taken to do it right then you are better off not fluting at all.  If the price seems too good to be true then it probably is.

The conclusion appears to be, fluting does help dissapate heat.  It's main purpose, however, seems to be weight reduction, which is valid on its own.  With respect to dimpling, the depressions are not as deep, so the heat release probably won't be as fast (yet faster than a plain HBAR), but the trade up is that barrel rigidity should be greater than the equivalent fluted barrel.  As a mental exercise, this suggests better accuracy retention, although the amount may or may not be significant, depending upon all the specifics (actual barrel thickness/diameter, metallurical composition, expertise of the machinist, etc.).

I think I'd almost rather go with dimpling over fluting -- on 416 stainless, as opposed to 410.  This is based upon a more or less equal weight reduction from either process, though.  There's alot of weight in the rifle and accessories making any savings a worthy consideration.  It would then become a matter of availability and cost.  As a side note, the photo of the KAC dimpled SR-25 only shows dimpling beneath the RAS.

Not related to this upper, any opinions out there on polygonal bore accuracy?

I've updated the specifications to include not only the M4 ramps on the barrel extension, but also the associated cuts in the upper, as one would seem to be of little value without the other.  Barrel steel was updated to 416 stainless.

So far, a Google search has turned up some small information on dimpling a barrel, including the off-hand mention from Knight's that it is provided on their SR-25s, but no machinists who provide the service.  If anyone should know of a source, I would be interested in learning of it for additional research.

Jason, please let me know if the quote you offered for this upper is still valid.  YHM has released the 9836A Single Rail Gas Block with Bayonet Lug, the last part prior to moving forward.  I've put out a general post on the Equipment Exchange board to industry partners for a price.  If I receive none by 1 December, I'll order directly from YHM.  My intention is to move into production at the first opportunity thereafter.  If you require that I resend the specs, I am happy to do so.

Assuming it arrives relatively quickly and my turn around to you with the KAC FF RAS is equally swift, could you suggest a production time for this upper?  I will add the caveat that my interest in quality of craftsmanship exceeds my interest in speed.  I would like to explore the possibility of having the barrel dimpled as opposed to fluted, as well, if possible.  Finally, any idea how much this monster might end up weighing?

E-maill on the way.

In your opinion, is it better to mount the scope using two throw lever rings or one mounting adapter with the rings built it?  In the end, both will have two throw levers and first consideration suggests that dual rings will weigh slightly less due to the absence of the aluminum rail connecting them.  Too bad noone makes Titanium rings/mounts!

I'm also considering a stock or this rifle.  Currently, it has an A2 stock and tube.  Possible alternates include the new MagPul PRS or the Fulton Armory adjustable buttplate stock.  Admittedly, though, stocks are not within the scope of this thread.

Sounds like you want something like an Army M16A4 Designated Marksman Rifle (DMR) or a Marine Squad Assigned Marksman Rifle (SAM-R).  The specifications for the Marine rifles I got from "Gunner" Williams of the Marine Precision Weapons Shop, Weapons Training Battalion at Quantico.

Start with a standard no-frills flat-top rifle, minus barrel.

The Army uses the Daniel Defense 12-inch free-float rail (with steel barrel nut -- Steve Thompson at ADCO carries these).  The Marines use the Knights.  The Army uses Knight 2-stage semi-only triggers.   The Marines use a tuned issue single-stage M16A1 trigger.  

The Army uses a variety of 1-7 or 1-8 twist stainless barrels.  The first thirty for internal school use we built with barrels in shop stock -- Krieger, Hart, Schneider, whatever was on hand.  Some we fluted (on a FADAL CNC machine center), some we didn't.  We use standard rifle extensions (no M4 ramps).

The 275 M16A4 Designated Marksman Rifles for the 3rd Infantry Division in Iraq use Douglas 1-8 stainless barrels.  The barrel under the handguards is 1-inch in diameter with 12 flutes (which lightens the barrel -- side by side the fluted 1-inch weighs less than an un-fluted 7/8-inch Armalite or Douglas).

The 40 we are building now for Mobile Training Team courses have Armalite 7/8-inch stainless 1-8 barrels we are leaving alone.

Black finish on the barrels was originally High Temperature Krylon Barbeque Grill Black.  All future barrels will be powder-coated in-house to customer's requested color.

The Marines use a stainless Krieger (I believe a 1-8) profiled from blanks in their shop.

Derrick Martin (Accuracy Speaks in Mesa, Arizona), Frank White (Compass Lake Engineering in Florida), and the Army use a "Short throat" barrel reamer for rifles shooting 77s only.  The Army uses what we call an "Infantry Trophy" throat specifically designed for 77-grain Sierra Match King bullets for rapid fire to 600 yards).

All Army rifles are proofed and bench-tested.  If they don't group minute of angle or better (10-shot group smaller than 3.28 inches at 300 meters) they go back to the shop.

Chrome-plating (generally) detracts from accuracy but increases barrel life.  I've never heard of a chrome-plated stainless barrel.

If you want to put a night vision sight (like the AN/PVS-22 Universal Night Sight) in front of your ACOG (Army) without any shading from the front sight tower you can use an Armalite gas block with a GG&G flip-down.  A folding front sight on the rail is useless (according to the Ranger Regiment).

A precision M16 is easy to build.  If you look in the Zediker book he puts it very concisely -- "Float a good barrel."

Quoted: A precision M16 is easy to build.  If you look in the Zediker book he puts it very concisely -- "Float a good barrel."

Unfortunately, I don't have the advantage of working with an M16, but Krieger has a fine reputation, as does KAC.  That said, the parallels between a civilian and a military rifle build for similar purposes are bound to be close.

Thanks for the profile on the DMR.  I'll store that away for future reference.

Quoted: All future barrels will be powder-coated in-house to customer's requested color.

Is this switch due to durability? I'm curious because there is a distinct heat resistance problem with PC. PC bake on finish will only resist about 425 degrees before it boils. I had a PC'd barrel that turned into a mess after 80 rounds of semi-auto rapid fire, another 80 rounds of full-auto had it smoldering.

The DMRs we build are semi-only.  We don't have a barrel oxiding capability in-house for stainless.

Quoted: It's a fairly interesting topic, not least because of the misconceptions involved.  Indeed, removing material has never had the result of increasing stiffness (moment of inertia, modulous of elasticity).  That notion has always bothered me.

Most people that state this go further and say that a fluted barrel is stiffer than a straight or tapered barrel of equal weight, which is a true statement.  If they leave out the equal weight qualifier then well they are wrong.  An interesting question is if you have a stiffer barrel per weight how does that effect barrel vibration?  It's an interesting read.

Quoted: That leads us to cooling.  Yes, the fluted barrel will cool faster, but it's not because there's an increase in surface area.  It's because cutting the flutes brings the surface of the barrel closer to the heat source (found at the bottom of the flute).  The process of conduction transfers the heat (a form of energy) through a conductive material and once it reaches the surface, it can be radiated into the environment beyond.  Cutting the flutes speeds that process, allowing the release of energy faster than a uniformly thick barrel.

Convection (Q) is proportional to surface area (A):

Q=hAΔT
Where:
Q=Heat transfer
h=Heat transfer coefficient
A=Area
ΔT=Temperature differential

Quoted: Convection (Q) is proportional to surface area (A):

Sure.  Of course, by itself, this can be a bit misleading.  Let's say I have a second barrel twice as thick as an original (neither fluted).  It will have a greater surface area than the original.  It follows that it will convect a greater amount of energy (heat) than the original, which would be true.  What this formula doesn't account for is how fast that process occurs or how much energy is building up within the material over the course of radiating it in the form of heat to the environment, which is to say, how long the barrel takes to cool.  This question of speed is the factor in favor of fluting for cooling, particularly with regards to hot loads and/or rapid firing.

Fluting (or dimpling) a barrel does, in fact, increase its surface area.  This does include the benefit of being able to radiate heat energy along a greater surface area.  It cools faster than a non-fluted barrel of the same diameter, but also faster than a non-fluted barrel of the same surface area.  As I understand it, the less material (mass) energy must pass through, the faster it reaches the surface to be convected away (all other factors being equal).  So, conduction plus convection equals a cooler and lighter barrel. ::smile::

In theory, one could construct a non-fluted barrel that has the same energy radiation as the fluted barrel in question.  It would, however, be rather more weighty than the fluted example.  This is the argument in favor of weight reduction being the primary rationale for fluting.

To me, it's rather like arguing over which side of a coin is better.  In the case of the coin, you get both sides regardless of which one you prefer and, in the end, it spends the same.

Quoted: Sure.  Of course, by itself, this can be a bit misleading.  Let's say I have a second barrel twice as thick as an original (neither fluted).  It will have a greater surface area than the original.  It follows that it will convect a greater amount of energy (heat) than the original, which would be true.  What this formula doesn't account for is how fast that process occurs or how much energy is building up within the material over the course of radiating it in the form of heat to the environment, which is to say, how long the barrel takes to cool.  This question of speed is the factor in favor of fluting for cooling, particularly with regards to hot loads and/or rapid firing. Fluting (or dimpling) a barrel does, in fact, increase its surface area.  This does include the benefit of being able to radiate heat energy along a greater surface area.  It cools faster than a non-fluted barrel of the same diameter, but also faster than a non-fluted barrel of the same surface area.  As I understand it, the less material (mass) energy must pass through, the faster it reaches the surface to be convected away (all other factors being equal).  So, conduction plus convection equals a cooler and lighter barrel. ::smile::

Yes there are more issues at work, the amount of mass and the conductivity of the material in question, but both of these effect delta T (temperature differential, for simplicity I'm holding the surrounding air temperature as a constant hence the variable in delta T is the surface temperature of the barrel)  and in no way diminish A (surface area).  My point wasn't factors that effect the temperature differential are not important but that you can not dismiss the effect of area.

Quoted: In theory, one could construct a non-fluted barrel that has the same energy radiation as the fluted barrel in question.  It would, however, be rather more weighty than the fluted example.  This is the argument in favor of weight reduction being the primary rationale for fluting.

How about this test, take two barrels, 1 of 0.850" dia. Fluted vs. 0.700" dia. non-fluted barrel, both have the same mass but the fluted barrel has 94% more surface area.  (I'm lazy, I used the numbers from here)  With the amount of mass we are talking about the amount of variance of delta T due to the differences in shapes (remember mass is the same) is going to be insignificant as long as the material is the same.

Speaking only for myself, I'm not prepared to dismiss anything out of hand.  What I was attempting to point out with the coin analogy was that determining what a person's primary reason for fluting would be is the subject of preference, not science.  If you choose weight reduction, you still acquire the cooling benefit.

Similarly, the comparison isn't about two barrels of different diameters.  Using different diameters in the example was to illustrate the difference between fluting and non-fluting to obtain the same surface area.  The question in this case is what benefit comes from or not fluting a particular barrel of a specific diameter, not whether to choose a HBAR and flute it over a thinner, non-fluted barrel.  Even in this case the fluted barrel will be reduced in weight closer to the non-fluted, thinner barrel.

Using this upper as a specific case in point (and the purpose of the thread), with accessories, it's getting weighty.  The heavy SS barrel was selected due to known performance characteristics, cleaning, and environmental factors.  Since it is unlikely to see a great deal of rapid fire (who can say "never"?), heat effect is less the focus than weight reduction.  Again, that's the point.  You get both whether you wish to dismiss the relationship, prefer one line of reasoning, or not.

As Jason has now pointed out, it seems DPMS does not manufacture an upper receiver with feed ramps.  While there's no outright necessity for them, even considering the M4 ramps on the barrel extension, receiver ramps do improve reliability under certain field conditions.  They seem to be desireable, where available.  With that said, the alternatives seem to be :

*Do without the feed ramps in the upper,
*Have feed ramps machined in (this is at least somewhat problematic, due to removal of the anodizing and getting the ramp angles correct),
*Replace the DPMS upper with a CMT upper and not get an exact color match with the DPMS lower receiver,
*Have the CMT upper refinished using Teflon for a color match.

While my understanding is this will not be a "show" gun, weapon operation not being effected, the investment does substantiate some degree of consistency in appearance.  Call it a sign of crafstmanship, if you will.  The questions now become, how does the CMT upper compare to the DPMS in terms of quality of manfacture and performance, first?  Followed by, how well will the halves mate?  There'll be no opportunity to check this until all assemblies are delivered from their respective sources, here.  Then finally, how close is the color match?

Deleted

Anyone else notice the JTAC website and e-mail account have gone offline?

Anyone know of a machinist capable/willing to dimple this barrel?

Watcher contact sr-47, I've talked with him about doing one for me.

Ok.  So, does anyone know how to search for a member?  Even the Search function doesn't let you search specifically for a member.  You can only search for posts from a member within a certain time frame and board.

email sent

Thanks for the e-maill address.  With the Christmas madness now out of the way, I've sent out a note.