Saltwater60
20×102mm Vulcan
I found this interesting and I wonder why you would want it except for a very few instances.
My AR-15 Is Longer Than Yours…
My AR-15 Is Longer Than Yours…
36” AR-15 barrel, yep it’s ridiculous
- Thread starter Saltwater60
- Start date
Beerman
6.5 Creedmoor
He’s pretty funny. (Pretty lucky, too, if you know his story.) I watch his videos, and Demolition Ranch, once in a while when I’m suffering on my indoor cycling trainer. Hickok45 videos, and Paul Harrell videos are also great, but educational AND entertaining.
camper4lyfe
.308 Win
Just put a thumb in it
Saltwater60
20×102mm Vulcan
I think you need the 36” barrel for your next build.
How much of an increase in velocity is there compared to a 20" rifle length barrel?
camper4lyfe
.308 Win
It's about a 400 fps increase from the 16" barrel and the monstrosity.
To go from 20 to 36 you only gain 400fps that's not a very practical tradeoff, unless you're going for longer range or trying to shred level 3 plates which that's should be able to do
Saltwater60
20×102mm Vulcan
I could only see this from a bench shooting steel, outside of that it’s a monstrosity as mentioned. There’s not much else practical for it.
FRANKIE
.308 Win
About as successful as the Paris Gun
Acer-m14
20×102mm Vulcan
i put the AR pistol brace on my Mosin M44 and holy crap it turned into a ELR 91/30 .. Extremely Long Rifle
Acer-m14
20×102mm Vulcan
FUDD LOL .
camper4lyfe
.308 Win
Not even remotely close
Beerman
6.5 Creedmoor
Ignorance is bliss.
dwa12479
.338 Win Mag
dwa12479
.338 Win Mag
How do you figure?
GOYABEAN
.950 JDJ
If a regular length AR15 will disinvolve you this longer version will be able to do it from east coast to west coast with a single pull of the trigger 
dwa12479
.338 Win Mag
I heard it takes down balloons at 40,000 feet even.If a regular length AR15 will disinvolve you this longer version will be able to do it from east coast to west coast with a single pull of the trigger
gun_slinger_boy
.338 Win Mag
At 36 inches you might actually be losing velocity. I think at about 24 inches you would have near if not 100% powder burn. I'm sure @meketrefe would know.
ETA: Nevermind. I actually watched it and there was about a 300FPS gain. I do know there is a point of diminishing returns though.
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These guys tested a .308 with a 6' barrel, cutting it down a bit at a time to 19" with the TL;DR results below:
meketrefe
.450/400 Nitro Ex
This is so hard to say because length alone is just one of many variables. the actual bore and bullet dimensions, the bullet construction and the type of rifling. Also the powder choice and whether if you factor in a gas autoloader you increase the dwell time like crazy and the line will drop.
A 223 with a faster powder and 55gr bullet is going to slow down faster than a slower powder with a heavier bullet. In general you will see little to no benefit after 25-26 inches and expect to go 10 more inches to gain only 100fps extra with a slower powder and deal with a pretty heavy and unpractical rifle. It might actually flatten sooner and even slow down.
A rifle with a higher volume case to bore ratio (overbored calibers) using slowest possible powder for the heaviest bullets might speed up 200 to +250 fps with an absurd barrel length of over 36ninches.
With autoloaders all those rules go out the window because of the gas system.
that is why a 24 223 makes sense and 21" is the sweet spot.
for 308 24 to 26 inches is very good with the sweet spot at 21;inches too.
None of those make much sense in less than 18 inches.
Again this is all a bit relative as you would imagine.