BS Busters Number 87: The Myth of the 18 Inch Barrel Velocity

[Molon]

BS Busters Number 87: The Myth of the 18 Inch Barrel Velocity



The BS: “20 inch barrels for the AR-15 are obsolete because 18 inch barrels produce muzzle velocities that are only a few feet per second less than the muzzle velocities of 20 inch barrels.”

The Facts: If you’re going to make a valid comparison between the muzzle velocity of an 18 inch barrel and a 20 inch barrel you need to use the same type of barrels, from the same manufacturer, made from the same steel, with the same type of rifling and twist-rate and the same chamber. Comparing the muzzle velocity of a stainless-steel 18 inch barrel that has a 223 Wylde chamber, polygonal rifling and 1:8” twist, to that of a 20 inch chrome-lined CMV barrel that has a 5.56mm NATO chamber, standard rifling and a 1:7” twist is asinine; yet that’s just what the Internet Commandos and other self-anointed SMEs do in order to make their ridiculous claim.

Secondly, in order to make a valid comparison, you need to chronograph a large enough number of shots from each barrel to remove the variable of the velocity deviations of the ammunition itself from the equation; piss-ant five-shot strings don’t cut it. Using quality ammunition with a low standard deviation is also a plus.

Thirdly, in order to make a valid comparison, you need to use a chronograph that has verifiable readings. Most of the non-industrial chronographs currently available simply aren’t up to the task; with one notable exception, that exception being the Oehler 35P chronograph with proof-screen technology.

The Oehler 35P chronograph is actually two chronographs in one package that takes two separate chronograph readings for each shot and then has its onboard computer analyze the data to determine if there is any statistically significant difference between the two readings. If there is, the chronograph flags the shot to let you know that the data is invalid. Obviously, testing should also be conducted under the same atmospheric conditions.


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The Data. On the same day, following the methodology described above, I conducted a muzzle velocity comparison between a stainless-steel Noveske 20 inch DCM barrel that has polygonal rifling, a 1:7” twist and a Noveske Match Mod 0 chamber, to that of a stainless-steel Noveske 18 inch SPR barrel that also has polygonal rifling, a 1:7” twist and a Novekse Match Mod 0 chamber. An Oehler 35P chronograph with proof screen technology was used for obtaining and recording the velocity data. (Velocities listed below are muzzle velocities, as calculated from the instrumental velocities using Oehler’s Ballistic Explorer software.


20 inch Noveske barrel
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18 inch Noveske barrel
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Thirty shots of Hornady’s 5.56mm 75 grain TAP T2 were fired over the chronograph from each barrel described above. (Thirty occurrences of the subject being studied are what the statisticians like to refer to as a “large sample.”) The shots were fired in a single-loading manner from a Bob-sled to allow the bolt to lock-back and the chamber to cool between each shot. A delay of 30 seconds between the loading of each round as well as the round being fired within three seconds of being loaded was performed in order to mitigate the effects of “chamber soaking” on the velocity readings.

The average muzzle velocity of the 30 shots of TAP T2 fired from the 20 inch Noveske barrel was 2885 FPS. The average muzzle velocity of the 30 shots fired from the 18 inch Noveske barrel was 2779 FPS; for a difference of 106 FPS.

I also conducted the same test in the same manner as above using the Black Hills 5.56mm 50 grain TSX ammunition. Thirty shots of the Black Hills load fired from the 20 inch barrel produced a muzzle velocity of 3414 FPS. The thirty shots fired from the 18 inch barrel had a muzzle velocity of 3305 FPS; for a difference of 109 FPS.



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Another valid method of comparing the muzzle velocities of different barrel lengths is to use the same single barrel for testing; that is, start the chronographing with the barrel at a length of 20 inches, then remove two inches from the muzzle of the barrel for a length of 18 inches, recrown the barrel and then chronograph at that length. I’m not about to lop off pieces from the muzzle of my Noveske barrel, but the good folks at Accuratereloading.com have conducted just such a test.

Using the barrel of a Sako bolt action rifle, they chronographed 70 shots of assorted 223 Remington 52 grain hand-loads from each length of the sectioned barrel. The difference in the average muzzle velocities of the 20 inch section of barrel and the 18 inch section of the barrel was 99 FPS.



....

[TGS]

The BS: “20 inch barrels for the AR-15 are obsolete because 18 inch barrels produce muzzle velocities that are only a few feet per second less than the muzzle velocities of 20 inch barrels.”

The Facts: If you’re going to make a valid comparison between the muzzle velocity of an 18 inch barrel and a 20 inch barrel you need to use the same type of barrels, from the same manufacturer, made from the same steel, with the same type of rifling and twist-rate and the same chamber. Comparing the muzzle velocity of a stainless-steel 18 inch barrel that has a 223 Wylde chamber, polygonal rifling and 1:8” twist, to that of a 20 inch chrome-lined CMV barrel that has a 5.56mm NATO chamber, standard rifling and a 1:7” twist is asinine; yet that’s just what the Internet Commandos and other self-anointed SMEs do in order to make their ridiculous claim.


Maybe I'm not privvy to the conversations you have been involved in, but to me it seems like you're misrepresenting what the common idea is. At least, what I've heard people commonly say...in that an 18" will often successfully fulfill the same application as a 20".

Given that, will the ~100fps difference make a huge difference in the efficacy of a medium range purposed 5.56 rifle?

[Duke2424]

It's so refreshing to read the truth from a scientific approach. Thank you for the time and effort Molon!

[DocGKR]

Well done sir!

I don't particularly like 18" barrels and have disposed of all my SPR/Mk12 type rifles.

[Little Creek]

[QUOTE=DocGKR;269005]Well done sir!

What is a few fps?

I have some 16" chrome lined barrels from Colt (carbine length gas system) and a DD (mid-length gas system). Both are lw. I recently bought a DDV11 S2W 18" barrel. I wanted the heavy barrel with rifle length gas system, but the 15" SLiM rail was the clincher. I have not yet shot it as I am saving up for a SSA-E or SD-E trigger and an optic suitable to 500 yard target/predator shooting. Why do others buy 18" barrels? Are they as popular in 3-gun as I think they appear to be?

[jetfire]

While this is all interesting in abstract, I once again find myself asking "what's the point?" 100 FPS isn't a significant enough loss to affect terminal ballistics, so unless it's hurting a metric that actually matters like accuracy, why does it even matter?

[Unobtanium]

What is the role of an 18" barrel?

A 20" barrel is pretty standard in 5.56 for a "rifle" (using the AR platform).
A 16.1" barrel is a creation to avoid NFA hassels from using a 14.5" barrel.
A 10.3" barrel is for people who want to look cool, or for a very VERY small niche of people. (the more I train, the more I realize how well my 16" carbines get it done, even in and around vehicles, often better than a 10.3" gun. I have both, I like to look cool...)

So what good is the 18" barrel, honestly? What is its purpose? IMO, It has none that is technically quantifiable, and remains an emotional/aesthetic purchase. Although, it can be argued that "I bought this 18" barrel because I wanted "X" configuration, and "X" configuration barrel was only available as an 18" barrel.

[DocGKR]

The 18" barrel SPR/Mk12 was a compromise. In an overly simplified narrative, one SOF group wanted a 20" DMR rifle for long range engagements with 5.56 mm; another wanted to stick with the home brewed "recce" style 16" configurations their unit armorers were building. Neither got what they wanted, as the procurement folks simply split the difference and standardized on an 18" configuration...

Personally, I find the 16" barrel does everything I want in a 5.56 mm carbine; if the weapon has a dedicated suppressor, then a 12" barrel works well.

[Tamara]

the Internet Commandos and other self-anointed SMEs

My irony meter just broke the stop peg off.

Otherwise, nice post. :)

[Maple Syrup Actual]

Great effort on the precision of the testing!

I've never really done any serious ballistics testing but this fits in with the general line I'd always heard about an inch of barrel being worth 50 fps until you get stupid short.

I'm not sure if that's actually a reliable rule of thumb but it's what I always heard.

Anyway if I had 20" or 16" barrels on hand I wouldn't race out and replace them with 18s...and vice versa. I can't imagine a thinking man that would, although I bet there's a metric Ftonne of that thing in some corners of the internet. What will an 18" barrel do? Pretty much everything a 20" will do, just a little slower. What will a 16" barrel do? Pretty much everything an 18" will do, just a little slower. What's fast enough? Depends. I have a BCM Recce 16 and if I ever find myself feeling like it's not enough gun, I'm not going to try to fix that problem with two inches of powder burn, I'm going to fix it with 20 more grains of powder.

Free lunches exist for everybody, everyday, everywhere, except those of us born to women either within, straddling the border of, or outside, Lithuania.

[shane45]

personally I go with the shortest barrel I can while achieving the things I want. Why carry any more than you need to. The result is that only 1 of my AR's has a longer barrel. My precision rig in .260 rem has a 22". Everything else is a 16" barrel, both 5.56 and 7.62's.

[Chuck Haggard]

http://rifleshooter.com/2014/04/223-remington5-56-nato-velocity-versus-barrel-length-a-man-his-chop-box-and-his-friends-rifle/

http://www.ballisticsbytheinch.com/223rifle.html

[Molon]

http://rifleshooter.com/2014/04/223-remington5-56-nato-velocity-versus-barrel-length-a-man-his-chop-box-and-his-friends-rifle/



That's a perfect example of the junk-science I was referring to in my original post. They used a bottom-of-the-barrel chronograph and only one five-shot string for the 62 and 68 grain loads and only two 5-shot strings for the 55 grain loads.

[rifleshooter]

That's a perfect example of the junk-science I was referring to in my original post. They used a bottom-of-the-barrel chronograph and only one five-shot string for the 62 and 68 grain loads and only two 5-shot strings for the 55 grain loads.

Hi,

I'm the author of the "junk science" article.

Considering i've run 4 different Magnetospeeds (which cost around $400 http://www.brownells.com/shooting-accessories/range-gear/chronographs/magnetospeed-barrel-mounted-chronograph-prod55606.aspx) and two different 35Ps, I wouldn't call them bottom of the barrel. In fact, if you look at objective research, you'll see that the Magnetto speed performs as well as any chrono out there. The spacing of the sky screens matters which is why the 35P has 2. It understanding a small amount of movement will change results. Magnetospeed solves this by have magnets fixed in place- they can't move. I regularly confirm the measured speeds with actual field data and they have been spot on. The little crappy ones are optimistic.

The 5 rounds per round per length limit is the result of time constraints. It tells as similar story as 10 and 20 shot group with less data. More rounds better, absolutely, but this was what we has time to do. Most of my graduate work dealt heavily with statistics, and yes, a larger sample size is always better, but you didn't control as much as you think you did in your experiment.

Back to your post.

Why didn't you use the same barrel and cut it? You could have controlled for headspace, bore diameter, chamber dimensions etc..
Did you slug both bores and measure the slug? All barrel makers will verify this changes within the same lot. Slug them both and let us know.
Did you measure the head space in both barrels? I doubt they are identical, refer to the combined gas law for why this would matter.
Did you make a chamber cast and verify the the same reamer cut both chambers? See above.
Did you measure both gas ports? See above.
Why did you only wait 30 seconds and fail to measure the barrels temperature? See above.

My view of the firearms industry is undoubtedly different than yours. The access to what goes on behind the scenes would call into question your assumptions of these barrels being the identical except for barrel length.

You did a nice job with what you had but I would hardly consider it definitive.

Trying to drag down what may be one of the most comprehensive barrel length pieces on 223 Remington available for free to shooters to make yourself feel better but is pretty lame dude. The other three articles on the subject were from accurate reloading, ballistics by the inch and Gemtech. http://www.dtic.mil/ndia/2010armament/WednesdayCumberlandPhilipDater.pdf

For what its worth, we've all spoken about our respective approaches and are happy to build on the data set.

I found your post because it was linked to mine. I read feedback and try to provide a better product for the next time around- that's how grown folks do it. I could have come and asked some hard questions you wouldn't have good answers for but that would have been immature.

Go google "223 Remington Barrel Length" and tell me who is the first hit. Actually read it and tell me how yours is better other than the sample size? You didn't control for temperature, bore dimensions, chamber, headspace.... (I did and had similar controls for temp as your arbitrary 30 seconds)

A better title for your post would have been "I bought 2 different barrels from the same manufacturer and shot them over my chronograph with two different types of ammo".

PM sent to you.

[rifleshooter]

Molon- I would also like you to post your complete data sets for further examination.

[rifleshooter]

The 35P was great in its day. Addressed a lot of short comings. That is why I bought one in 1994. Yep, 20 years ago. Same thing.


I am taking the liberty to post the reviews from Magnetospeed's website:

Reviews & Tests

Reviews:

http://www.realguns.com/articles/360.htm

http://www.realguns.com/articles/391.htm

http://snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=3240356&page=1

http://68forums.com/forums/showthread.php?27824-MagnetoSpeed-Chronograph-Review

http://randywakeman.com/MagnetoSpeedBarrelMountedBallisticChronograph.htm

http://highcountryshooters.com/forum/ubbthreads.php?ubb=showflat&Number=5556

http://forum.accurateshooter.com/index.php?topic=3784927.0

http://www.youtube.com/watch?v=MgL9T2ZO2wc&feature=related

http://www.youtube.com/watch?v=rwFnrFf6DrU



Magnetospeed custom prototype testing @ Southwest Research Institute
*Data provided by Southwest Research InstituteMagnetoSpeed LLC worked with Southwest Research Institute(SwRI) to quantify the ballistic accuracy of the MagnetoSpeed sensor package. SwRI uses a bench top range setup to do ballistic tests on ammunition and uses a double Oehler sky screen setup to accurately capture bullet velocity. In the table above, seven shots were taken all using the same ammunition (7.62x51mm M80) with the MagnetoSpeed sensor as well as the double skyscreen Oehler setup. Velocity 1 above is the velocity measured at the first set of skyscreens and Velocity 2 is read at the second set. When the muzzle velocity is back calculated based on the distance between skyscreen sets and the muzzle we obtain the "Calculated Muzzle" velocity in feet per second (fps). Comparing this to the Magnetospeed data we get percent differences of less than half a percent. This data series shows that the MagnetoSpeed is at least 99.6% accurate to the Oehler system used at SwRI for this test.

Magnetospeed Prototype: High-Speed Video Testing

.243 round just out-running muzzle blastThe SwRI data was good but we wanted to know with more certainty how the MagnetoSpeed sensor stacked up against actual bullet velocity. In order to do this we had to see the bullet and calculate the velocity based on actual measurements. It was time to implement high speed video into our testing scenario. The picture to the left shows a slide of the high speed video we took and the home page shows a slow motion animation of a .243 caliber bullet exiting the barrel and crossing the sensor deck. Based on visual calculations and several test shots we were able to document a 99.9% average accuracy level with the MagnetoSpeed sensor with respect to the video evidence.

Magnetospeed prototype tests by Legion Firearms LLC with AR-15

*Picture provided by Legion Firearms LLCJamie Wehmeyer at Legion Firearms agreed to do some testing for us on the MagnetoSpeed prototype. Using one of his custom AR-15 firearms he put over 100 rounds on the MagnetoSpeed sensor in about 5 minutes. He reported that velocity was right within the expected published range and all the shots had a max spread of 100 fps. When asked about the user interface he reported that it was "super easy to use". They shot mostly single shot and 3-round-bursts and had no problem picking up the velocity. Even though the MagnetoSpeed sensor was not really designed for rapid fire we had them do several series of continuous rapid fire. The sensor would record data up until about the 18th shot in the rapid fire series before giving an error. All in all a good test for the MagnetoSpeed and AR-15.


KMW- Long Range Solutions tests the MagnetoSpeed prototype
*Picture provided by KMW-Long Range Solutions.
Terry Cross at KMW-Long Range Solutions is an expert when it comes to building high precision rifles. We had him do some initial testing with the MagnetoSpeed Rifleman and one of his custom .308 rifles. He used Federal ammo and got a "100% read rate on every projectile". He said his next test will be to shoot the MagnetoSpeed sensor in tandem with a PACT chronograph using standard screens to see how the two compare. We are anxious to hear the results.

[TGS]

Internet Commandos and other self-anointed SMEs

As Tamara said about the irony....

and with the last few posts, that's gotta sting by now.

lulz.

[JodyH]

"What difference does it make?"
-Hillary Clinton-

[jetfire]

Well, now this thread is interesting!

[rifleshooter]

bump.

[Lost River]

My irony meter just broke the stop peg off.

Otherwise, nice post. :)

Crystal ball....?

This reminds me of a story..

I was at a facility that was soliciting future .gov business (long range precision rifle stuff for guys working down range). The owner was saying some incredible stuff in regards to abilities and a particular rifle's round count with apparently no measurable wear and world class groups on a regular basis..

Lots of guys kept looking back at me, waiting for me to throw the BS flag, but I was biting my tongue, as argument was pointless and knowing my boss would prefer that I just be polite, professional and most importantly, be quiet.

The real gem was when the owner said that tier 1 group xxx trained at his place. Little did he know, a number of people he was speaking directly to were recent former members of the that organization. To be clear, I'm not a former or current member of the organization.

Anyways, after glances exchanged, and a break occurs, phone calls were made and nobody knew anybody from that unit that had trained there, either individually or as a unit.

Pays to know your audience.

We were locked on to do a mandated course there, ( however a former SOTIC instructor now working for our organization was the primary instructor and made the best of a crap situation) but shortly thereafter the program got moved to a more suitable location. This one did not have self anointed experts, but actual guys with credible backgrounds..

[JodyH]

bump.


http://youtu.be/EbZIpyQK_VU

[jetfire]

I doubt that the OP will revisit this thread.

[rifleshooter]

I doubt that the OP will revisit this thread.

I'm hoping he does and posts his data.

I'll show you some pretty neat statistical stuff, plus we can have a couple of discussions about ballistics. Should make everyone involved more knowledgeable.

[ssb]

I'm hoping he does and posts his data.

I'll show you some pretty neat statistical stuff, plus we can have a couple of discussions about ballistics. Should make everyone involved more knowledgeable.

While I understand the need to offer a rebuttal -- which it appears that you've done -- I'm curious as to what benefit there is to continuing the bumps? If Molon wants to reply, he'll reply. If he doesn't, he won't. I doubt that posting a high-level version of "come at me, bro" is going to change that.

[rifleshooter]

While I understand the need to offer a rebuttal -- which it appears that you've done -- I'm curious as to what benefit there is to continuing the bumps? If Molon wants to reply, he'll reply. If he doesn't, he won't. I doubt that posting a high-level version of "come at me, bro" is going to change that.

Point taken and you are right. Got caught up in the moment.