onger barrel= higher velocity

[Modificationvt]

I know that a 3" barrel gives the round less velocity than out of a 5" barrel of the same cal. I was wondering do you get more velocity out of a 6" and 7" barrel? if so at what point does this get redundent? does it work with any round? rifles?

sorry if this is a real noob question, ive been trying to read up on it online but have been having issues finding anything.

I ask because I am contemplating using my lathe to make a new barrel for my Mosin Nagant, and am wondering if I make the barrel a couple inches longer than stock would i improve velocity and improve accuracy.

edit: sorry for the typo in the title....

[gotnspikes]

I am not sure how a longer barrel would give added velocity. I can understand better accuracy due to keeping the bullet in a straighter line longer by coming out of the longer barrel. I just don't understand more velocity as there is nothing in the longer barrel that adds prepellant. Actually as I think abou it, it seems to me that the longer barrel would reduce velocity as the bullet would have to sustain friction longer. If I am incorrect(which is completely possible) please help me to understand.

Thank you.

[Joeywhat]

Originally Posted by gotnspikes

I am not sure how a longer barrel would give added velocity. I can understand better accuracy due to keeping the bullet in a straighter line longer by coming out of the longer barrel. I just don't understand more velocity as there is nothing in the longer barrel that adds prepellant. Actually as I think abou it, it seems to me that the longer barrel would reduce velocity as the bullet would have to sustain friction longer. If I am incorrect(which is completely possible) please help me to understand.

Thank you.

Longer barrel means the bullet is being pushed by the gases for a longer period of time...so it moves faster. This is why rifles chambered in pistol cartridges (or vice versa) show the rifles with a higher velocity then through a pistol. There is also a significant difference when comparing say a 2" .357 snub, with a 6" hunting pistol. You can find bullet velocities for certain barrel lengths online.

Also, a shorter barrel does not have a profound effect on accuracy in itself...you lose more accuracy with the shorter sight radius then due to the barrel itself. I'm sure if you were able to get a 16" sight radius on a gun with a 3" barrel, it'd be a hell of a tack driver...and the opposite is also true.

[gotnspikes]

Originally Posted by Joeywhat

Longer barrel means the bullet is being pushed by the gases for a longer period of time...so it moves faster. This is why rifles chambered in pistol cartridges (or vice versa) show the rifles with a higher velocity then through a pistol. There is also a significant difference when comparing say a 2" .357 snub, with a 6" hunting pistol. You can find bullet velocities for certain barrel lengths online.

Also, a shorter barrel does not have a profound effect on accuracy in itself...you lose more accuracy with the shorter sight radius then due to the barrel itself. I'm sure if you were able to get a 16" sight radius on a gun with a 3" barrel, it'd be a hell of a tack driver...and the opposite is also true.

Thank you. I didn't think of that...obviously

[Modificationvt]

Originally Posted by Joeywhat

Longer barrel means the bullet is being pushed by the gases for a longer period of time...so it moves faster. This is why rifles chambered in pistol cartridges (or vice versa) show the rifles with a higher velocity then through a pistol. There is also a significant difference when comparing say a 2" .357 snub, with a 6" hunting pistol. You can find bullet velocities for certain barrel lengths online.

Also, a shorter barrel does not have a profound effect on accuracy in itself...you lose more accuracy with the shorter sight radius then due to the barrel itself. I'm sure if you were able to get a 16" sight radius on a gun with a 3" barrel, it'd be a hell of a tack driver...and the opposite is also true.

Thanks for that, I am planing on putting a scope on it when its done, so sight raidus isn't an issue, I just thought that increased velocity would counter (maybe not counter, but minimize) the effects of wind and gravity over long range.

[amishclark]

i haven't really done the math, but you can think of the pressure generated behind the bullet in a length of barrel as a bell curve:



think of the x axis as barrel length, and the y axis as pressure behind the bullet. as you move towards the right on the x axis (barrel length) pressure increases; in other words, pressure has a chance to build before being lost by the bullet exiting the barrel. there is a point (again, didn't do the math) where the pressure generated just prior to the bullet leaving the is at a maximum (ideal for a given round!!!). that is the high point in the curve. further towards the right, the pressure decreases...think of it as the volume between the breach and the bullet just prior to exiting the barrel being too large...you will start to lose velocity (to the point of the bullet not exiting) as the volume in this space increases...

eta: it's like a balloon. you blow and blow, and finally you reach a point where you can't blow any more (assuming the balloon doesn't pop). that is the high point on the curve. now...imagine that you're at this point, and suddenly the balloon decides it can stretch further, but you aren't blowing any more...tada! decrease in pressure!

[Kaboom]

i am the grammar police. all your bad grammars are belong to me!

ROFLMAO, just saw that.

[vafish]

Originally Posted by amishclark

i haven't really done the math, but you can think of the pressure generated behind the bullet in a length of barrel as a bell curve:



think of the x axis as barrel length, and the y axis as pressure behind the bullet. as you move towards the right on the x axis (barrel length) pressure increases; in other words, pressure has a chance to build before being lost by the bullet exiting the barrel. there is a point (again, didn't do the math) where the pressure generated just prior to the bullet leaving the is at a maximum (ideal for a given round!!!). that is the high point in the curve. further towards the right, the pressure decreases...think of it as the volume between the breach and the bullet just prior to exiting the barrel being too large...you will start to lose velocity (to the point of the bullet not exiting) as the volume in this space increases...

eta: it's like a balloon. you blow and blow, and finally you reach a point where you can't blow any more (assuming the balloon doesn't pop). that is the high point on the curve. now...imagine that you're at this point, and suddenly the balloon decides it can stretch further, but you aren't blowing any more...tada! decrease in pressure!

That explains it pretty well.

Generally speaking a longer barrel will give you more velocity (but not necessarily more accuracy). But, there is a point where the gases stop expanding and the longer barrel actually slows the bullet down through friction. It really depends on the burn rate of the powder being used.

Handgun ammunition is made with relatively fast burning powders to propel a bullet in a short barrel.

With your Mosin I doubt you could make the barrel long enough, and still have it be usable, to reach the point where friction starts to slow the bullet back down. If you have an M44 or M38 with the 20" barrel going to a 24" barrel would give you a bit of a boost in velocity. Going much beyond a 24" or 26" barrel is going to make it pretty unwieldy.

[einheit 13]

Originally Posted by vafish

That explains it pretty well.

Generally speaking a longer barrel will give you more velocity (but not necessarily more accuracy). But, there is a point where the gases stop expanding and the longer barrel actually slows the bullet down through friction. It really depends on the burn rate of the powder being used.

Handgun ammunition is made with relatively fast burning powders to propel a bullet in a short barrel.

With your Mosin I doubt you could make the barrel long enough, and still have it be usable, to reach the point where friction starts to slow the bullet back down. If you have an M44 or M38 with the 20" barrel going to a 24" barrel would give you a bit of a boost in velocity. Going much beyond a 24" or 26" barrel is going to make it pretty unwieldy.

Any 7.62x54r ammo made after 1934 is typically designed for 28" barrel...or 91/30s.
Any 7.62x54r ammo made after 1965 is typically designed for 24" barrels...or SVDs and PK/PKMs.
1940s and up ammo drops muzzle velocity by about 100fps when fired out of an M91. About the same when its shot out of a carbine. This is for Russian spec ammo only, every country has their own 'mix' for powders and bullet weights. The giant flash out of the carbines is wasted energy. M91s barely makes a flash, but at 250yds they are dropping faster than the same ammo fired from a 91/30. Accuracy is relative to each rifle/pistol and the ammo.