optimum barrel length

[evollep3]

Yes, you can shorten a barrel and it will stay accurate or even become more accurate.

The second upper I used for 3 gun was a 20 inch Bushmaster that was cut down to 18. It was a great upper and very accurate.

The upper I just had done started life out as a 24 inch bull barrel Rock River Arms varmint upper. The barrel used to be 24 inches long and 1 inch thick. The calgunner I bought it from had it cut down to 18 inches and re-countoured it to .850 under the handguards but he left it 1 inch thick from the gas block forward with a target crown.

I had it recountoured and lightened to .750 from the gas block forward. Had a PRI adjustable low profile gas block put on, threaded, recrowned and put on a MSTN QC Muzzle Brake.

When I got the upper it shot pretty good. I tried it with Black Hills 75gr, 60gr V-Max, and 52gr Match Hollow Point. All the 5 shot groups hovered from .75 to 1 inch at 100 yards.

After I had the work done, It shot the same or better. Here is the last group I shot while testing it out. From a bench, 5 shots at 100 yards, using a bipod and sandbags in the back, Black Hills Factory Seconds 223 52gr Match Hollow Point Moly Coated

have to ask who did the work?

[tiger222]

I like 18.5" personally

[cal3gunner]

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[evollep3]

Its a secret!!!

I would love to give credit where credit is due, but they wish to remain anonymous for the time being.

i have a simular upper but a 18" that i want to do something simular and thread the end for a comp

[CSACANNONEER]

I have not had a SINGLE complaint or comment EVER about a degradation in precision on the hundreds of barrels I have shortened.
I have had LOTS of accolades for them getting more precise though.

I attribute most of this to the poor quality of factory crowns.
The increase in rigidity from making a barrel shorter is probably a much smaller part of the equation.

Yea, you know what you are doing. I put that comment out there just to clarify that it can happen if the work is done poorly etc.

[ar15barrels]

i have a simular upper but a 18" that i want to do something simular and thread the end for a comp

I might know a guy that knows a guy...

[Beelzy]

The Barrel Rule is: As long as possible, as short as practical.

You lose speed downrange when you shorten barrel length.

[CSACANNONEER]

The Barrel Rule is: As long as possible, as short as practical.

You lose speed downrange when you shorten barrel length.

You can also loose speed if the barrel is too long. Once the powder is 100% burned, if the bullet is still in the barrel, friction will start slowing it down. This is part of the reason that you don't see 6ft barrels on benchrest guns. They just don't shoot as well as shorter ones. There is an optimum length for any given barrel but, to calculate that length, you need to know all your other variables including your exact powder type, load, burnrate (of that lot), how tight your bullet is crimped, the reletive humidity and elevation you are shooting at, etc. So, in all reality, you can only get close to choosing an aproximate and apropriate barrel length for the task at hand.

[ar15barrels]

You can also loose speed if the barrel is too long. Once the powder is 100% burned, if the bullet is still in the barrel, friction will start slowing it down.

This is a common misconception.
It's NOT the powder burning that is pushing the bullet down bore.
It's the gas pressure that the burning powder creates.
As long as the pressure behind the barrel exceeds the level required to overcome dynamic bore friction, the bullet will continue to accelerate.
Once the pressure drops below the dynamic bore friction value, then the bullet will begin to slow down.
So, your powder can finish burning 2/3 down the barrel, but as long as there's still more pressure than needed to keep the bullet moving all the way to the muzzle, it will not slow down in the barrel.

[CSACANNONEER]

This is a common misconception.
It's NOT the powder burning that is pushing the bullet down bore.
It's the gas pressure that the burning powder creates.
As long as the pressure behind the barrel exceeds the level required to overcome dynamic bore friction, the bullet will continue to accelerate.
Once the pressure drops below the dynamic bore friction value, then the bullet will begin to slow down.
So, your powder can finish burning 2/3 down the barrel, but as long as there's still more pressure than needed to keep the bullet moving all the way to the muzzle, it will not slow down in the barrel.

Thanks Randall. As I typed that I was thinking that the gas pressure would keep accelerating the bullet after the powder was completely burned but, I was trying to simplify my reponse by repeating what I've commonly heard.

So, you're saying that after the powder has completely burned, the pressure will remain constant? I bet there would be a slight drop in pressure caused when gasses passed the projectile in the rifling groves. This is hypothetical and may not happen or just be too small to measure. I may be far too simple minded to fully understand what you are saying about friction. The way I see it is that once the bullet is traveling down the barrel, it is consuming energy to combat the friction. There is energy loss due to friction. This loss of energy would have to allow the projectile to decelerate while still in the barrel. Would it not? Can you point me to a book or other source where I might learn about "dynamic bore friction"? Is it a complicated concept?

[ar15barrels]

So, you're saying that after the powder has completely burned, the pressure will remain constant?

I bet there would be a slight drop in pressure caused when gasses passed the projectile in the rifling groves. This is hypothetical and may not happen or just be too small to measure. I may be far too simple minded to fully understand what you are saying about friction. The way I see it is that once the bullet is traveling down the barrel, it is consuming energy to combat the friction. There is energy loss due to friction. This loss of energy would have to allow the projectile to decelerate while still in the barrel. Would it not? Can you point me to a book or other source where I might learn about "dynamic bore friction"? Is it a complicated concept?

Oh no, pressure is not constant.
If you have a fixed volume of gas (you do once the powder stops burning), the pressure it creates depends on the volume that you contain that gas within.
Once the burning powder stops ADDING pressure, the movement of the bullet down-bore is increasing the volume of the barrel, thereby reducing the pressure.

The dynamic pressure level is different for each bullet.
I have heard the figure 7500psi thrown around for typical 55gr and 62gr 5.56 bullets.
What dynamic pressure level means is that you throw away the pressure BELOW that threshold and then figure that any pressure ABOVE that level will continue to accelerate the bullet.
When you have more pressure than the dyamic pressure level, the bullet accelerates.
When you have less pressure than the dynamic pressure level, the bullet decellerates.

Here is a graph of a 5.56 load in a 100" long barrel:

[scidx]

Wow, thanks Randall. That's the direction I was looking for.

[evollep3]

[Army]

The 5.56 was originally designed to be optimal through a 20" barrel.

[cal3gunner]

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