Barrel contours and heat

tylerw02

Regular Member
Does anybody have any scientific testing to show how heat affects barrel contours? How long to heat, how long to cool, and accuracy deviation between contours? Please, no anecdotal evidence, let’s keep this scientific.

I’d like information on semi-auto platforms, as I know there is info out there about Socom contours. It would be nice to see government, LW, BCM or others’ continuous taper, and SOCOM contours. I’m certain the results would be interesting.


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Dr. Cornwallis

Regular Member
Does anybody have any scientific testing to show how heat affects barrel contours? How long to heat, how long to cool, and accuracy deviation between contours? Please, no anecdotal evidence, let’s keep this scientific.

I’d like information on semi-auto platforms, as I know there is info out there about Socom contours. It would be nice to see government, LW, BCM or others’ continuous taper, and SOCOM contours. I’m certain the results would be interesting.


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Tyler, I don’t have anything “scientific” to provide and it may be difficult to source something scientific with so many barrel profiles but it’s widely accepted/proven fact that heavier profile barrels take longer to heat up, but consequently take longer to cool down, however, they retain accuracy longer.

Government profile offers no advantage offer an LW profile barrel, as behind the FSB If is a LW barrel.

My KAC utilizes their proprietary profile (light weight ish) and I’ve found that under normal firing conditions there is no noticeable degradation of accuracy.




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tylerw02

Regular Member
Those are my anecdotal conclusions as well. Unfortunately, many believe government barrels will retain accuracy longer than LW barrels but I haven’t got anything to back up my anecdotal experience. Perhaps some personal experimentation is called for.


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Dr. Cornwallis

Regular Member
Those are my anecdotal conclusions as well. Unfortunately, many believe government barrels will retain accuracy longer than LW barrels but I haven’t got anything to back up my anecdotal experience. Perhaps some personal experimentation is called for.


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I wish I could do a side by side but I no longer have a rifle with a .gov profile. Logic and reasoning tell us there should be no reason why a gov profile would retain accuracy any longe than an LW. If it were thicker behind the gas block that would make sense, however, it’s an LW profile for the 2/3 of the barrel and it’s the 2/3 that would matter most in maintaining accuracy under sustained fire.


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tylerw02

Regular Member
Good information, I’ve seen this test but not quite what I’m looking for.


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Dr. Cornwallis

Regular Member
What is the need for such scientific data? Are you employed by a department or agency with whom you’re trying to influence a decision and they’re requiring such evidence? What are you trying to prove with such evidence?

I would recommend searching username “Molon” over on m4c. I believe he has some pretty in depth accuracy reports on various barrels.


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pointblank4445

Established
I know it's not AR-specific, but if you're interested about barrel accuracy vs. contour, look into the history of the Palma barrel contours. The theory is to have a very sharp reduction forward the shank, but continues on with less contour from that point to the muzzle.

While it's a bit dated, there's some good info here:
http://precisionrifleblog.com/2014/11/18/rifle-barrels-what-the-pros-use/

You will note in the profile comparison that the Heavy Palma 30" barrels come in lighter than the 26" MTU profile. Even when cut to shorter length's the Palma's do wonderful work (Hint: Accuracy International has used Palma profiles all the way back to the "Green Meanie") and illustrate that more mass does not equal "more better". The trick is knowing where you can trim it. Similar trends are showing up in AR barrels. Dr. C notes his KAC; there's also BCM's ELW profile as well as what Jim Hodge has going on with his Mod 2 profile to name a few.

In my non-stainless AR barrels, I've had: HK Socom profile, Colt Socom profile, Colt Govt, Centurion/FN medium, FN Govt, FN LW, Criterion medium, (slightly lighter forward GB than Gov) that have been accuracy tested with with nearly the same criteria (LPVO's, Geissele SSA or SSA-E triggers, identical ergonomic setups, all 77gr OTM Black Hills). I've had many more AR's (KAC, BCM, DD, etc) but have only recently started this process. Of those above, the widest variance between the best and worst is probably about 0.5 MOA at most shooting 100y off of a bipod with a rear bag. I've noted just as much if not more accuracy deviation in rifles on occasions when I have changed muzzle devices or opted to shoot from a pack (rail flex/torque dynamic), and I suspect that the who/how the barrel is made may also be more of a factor than contour alone.

Some Molon for ya:
https://www.ar15.com/forums/ar-15/A...From_Colt__Noveske_and_more______/118-687913/
 

tylerw02

Regular Member
What is the need for such scientific data? Are you employed by a department or agency with whom you’re trying to influence a decision and they’re requiring such evidence? What are you trying to prove with such evidence?

I would recommend searching username “Molon” over on m4c. I believe he has some pretty in depth accuracy reports on various barrels.


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I would like to have solid information to point people toward when it comes to making a decision, not necessarily for a department. Perhaps just a personal interest; wanting to be more knowledgeable as a whole. There is a lot of hearsay about the topic.


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Arete

Regular Member
More:


"Daniel Defense 16" CHF lightweight barrel from Rainier Arms RUC rifle. 100 total rounds--10 shot initial group at 50 yards--80 rounds through the gun quick to heat up the barrel--another 10 shot group at 50 yards to see if there was any shift of the zero or if the group opened up"

(Faxon lightweight barrel test)
 
Daniels Defense S2W profile is a tapered profile, its very thick behind the gas block and slowly tapers down to the gas block. I have one of the 16" S2W profile barrels in 6.8 SPC II, but I used an AZ61A Magnesium Alloy handguard to off-set the added weight, so it handles like a typical 16" gov profile 5.56 upper with a typical 6061 T6 handguard (comparable lengths).

36.9 oz for the barrel (ouch!!!) but only 6oz for the 15" mag alloy handguard and 7075 barrel nut = 42.9 oz

I also have a Daniels Defense 16" Light Weight profile barrel in 5.56 (NATO chamber, 1:7). Basically it's Gov. profile behind the gas block, but thinner out front like the original M16 pencil profile. Weighs the same 24oz as a 14.5" Gov profile in the URG-I.

24 oz for the barrel but 14.9 oz for a typical 13.5" Geissele SMR handguard + 7075 barrel nut = 38.9 oz

1/4 lb or 4oz difference is fairly small, most of that being back towards the receiver (due to the barrels profile).

Although a Scout profile 6.8 barrel is 28-29oz, so I could have made the 6.8 upper lighter than even the 5.56 upper. Likewise, with the 5.56 upper, I could have used the lighter handguard paired with the LW barrel for an ultra light setup, but I think it would be too lightweight at that point. I prefer mid-weight guns vs. very heavy or very light.

My goal with the 6.8 upper was to be at a comparable weight to a typical 16" 5.56 setup, but with harder use capability and maximum fire power in an AR-15 frame size, while the 5.56 upper was closer to a general purpose URG-I type setup but with 16" barrel to avoid having to pin the brake. I am quite pleased with both for their roles.

Here is what I got at 50 yards during zeroing of two uppers, both equipped with 1-6x LVPO's from Vortex. Razor HD Gen 2E on the 6.8 SPC upper, Viper PST Gen 2 on the 5.56 upper, both on Badger Ordnance C1 1.70" mounts.

Surprisingly, the scope just happened to be nearly dead on starting out with 6.8 SPC (lucky):
IMG_20210904_213901.jpg
Confirming 10 shot group with 110gr OTM after minor adjustments with VMax (Black) (it's .2 MRAD, so that's about as center as it can get):
IMG_20210904_213840.jpg

Since I ran out of those targets, I did have to use a training target for 5.56 vs. a zeroing target (and it started raining right at the end, so it became polka-dotted), so that MAY have had a small impact on my POA and final accuracy results:
IMG_20210609_213735.jpg

Very comparable velocities, even the BC's were comparable (.363 for 77gr OTM, .360 for 110gr OTM and .370 for 110gr VMax). Same manufacturer (Daniels Defense) of the barrels in their respective calibers, same barrel lengths, same mil-spec forged uppers (squared and bedded), same torque on the barrel nuts (55 ft-lbs with a calibrated wrench), same day, same magnification, same distance, same chrono...you get the idea. I did pull at least one shot on each target due to running out of time and starting to rush. Cadence was about 1-2 seconds per shot, so not exactly eeking every oz of precision either, just practical shooting with decent holds.

Note the upper right circle (last photo) is where I did most of the rough sighting in, then once I got it close to center, I shot the lower left circle (lower left four holes), made one final adjustment and then shot a final 10ish shot group (the center cluster, low left circle). BTW the muzzle velocity for 77gr Razor Core was 2586 FPS average, but I forgot to write it on the target.

These results seem pretty well inline with pointblank's results and the article I attached comparing a Colt SOCOM profile to a Cold Gov profile barrels accuracy seems to further that notion. Not a huge difference for slow fire precision or light duty use at closer ranges. I think the heavier barrels come in to play with recoil (more mass makes for a softer shooting gun) and rapid firing schedules and when pushing longer ranges (300 to 600 yards). Heat is also the enemy of accuracy, hot barrels aren't accurate in any caliber or profile.

Thinner profiles whip more during rapid fire bursts (suppression) weather semi or full auto, while that may not matter when standing (due to recoils affects on POA being far greater), it does start to show up when prone, supported on a bipod, heavy barrels start to really shine. That's exactly why the M27 IAR uses a really have profile between the chamber and the gas block. Less whip, more thermal mass for bursts of fire, while maintaining reasonable accuracy under heavy use but at the expense of weight / balance.

Holy barrels batman, those suckers gotta be nearly 3 lbs! (M27 IAR aka HK416 Heavy Barrel).
DSCN0449.JPG

So it all depends on what your doing. I will say that heavy profiles are slightly more accurate on average even when cold, but it's only about 1/2 MOA with all other things being equal. If you can afford it, why not have one of each? I really like lightweight profiles in 5.56, it suites the low recoil and weight of the cartridge. I like to capitalize on accuracy and heavy use for more potent cartridges (6.8 SPC, 7.62x39, .308 etc.) or that may see use for game animals as well.

If you roll your own uppers, you can achieve comparable weights by using ultra light handguards to mostly off-set the weight of the heavier barrel profiles, gaining the best of both worlds.
 

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pointblank4445

Established
Holy barrels batman, those suckers gotta be nearly 3 lbs! (M27 IAR aka HK416 Heavy Barrel).
View attachment 8551

Those appear to be MR556 barrels (I can see the tab that mates with the carrier on 2 of them).
The M27/416 Socom profile is slightly different than the MR556/MR223.

Fun fact: my legit "M27" countour barrel I used to have was the WORST shooter of any 416 upper/gun I had.

And at this point after doing multiple builds identically but seeing vastly different results in barrels off the same production runs with the same lot match ammo...you just don't know till you know.
 
It seems to me that variations in tolerances, clearances and materials have the biggest impact and any barrel configuration can be sub-par. Maybe HK has some quality issues or they are putting out seconds to the civilian markets? Not sure, I haven't see those issues with Daniels Defense, ARP or even Adams Arms (back when 5.45 training uppers were a thing).

Assembly is also critical, take a good barrel in a loose fitting receiver and accuracy is going to be all over the place. Both of my DD's are good shooters, but back when I started building AR's I was originally using receivers with a loose fit to the barrel extension (Adams Arms), didn't bed the barrels (even though I did torque down the barrel nut) and I'd get random flyers I could never tame. Maybe some of that was the AA piston system, but I know some of that was the clearances between the barrel extension and receiver.

Did you ever check to see with a mic if that M27 contour barrel extension was a tight fit to the receiver? Both my DD barrel extensions were .996 to .997 and both my poverty pony Anderson's were also .996 to .997. So it's a snug fit, but not thermo-fit like a BCM or JP receiver, so I bedded them both with rocksett ceramic engineering adhesive. I also squared the receiver faces with lapping compound and the brownells tool and finally torque the barrels nuts to the middle of the range (I use 55ft-lbs).

Good building techniques seem to really produce some good results overall (I've done this with many barrels and even some cheaper nitride cut rifled barrels shot better after an idealized build process). I'm not sure if HK uses a quick change barrel setup like LMT etc., so in that case you may be at the mercy of the system limitations. I'm not suggesting you did anything wrong, I'm just wondering if that could be a factor in the variations.
 

pointblank4445

Established
Can't say my experience has been any different with DD...either factory built or using thermo/tight-fitting Hodge parts.
One 16" gov profile is now my SPR and can stack 10 rounds in an inch or less @100y. It's twin is more than 2-2.5x times that same conditions, same ammo.

We saw the same with factory DD mk12's and I might still have the pic around to show between the two.

All this being said, I haven't had ANYTHING that hasn't at least been able to consistently print anything decent under 3MOA and something at 2MOA...it just may not be a "match" round and not everything loves 77gr.
 
Are you using Black Hills or IMI? My 77gr stuff is IMI that I bought from SGAmmo, it has been quite a bit more consistent than 62gr Green Tip from my 5.56 upper. Here is what I got with green tip on that same day (keep in mind, these are 50 yard sight ins, so double the group size for 100 yards):

IMG_20210609_213731.jpg

I think the circles are 1 MOA (scope is .2MRAD, so .36 in adjustments at 50yds), so that's about 1.5-2 MOA for green tip (some of the shots in the middle were some of the first hits aiming at the lower left circle before I got it dialed in) on a 1-6x, off S-BRM bipod. Over adjusted, with the bottom three, tweaked elevation with that small left cluster and then made a final windage adjustment as I shot the center circle cluster.

I think that one lone shot in the upper right was literally my first shot at the center target just to see if it was printing somewhere on the paper. Now these are just quick groups, I'm not slow firing, my cadence is about 1 second apart just focusing on a good solid hold. Maybe I could squeeze a bit more out of it with a slower firing schedule, but I think that's a somewhat decent approximation for a more realistic use for SHTF.
 
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