View Full Version : Barrel profile question.
I'd like some thoughts on the difference between a govt profile barrel, and a lightweight barrel. Not in the shape etc, but from more of a technical standpoint. My new 14.5" lightweight barrel is turning out to be a good shooter. I was thinking of doing another but using a 14.5" govt profile one. What I am wondering though, is if the govt profile will really offer any advantages over the lightweight one, disadvantages, or no real difference at all.
One theory I had, was that the govt one might be actually not as good as a full light weight barrel and here is why. Under the handguards, they are basically the same. However, I wonder if the heavy profile on the end of a govt one, may cause the accuracy to drop a bit more than a lightweight once it really gets heated up, due to that extra weight hanging out at the end of the barrel and having more leverage there. I am no engineer, so I dont know if thats even feasible, hence the question.
It was my understanding that when the govt. went to the A2 rifle, the heavier barrel(at the end) was supposed to be adding accuracy. I guess the logic would be less barrel whip? Less vibrations, etc.
I don't know, it's a start--
Mark
I thought that to help eliminate barrel whip, you needed it to be heavier closer to the chamber area than at the muzzle, this making the barrel stiffer and more rigid. I could be wrong though.
I think with the harmonics, it's more at the end. Things such as the length, diameter, type of steel and maybe twist may affect the level of it.
I don't have any technical knowledge on this.
Mark
Long Range Trigger Monkey
12-31-06, 00:03
I don't think that barrel whip really comes into play with an AR15, it's more in the realm of free floated barrels where the barrel can react the same way everytime. With an AR barrel, typically there's all kinds of stuff that will affect the harmonics of a barrel, sling pressure, grip pressure, whether or not you're shooting off a bag or not, all of these things will make the barrel respond differently. Also barrel contours really don't do much for accuracy and this is true for bolt guns and gas guns, it's all in the ammunition and how good the bore/rifling is. The only thing a heavier barrel will have over a government or pencil contour barrel is heat dissipation which would be able to keep the barrel cooler longer but the accuracy gain would be marginal. If you were shooting match ammunition out of a government profile barrel and a pencil contour barrel, I'm sure you wouldn't be able to see much difference in accuracy.
FWIW, and if I remember correctly, JP Enterprises uses a thicker profile forward of the gas block compared to under the handguards because for the same weight, it provides a little more accuracy.
Hmm.... interesting. Thanks Zak.
deadwood83
12-31-06, 13:21
Alright so now i'm going to throw in the confusing part. If less weight under the handguards provides beter accuracy for the weight then why on the SPR and SDM rifles is the barrel heavier under the handguards?
Sorry, I can't find where I might have read that, unless it was in a conversaion with them, or I am imagining it. I don't think obsessing over it is worthwhile.. If you get a quality barrel and have someone who knows how to put a solid upper together build it, just tune the profile for the weight you want. Most people think you need a lot of metal to make an accurate barrel. Paul (MSTN) and I spec'd my 17" to survive an "ITRC" stage (300-400 rounds in 15-20 minutes) and it is around 0.70" under the handguards. Last time I had a big scope on it, it shot 1/3rd MOA.
-z
K.L. Davis
12-31-06, 13:32
Need to keep in mind that we are talking about battle weapons... Match rifles are a totally different critter. As it was told to us when we went to the A2, that thicker section of barrel was there because the thinner barrels would bend in front of the sight... yes, the conventional handguards do help keep the barrel from bending from the sight back, but PFC Dilligaff was able to figure out how to bend the barrel out front.
Yes, muzzle weight can/does add to accuracy if properly tuned. The static weight difference of having to "whip" the weight, can help to reduce the harmonics of the barrel.
SPRs and family use a heavier barrel because it is free floated and not partially supported by the handguards at the front sight.
KL, you hit on part of my concern/question/interest. On my latest build, I used a 14.5" full lightweight barrel, and standard handguards. I've only had a chance to shoot it at 50 yrds, but at that range using M193, its really nailing the shots nicely, even after its had a few mags through it and gotten heated up a bit. I like the gun so much, I am thinking about building a second one, and free floating it, but I cant decide if I should use a govt profile one, or just go with another full lightweight one.
Not AR related but alot of rimfire shooters will install an adjustable screw pressure pad at the front end of the stock and "tune" the barrel to it's most available accuracy in conjuction with the load used. Each rifle is different.
So, alot of variables involved when shooting the AR as well. I would say for the 14.5 lw barrel with the intended role, no worries, just shoot it.:cool:
deadwood83
12-31-06, 14:40
Thanks for the explanation KL. That makes a lot of sense when I think about it (but I never would have put it together like you did lol.)
mark5pt56 I know exactly what you're talking about, being a rimfire shooter myself.
However AR shooting is different in that most of us aren't sitting 25 feet from the target with a 14x scope trying to get 5 consecutive groups under .25 inches. I say most because I am guilty of that very act, not that there's anything wrong with it.:D
Thanks for the explanation KL. That makes a lot of sense when I think about it (but I never would have put it together like you did lol.)
mark5pt56 I know exactly what you're talking about, being a rimfire shooter myself.
However AR shooting is different in that most of us aren't sitting 25 feet from the target with a 14x scope trying to get 5 consecutive groups under .25 inches. I say most because I am guilty of that very act, not that there's anything wrong with it.:D
Do you mean 25 yards? I never believed the heartbeat crap until I had a 24x at 25 yards and saw the cross hairs flutter with my heartbeat when I gripped the pistol!(TC in .22lr)
Being somewhat old school, and in the service during the transition from the A1 to the A2, I heard that one of the main reason for bent barrels was that some of the grunts were using the barrel of their A1's to bust the straps on the MRE cases, thus resulting in bent barrels.
Not sure if it's true, but sounds about right.
Sorry, Hawkeye, didn't mean to hijack your thread.
No highjack at all my friend. I had an A1 in basic, and my first rifle at my NG unit was an A1, then I got an A2 later. Heard the same thing regarding the front of the barrel being heavier. Grunts using it as a pry bar.
FWIW my #1 rifle is a 16" "pencil" barrel from a Colt 6520 and it is completely free-floated inside of a Daniel Defense 9.0 rail with a cut down stock hidden underneath. I have not had any accuracy issues with surplus-quality ammo. I do not think that free-floating a 16" lightweight barrel has an adverse effect on accuracy. I guess I could maybe see how free-floating a 20" GI profile barrel might, with the extra weight at the end and the thinner profile in the middle, but I have no way to test that.
My "built" 6520 #1 gun is no more and no less accurate than my stock 6520 #3 gun.
I thought the .gov went with the thin profile under the handguards for the A2 because they needed it to fit the M203s, which were designed for A1 barrels.
greyman556
06-17-15, 07:32
I have seen "pencil barrel" AR 15's that shoot some amazing 100 yard groups. There are advantages to lighter rifle especially if it is carried day in and out.
Barrel profile is interesting topic. JP Enterprises uses barrel contour (free floated) that is counter-intuitive; barrel is smaller near chamber (hottest point) to readily dissipate heat and barrel is larger at end (forward of gas port) to aid accuracy (must be harmonics). Believe AMU uses JP Enterprise rifles.
"Barrel myth" stories are entertaining also.
I heard that one of the main reason for bent barrels was that some of the grunts were using the barrel of their A1's to bust the straps on the MRE cases, thus resulting in bent barrels.
Urban legend.
From David A. Lutz, Lt. Col. USMC (Ret'd):
“Since The M16A2 Product Improvement Program (1980-1983) was my program, this is the down & dirty on the barrel thickness issue. We (Marines) were replacing a lot of "bent" barrels that were determined to be "bent" because the Armorer's Bore Drop Gauge would not freely pass through some barrels during Ordnance Inspections (LTI's). So the Logisitcs people had "Barrels Bending" on their list of "M16A1" things to "Improve" right after listing "Handguards Breaking."
We "experts" thought this bending was from rough handling like during bayonet drills, etc., as an absence of any mid-barrel handguard damage in these rifles made one assume the fulcrum of such bending was the bayonet lug. So we made that part of the barel thicker because we did not want the excess weight of a full length heavy barrel. In testing using the bayonet lug as a fulcrum, and applying calibrated mechanical pressure to the muzzle, the new barrel was about 9 times more resistant to bend and take a set than an M16A1 profile. So we went with this "improvement."
However, soon after I started using a bore scope with a video recorder and monitor to inspect "bent" barrels. What I found was a mound of bullet jacket material at their gas ports. This build up was caused by a burr left from drilling/reaming the gas port. This was where the Armorer's Drop Gauge was geting stuck. When we removed this "mound", the barrels would all pass the Drop Gauge. We let Colt know what we had deduced, and that is one reason they kept models of "A2's" in their line-up with A1 profile barrels. However, the A2 profile was already down the road for the US Military. So about the only advantage of the A2 profile was to give the rifle a little more muzzle hang. This was noted by most all the Operational Test paticipants, especially when they fired the standing/off-hand leg of our rifle qualification course.
So my advice to mlitary armorers is to never replace a bent barrel until you visually check the gas port, or at least scrub the hell out of the gas port area with a new bore brush and an electric drill. And thank God for chrome bores!”
I have seen "pencil barrel" AR 15's that shoot some amazing 100 yard groups. There are advantages to lighter rifle especially if it is carried day in and out.
Barrel profile is interesting topic. JP Enterprises uses barrel contour (free floated) that is counter-intuitive; barrel is smaller near chamber (hottest point) to readily dissipate heat and barrel is larger at end (forward of gas port) to aid accuracy (must be harmonics). Believe AMU uses JP Enterprise rifles.
"Barrel myth" stories are entertaining also.Wow, talk about a necro post...
A thinner section doesn't really dissipate heat any better. Yes, it cools down faster than a thick section, but it also heats up faster (and therefore tends to get hotter). If you're firing a moderate pace for a long time to get the barrel warm but not scorching hot, continuing that pace until it reaches and stays at a given temperature, the greater surface area of the thicker barrel should result in a lower equilibrium temperature vs the lightweight barrel. Fluting can increase surface area while decreasing weight, and for the same situation the fluted barrel would end up cooler than the heavy barrel. The cross section of a fluted heavy barrel is more efficient when it comes to stiffness per weight as well, when compared to an equal weight medium barrel - the farther from the center the metal is, the more it does for rigidity. The most efficient (for stiffness vs weight) fluting pattern would remove as much material near the center as possible while leaving material farther out - fluting with a dovetail for example. Of course, sharp interior corners cause problems, so more realistic is to cut with a square end mill with slightly rounded corners (to avoid stress risers).
Also barrel contours really don't do much for accuracy and this is true for bolt guns and gas guns . . .
False.
https://app.box.com/shared/static/h9jeod1yjx.jpg
Molon, are these samples of one, or were several barrels from the same maker tested?
I think you are right about this, BTW, but I've seen a lot of difference between individual barrels to take a simple of one as representative.
If Precision were ever a concern, especially once things started getting Heated up, I'd never willingly opt for a Pencil or Lightweight Barrel.
I thought the .gov went with the thin profile under the handguards for the A2 because they needed it to fit the M203s, which were designed for A1 barrels.
The real reason.....
Powered by vBulletin® Version 4.2.5 Copyright © 2024 vBulletin Solutions, Inc. All rights reserved.