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I guess we're all pretty much aware that up to a point, a longer rifle barrel generally produces higher velocities. To my surprise, I just discovered, "Tain't necessarily so!"

We know that the velocities spec'd on factory ammo is based on a SAAMI specified 24" barrel. That usually means we aren't going to see those box velocities from our 16" ARs.

Recently I decided to go with an 18" barrel just to get a bit more velocity (and energy) from an AR. I chose a Daniel Defense S2W 18" barrel with a 1:7 twist. Imagine how surprised I was when two BCM 16" barrels shot 5% higher velocities with 5 different brands of ammo than the longer DD 18"!

Here's a pic of the three rifles, not that it means much, but...the 18" is in the middle - - - and I had to make the pic in low light so it's grainy:



Here is the chrono results. I emboldened the fastest for each brand of bullet. Notice in every case the 16" barrels were faster for all brands of bullets than the 18":

............................Dan Def 18"....BCM 16" SS410....BCM 16" LWE
HPR 55 gr VMAX........2658..................2759.................2935
IMI 77 gr..................2654..................2624..................2728
IMI 69 gr..................2817..................2895..................2863
Aus Outback 69 gr......2458.................2590..................2671
Aus Outback 55 gr......2764.................2786..................2894

The devil is in the details:

- All are mid length gas systems
- All three had adjustable gas blocks that were tuned to the ammo being shot
- The DD 18" has a salt bath nitride internal with a 1:7 twist, DI
- The BCM SS410 16" is ion bonded with a 1:8 twist with Adams Arms piston drive
- The BCM 16" Light Weight Enhanced is chrome lined with a 1:7 twist with Adams Arms piston drive
At first, I thought the difference was in the twist rate, but then I realized the BCM LWE 16" with the tighter 1:7 was faster than both the DD with 1:7 and the BCM SS410 with the 1:8???

So, what did DD have to say about this?

"The differences you are seeing are in relation to the gas port hole size and the length of the barrel. The dwell time on your 18" Mid-Length S2W and the dwell time on the BCM 16" will be different due to the difference in length and also the differences in the manufacturing process of each barrel. The gas port hole diameter being the biggest contributing factor.

Our 18" 5.56 Mid-Length S2W barrel has a gas port hole diameter of 0.07. I am unsure as to what exact dimensions the BCM 16" barrel is made to.

Another factor in this will be the gas block used and ensuring both gas blocks are properly installed. If the gas isn't cycling as it should, you would see a difference in velocity (over pressure or under pressure). "


I presume there is some factual basis for their claims, but starting with the last first, the gas blocks, all three had adjustable gas blocks and were adjusted to the ammo. Not buying the dwell time claim at all. Gas port hole diameter could play a part I suppose.

And I suspect manufacturing tolerances and barrel treatments, i.e. chrome lined etc. could make a difference too.

But the lesson here is, just because you get a longer barrel doesn't necessarily mean you'll gain any velocity. In fact, my results show the 18" barrel is about 4-5% slower than the two 16" barrels.

So the question is, why would I want to tote around 2 extra inches of barrel to get less velocity???
 

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When I was an avid (young) black-powder shooter, we'd put a white sheet on the ground in front of the muzzle. It was a ******* chronograph. Increase the powder charge until un-burnt specks started sprinkling down. Then we knew that we were gettin' the maximum FPS. (*and quit wasting powder for simply more...boom & smoke) :biggrin2:
 

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So the question is, why would I want to tote around 2 extra inches of barrel to get less velocity???
The answer is that you don't!

Thanks a lot tangle. now I need to invest in a chrono.
 

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Another variable is powder burning rate, faster burning powders tend to give higher velocities in shorter barrels whereas slower burning powders need longer barrels to fully consume all of the powder for higher velocities.
 

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I seem to remember sometime back in the 1980s someone did a cartridge study (I think it was a .357 mag but don't quote me) wherein he chrono-graphed a series of shots from the same barrel, cutting off an inch at a time, re-crowning, then shooting again. He determined there was a point where the friction from the barrel started slowing the bullet. From that one could theorize that modern ammo optimized for the shorter barrel of the M4 might yield similar results independent of the effects of the gas system. It might also be that the internal friction is negligible compared to the effects of the gas system.

It might be interesting to compare some ammo from the '70s to some of current manufacture.
 

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I've read that tolerances on the chambers can make a difference, for revolvers anyway. The tighter fit the higher the velocity, is the theory, as expansion of the case uses energy. Not sure if that's a factor for rifles.
 

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Discussion Starter #8
Ironically, I had just read an article in AR 15 magazine where they shot 19 brands of ammo through a gun as they decreased the barrel in 1 inch increments. The article pretty much found the same thing the BBTI did with on exception. There was one case where the velocity increased by 1 fps going from 18" to 16".

But this is a bit different in that those two resources use the same barrel so all the tolerances remain constant. I was using two barrels of different lengths from different manufacturers. Hence there could be variations in barrel characteristics. In fact, must have been because I can't think of another reason why the longer barrel would be slower????
 

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Another variable is powder burning rate, faster burning powders tend to give higher velocities in shorter barrels whereas slower burning powders need longer barrels to fully consume all of the powder for higher velocities.
I believe this is a key element.
 

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I came across this same thing a few weeks ago. I loaded up 240 gr XTPs with the same powder charge, and the 4 5/8th inch barrel was getting 30 fps faster than the 6 inch barrel with my 44 mags. Mind you the 6 inch barrel is a S&W and the 4 5/8th is a Ruger.

About 6 months ago I was chronographing factory ammo and the 6 inch barrel was about 40 fps faster. There's a lot of factors at play with this.
 

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In economics, its called the Law of Diminishing Returns.
 

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Discussion Starter #12
I hear ya guys, but let's remember that the ammo I'm using is chrono'd and spec'd by the manufacturer from a 24" SAAMI standard test barrel. If going from 16" up to 18" drops the velocity by about 4%, can you imagine how much it would drop going from 16" to the 24" std barrel length!

Plus, in a study linked by 10th, and another study I referenced, both showed that as the barrels got shorter, the velocity consistently decreased. Since they used factory loads, burn rates cannot account for the decease in velocity going from 16" to the longer 18".

And in yet another barrel shortening study, it was shown that the optimum length for the 5.56 is 20" and velocities decreased as the barrel was shortened - and it decreased for a longer barrel, but that was because the M193 ammo was developed for a 20" barrel of the original AR.

I can't prove it, but my theory is that the bore dimensions and bore treatment, i.e. chrome vs nitride is at the root of this.
I guess one thing I could try is a BCM 18" barrel like the BCM 16" I have and see if it's slower. Please send PMs and I'll tell you where to send contributions :tongue:
 

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But this is a bit different in that those two resources use the same barrel so all the tolerances remain constant. I was using two barrels of different lengths from different manufacturers. Hence there could be variations in barrel characteristics. In fact, must have been because I can't think of another reason why the longer barrel would be slower????
My gut says the answer is buried in DD's response. I'm not a dyed-in-the-wool pneumatics guy, but among the aerospace stuff I test are all sorts of valves, many of which control bleed air flow from jet engines. Most of my company's valves are pneumatically operated, taking higher pressure air as "muscle" to open and close the butterfly or poppet which control the air passing through the valve.

Certain valves require very fast response time to manage things like turbine overspeed, compressor surge & stall, etc. The length and volume of the tube conveying the servo (muscle) pressure to the valve actuator are critical factors in determining that response time. The servo supply pressure and temperature are important as well, as is the size of the orifice (or orifices) feeding the servo air from the engine to the valve's actuator. You could have a short, large diameter tube from the engine to the valve actuator, but if the hot air is being fed through a tiny orifice, the pressure rise in the combined volume of the tube and actuator will be very slow. Likewise, if the orifice is large but it feeds the valve through a long, narrow-diameter tube, the actuator will be slow to respond. The valve designer's challenge is to optimize the actuator characteristics to the valve force needed and the servo supply pressure, temperature and flow available.

Bringing this back to your test guns: I think some uncontrolled variables involved here are the size of the ports in the gas block and the effective orifice of the barrel port combined with your adjustable gas blocks, the volume of the gas tubes, and the effect of the piston with its "stiction" and inertia versus the DI actions.

You noted that your adjustable gas blocks were "tuned to the ammo being shot." Was that to optimize velocity or to ensure adequate bolt operation? My inquisitive mind is wondering about the combination of the adjustable gas block and the barrel gas port - could a condition be created where enough gas is bled off to drive the piston that the effective pressure driving the bullet is diminished as the bullet is still in the barrel? As another post alluded, the burn characteristics of different powders could be another factor as well.

This is all highly speculative, just food for thought from another engineer. My "design of experiment" to chase this would be look at different gas block adjustment settings, using just the DD and the BCM LWE with the Vmax, IMI 69 gr and the Aus 68 gr just to cover the corners of the box.

Great post, and very informative.
 

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Discussion Starter #14
My gut says the answer is buried in DD's response. I'm not a dyed-in-the-wool pneumatics guy, but among the aerospace stuff I test are all sorts of valves, many of which control bleed air flow from jet engines. Most of my company's valves are pneumatically operated, taking higher pressure air as "muscle" to open and close the butterfly or poppet which control the air passing through the valve.

Certain valves require very fast response time to manage things like turbine overspeed, compressor surge & stall, etc. The length and volume of the tube conveying the servo (muscle) pressure to the valve actuator are critical factors in determining that response time. The servo supply pressure and temperature are important as well, as is the size of the orifice (or orifices) feeding the servo air from the engine to the valve's actuator. You could have a short, large diameter tube from the engine to the valve actuator, but if the hot air is being fed through a tiny orifice, the pressure rise in the combined volume of the tube and actuator will be very slow. Likewise, if the orifice is large but it feeds the valve through a long, narrow-diameter tube, the actuator will be slow to respond. The valve designer's challenge is to optimize the actuator characteristics to the valve force needed and the servo supply pressure, temperature and flow available.
Interesting stuff!

...Bringing this back to your test guns: I think some uncontrolled variables involved here are the size of the ports in the gas block and the effective orifice of the barrel port combined with your adjustable gas blocks, the volume of the gas tubes, and the effect of the piston with its "stiction" and inertia versus the DI actions.
I think so too, but I have to admit, I don't have a good feel at this point why longer is slower???

I considered that some of the difference might be the DI vs piston, but then I recalled I got the same results some time back when both guns were DI.

As for burn rate, I just don't see how the burn rate is a factor - because, in every study I've seen, longer barrels, up to 26" produce more velocity than shorter barrels with factory loads. For example, in the AR-15 Magazine study where they chrono'd 19 factory loads in barrel increments of 1" starting at 26", in all cases the 26" velocity was significantly higher than 18" velocities. That tells us that the additional powder burn/burn rate etc. in the longer barrels is more than enough to overcome any additional drag caused by the longer barrels.

There were a few exceptions but they were very slight. E.g. a couple of loads showed very slightly higher velocity at 24" than 26", and we're talkin' like 2-3 fps. And, we should note that manufacturers maximize their "box" velocities around a 24" barrel as specified by SAAMI.

So with 19 factory loads, longer barrels produced higher velocities. But that is based on one barrel being shortened rather than two different barrels like I'm using.

Good point about the gas tube and barrel port. The gas port (orifice) on the 18" is 0.07" according to DD and that's pretty much a norm for 16" - 18" barrels. I'll have to measure the 16" just for the sake of knowing, or maybe BCM can tell me.

...You noted that your adjustable gas blocks were "tuned to the ammo being shot." Was that to optimize velocity or to ensure adequate bolt operation? My inquisitive mind is wondering about the combination of the adjustable gas block and the barrel gas port - could a condition be created where enough gas is bled off to drive the piston that the effective pressure driving the bullet is diminished as the bullet is still in the barrel? As another post alluded, the burn characteristics of different powders could be another factor as well.
The tuning was to minimize recoil while yielding reliable bolt operation. I tuned both the DI and piston the same way.

The piston is on the 16" which is the faster of the two barrels. But some time back I did this same thing with both guns set up as DI and got the same results. :(

...This is all highly speculative, just food for thought from another engineer.
And it is most welcome - I appreciate any thoughts you have - they've all been excellent!

...My "design of experiment" to chase this would be look at different gas block adjustment settings, using just the DD and the BCM LWE with the Vmax, IMI 69 gr and the Aus 68 gr just to cover the corners of the box.
That's a good thought! I can chrono the two rifles at two different gas settings and see how the different gas settings affect velocity - that should be interesting!

...Great post, and very informative.
Thanks! What would really make it a great post is to figure out why this is happening. Right now, I'm thinking it's got something to do with bore tightness and/or internal barrel treatment, i.e. chrome (16") vs nitride (18"), but that's purely guessing at this point.
 

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Discussion Starter #15
OK, here's an update. This was inspired by gasmitty. I did this to see how much effect the gas bleed-off has on velocity. I shot the DD 18" DI and the BCM 16" AA piston with the three brands of ammo listed in the table below. I shot and chronographed each gun with each ammo with the gas ports fully closed and generously opened. The table is necessarily complex because of all the info and conditions, but that's the nature of the beast I guess. Also, I had to use "_" to force desired alignment of columns, etc.

So basically, I shot 5 shots with the gas block closed, be it DI or piston, and 5 shots with it open such that it heartily cycled the action. Then I went to the next brand of ammo and repeated the drill.

I calculated the % difference in the closed and open gas port for each gun and ammo brand. So here's the results:

Ammo: Aus Outback 69 gr BTHPIMI 69 gr BTHPHPR_55_gr_VMAX
gunclosed___SD____open___SD_closed____SD___open____SD_closed____SD___open____SD_
Daniel_Defense_18"_2539__32.0___2518___29.6_2827___17.6___2828__ 34.32707____42.8___2663___42.2
% diff: open-clsd0.83%-0.04%1.65%
gunclosed___SD____open___SD_closed___SD____open___SD_closed___SD____open___SD_
BCM LWE 16"_2641__19.8___2642___23.2_2877_._30.1___2905___19.7_2937_._82.1___2912_._55.8
% diff: open-clsd0.0%-0.96%0.86%

Unfortunately, I'm not sure that this sheds much light on the problem, but maybe as we think about it...
 

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There are always going to be faster and slower barrels. That is why I disagree with people who give blanket indications of velocity from X" of barrel. The barrel rifling may be "cut" differently, rougher or smoother, one may have a .001 tighter bore.
 

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Tangle, you move fast!

The biggest velocity delta from gas block closed to open is 1.65% - a whopping 44 ft/sec out of 2600. All the other deltas were under 1%... I think that says we can rule out the effects of gas port size (which includes gas block open or closed). In fact, the differences are so small that in once case (BCM with the 69 gr Aus ammo) the velocity was lower with the gas block open, which runs counter to what we'd expect. Most likely the variability in the ammo is masking any small effect that might surface after maybe a few thousand rounds. I think it was worth the time to run the experiment, though.

I think the velocity anomaly between the guns (shorter being faster) must be due to the barrels. I'd propose "slugging" the barrels with a lead ball (or maybe a .22 rimfire bullet) but I bet you'd have to measure to 0.0001" or better to see a difference, which means an optical comparator or other pricey instrument few of us have in our toolboxes. That would also not reveal any difference in friction between barrels, which could also be a contributor.

What's next, professor?
 

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Discussion Starter #18
Tangle, you move fast!
LOL - not as fast as it might seem - there were some guys at the range and they would shoot a bit and then want to walk out to their target to inspect it. Took for ever! I guess they hadn't heard of a spotting scope (or couldn't afford one).

Oh, it's Spring Break for me and my wife and I are at Fairfield Glade, TN vacationing. I wouldn't hear of coming here because I don't play golf or bingo, and then I discovered a totally separate Fairfield Glade Sportsman's Club with a rifle range out to 300 yards and they are going to expand it to 500 yards sometime this year! So I joined and my wife gets some badly needed down time and I take off to the range that's only 6 miles from where we stay. That way she gets to rest or whatever without having to hear, "What do you want to do now?" every 10 minutes - anyway, that's why I could move so fast.

...The biggest velocity delta from gas block closed to open is 1.65% - a whopping 44 ft/sec out of 2600. All the other deltas were under 1%... I think that says we can rule out the effects of gas port size (which includes gas block open or closed). In fact, the differences are so small that in once case (BCM with the 69 gr Aus ammo) the velocity was lower with the gas block open, which runs counter to what we'd expect. Most likely the variability in the ammo is masking any small effect that might surface after maybe a few thousand rounds. I think it was worth the time to run the experiment, though.
My friend you have a keen eye and analytical mind! I was quite surprised to see such little difference between open and closed.

I concur with your assessment. The differences are too small to be significant, except for that 1.65% and that may be a statistical anomaly.

And yes sir it was worth running the experiment. It's pertinent to know that gas ports have very little effect on velocity.

...I think the velocity anomaly between the guns (shorter being faster) must be due to the barrels. I'd propose "slugging" the barrels with a lead ball (or maybe a .22 rimfire bullet) but I bet you'd have to measure to 0.0001" or better to see a difference, which means an optical comparator or other pricey instrument few of us have in our toolboxes. That would also not reveal any difference in friction between barrels, which could also be a contributor.
Everything does seem to be pointing to barrel characteristics.

I guess we have to accept that there can be enough differences in barrel characteristics to swamp out increased length. I know I've learned a lesson from this - buying a longer barrel doesn't guarantee more velocity. It probably would if one bought the same type of barrel from the same manufacturer, but I would not be surprised if even that isn't a sure thing.

...What's next, professor?
Hmmm, good question :tongue:

BTW, I have an answer to the question I asked, "So the question is, why would I want to tote around 2 extra inches of barrel to get less velocity???" but I'll have to post that in a thread tomorrow.
 

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My foray into long range shooting, and in a new caliber has taught me several things.

1 - Not all barrels are created equal - even by the same MFG. Multiple Reports of two barrels from the same company having anywhere from 50 to 125 fps variance.
2 - You are shooting a virgin barrel against two "broke in" barrels. Barrels get faster with age to a point then start slowing down as they get worn out.
3 - As it's been pointed out, barrel manufacture. You are comparing Bananas to Radishes here.
4 - {I am speculating here} Just like chamber reamers - as they get worn, the chamber get tighter. I would say the same for barrel cutting.
5 - You can post pics of 3 rifles here, but nothing on your lube test. For shame.
6 - You have too much time and recreational income at your disposal, starting to make some of us jealous....by us I mean Me.

I do agree that what you have is a bit odd. Typical FPS drop is (IIRC) 10 per inch from 24"-18" then north of 30 from there.
 

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My foray into long range shooting, and in a new caliber has taught me several things.

1 - Not all barrels are created equal - even by the same MFG. Multiple Reports of two barrels from the same company having anywhere from 50 to 125 fps variance.
Based on this recent experience - Right on!

2 - You are shooting a virgin barrel against two "broke in" barrels. Barrels get faster with age to a point then start slowing down as they get worn out.
True! I recall two barrels I bought and both exhibited awful accuracy; I was ready to send them back to the manufacturer. But I kept shooting them and they became tack drivers - just from shooting them.

3 - As it's been pointed out, barrel manufacture. You are comparing Bananas to Radishes here.
4 - {I am speculating here} Just like chamber reamers - as they get worn, the chamber get tighter. I would say the same for barrel cutting.
I have to agree. we've about eliminated any other variable. And you are right about the tolerances. There is bound to be a difference in tolerances in the first barrel produced and the thousandth.

5 - You can post pics of 3 rifles here, but nothing on your lube test. For shame.
I know, I know - I'm workin' on it. I'm out of town until this afternoon and to my surprise didn't have the pics with me when I started to post them.

6 - You have too much time and recreational income at your disposal, starting to make some of us jealous....by us I mean Me.
Well, if it helps, and I doubt it will, you have to get old and get things paid off and draw full SS.

I do agree that what you have is a bit odd. Typical FPS drop is (IIRC) 10 per inch from 24"-18" then north of 30 from there.
I can't say that it's odd, surprising is how it struck me, with some disappointment in the mix too. But I think you identified the problem as manufacturing variances be it from one manufacturer or more than one. In fact, that's what I said back in the OP, "And I suspect manufacturing tolerances and barrel treatments, i.e. chrome lined etc. could make a difference too." and in post #8, "Hence there could be variations in barrel characteristics. In fact, must have been because I can't think of another reason why the longer barrel would be slower????" - great minds think alike.
 
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