The elusive 20inch AR15 BBL
- Thread starter jdman
- Start date
Bolivar
CGN Ultra frequent flyer
Damn, I guess I had better trim down my 26 inch bolt action barrel then.
Wait a minute, I used to have a 24 inch barrel on my AR. I had it cut to 20 inches and lost about 100 fps. An my 14.5 inch barrel shoots the same load slower still . I guess my barrels must must be some weird freak of nature that obey the laws of physics.
If your barrel is longer, gas pressure acts on the bullet longer, accelerating it to higher velocity. Why do you think long barrels are are used for long range rifles. Talk to an F class or TR shooter. None of them are using shorties.
I happen to find my 20 inch to be a handy length, not too long or awkward. Other use 24 inchers well too. I don't agree that a 20 inch barrel is a fail in anyway.
I am only stating the results of testing where they took a 24" barrel and cut it down in 1" increments down to 5" measuring the velocity and pressure as the length decreased. Their results showed a decrease in both muzzle velocity and pressure for lengths greater than 20". They stated the result was expected as a result of the powder being fully ignited by the 20" barrel length (as the .223 cartridge was specifically designed). Their results obey the laws of physics. I have not tried to replicate their results. I would if I had the coin. They even re-crowned the barrel after each cut too. Now results of an experiment must be repeatable so take them with a grain of salt.
Been shooting for over 30 years 2bad4u2.
Been shooting for over 30 years 2bad4u2.
Oh and on April 15th I brought up my doubts/curiosity about their results in the following post:
Bolivar did you actually chrono your barrels? Did you take photos and have the results from before and after? Right now I will take the magazines results over yours; as far as I know you could be using theoretical charts for you values...
Bolivar did you actually chrono your barrels? Did you take photos and have the results from before and after? Right now I will take the magazines results over yours; as far as I know you could be using theoretical charts for you values...
blaxsun
CGN Ultra frequent flyer
Naw i want distance, not mirror commando
I know, just giving you a rough time.
My pick would be the PWS Wraith. It's only 18", but it looks like a great setup...
can you point out on this chart where the velocity decreases with the longer barrel? i cant seem to locate it
curtosy of Firearmsforums.com Http://firearmsforum.com/firearms/imageuser/27/19331
also a 2nd set of data www.ballisticsbytheinch.com
http://www.ballisticsbytheinch.com/docs/2011data.pdf
can you point out on this chart where the velocity decreases with the longer barrel? i cant seem to locate it
curtosy of Firearmsforums.com Http://firearmsforum.com/firearms/imageuser/27/19331
Wow, reading fail or what. Did you not read where I stated EVERY single chart I have seen outside of an actual physical test done by gunsmiths for a magazine article contradict said magazines results. Perhaps you also failed to read where I questioned not only their results but perhaps the results of other charts as most of them will be the result of theoretical computations, not EMPIRICAL data. Empirical data trumps theory each and every time...what do I know I'm only an aerospace engineer.
Wow, reading fail or what. Did you not read where I stated EVERY single chart I have seen outside of an actual physical test done by gunsmiths for a magazine article contradict said magazines results. Perhaps you also failed to read where I questioned not only their results but perhaps the results of other charts as most of them will be the result of theoretical computations, not EMPIRICAL data. Empirical data trumps theory each and every time...what do I know I'm only an aerospace engineer.
sorry Van;
i jumped the gun and stopped reading at post #21 where you stated lower muzzle velocities in barrels longer than 20"
Sorry
Bolivar
CGN Ultra frequent flyer
Yes, of course I chronoed my results. I stated that I lost approximately 100 fps. I did not say "I think I lost 100 fps" or " according to my calculations I lost 100 fps". I lost 100 fps after cutting 4 inches off of my barrel, period, full stop. My experience with exactly one barrel with the same ammunition before and after.
Did I take picture? Of what? The rifle? The chronograph? What would you like for proof? Sorry, I did not shoot a youtube vid of me chronographing the long and shorter barrel lengths and I can't put the missing 4 inches back on to test the original condition. All I have are my written notes.
By the way, you are not the only engineer on this forum and flashing your credentials to make people think they should believe what you are saying beneath the profession.
Longer barrels in a standard AR recevier are generally bad juju.
SOCOM via Crane (but out to a lot of the SFG's) tested a variety of barrel lengths when they descided on the 18".
Longer Heavier Barrels can flex the AR uppers - which can lead to several issues, reduced reliability being a minor part of that.
Frankly if you want a longer barrel than 16" in 5.56mm I think you need to re-think your caliber.
SOCOM via Crane (but out to a lot of the SFG's) tested a variety of barrel lengths when they descided on the 18".
Longer Heavier Barrels can flex the AR uppers - which can lead to several issues, reduced reliability being a minor part of that.
Frankly if you want a longer barrel than 16" in 5.56mm I think you need to re-think your caliber.
Fair enough. The fact that you lost only 100 fps with a 4" cut in barrel length makes me believe it is quite possible that the Surefire magazine's results are valid. At shorter barrel lengths guaranteed you would see a larger difference.
If you know as many engineers as I do you'd realize that NOTHING is beneath the profession.
Bolivar
CGN Ultra frequent flyer
But the original barrel length was 20 inches. Does that mean that they were flawed from the get go?
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Longer barrels mean more velocity, I believe 55gr needs to be at least at 2,700FPS to fragment. The shorter your barrel, the shorter your killing range. That's why the Marines stuck with their full sized M16's.
Also, the shorter the barrel the harder it is erosion wise on the gas block. Many of the M4's shortcomings can be attributed to the extra wear caused by the shorter barrel when using a DI system.
Clobbersauras
CGN Ultra frequent flyer
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