barrel length vs pressure

B

BrandonK

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Hi all, I was wondering if or how barrel length effects pressure? I was given some loads that were for a 24" barrel that are 5k to10 k under SAAMI specs for pressure. If the barrel length increases does the pressure? Its a dasher Im talking about. Thanks:)
 
I had better leave this one to the ballisticians-- we have some very brilliant guys who visit this forum.

Now for my ignorant two-cents worth.

I believe pressure peaks during the first 12 or 14 inches of the bullet's travel down the barrel. Obviously, subject to variance depending on the powder.

If that is correct, a long barrel will make little or no difference to PEAK pressure.
 
I was just reading Harold Vaughn's "Rifle Accuracy Facts" some last night and I think the chapter I was reading addressed this question some.

I'll check again and see if it can be of any help.
 
Pete's right here, and even a bit generous in the space it takes to reach peak pressure. There's some very good info on this in Hatcher's Notebook on this, but assuming that your powder choice is anything close to appropriate, you're good. You're changing expansion ratio when increasing/decreasing barrel length. This, in turn will change your velocity, but pressures aren't really the issue there.

Hope this helps, and good shooting!

Kevin Thomas
 
Pressure peaks in the 1st couple inches of the barrel and drops rather fast as bullet moves down the bore opening up more space for the gases to fill. You will have, theoriticaly, the same peak pressure in a 5" barrel as a 30" all else being equal. The slower burning powders are still adding gas to the mix farther down the barrel and higher pressure levels are retained at a given lenght due to it. Peak pressure is the result of the powders burn rate, amount of powder, chamber volume, and resistance of the bullet.
I`d be interested in how the loads you were given were determined to be 5-10K below SAAMI?
 
Bullet travel at Pmax

According to QuickLoad, the maximum pressure (61,000 psi)is reached at 1.31" of bullet travel in my 6mmBR load (VARGET). At exit from the muzzle the pressure is 6257 psi. This does not mean taht thevelocities will be the same with a shorter barrel.

BTW, if you want to see a fault in the programming of QuickLoad, put a rediculously long barrel length and watch the velocity prediction: ie the above load with a 29" barrel is predicted to have a MV of 2919 fps, change the barrel length to 200" and the predicted MV is 3632 fps - NOT GOING TO HAPPEN.
 
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K Thomas said:
Pete's right here, and even a bit generous in the space it takes to reach peak pressure. There's some very good info on this in Hatcher's Notebook on this, but assuming that your powder choice is anything close to appropriate, you're good. You're changing expansion ratio when increasing/decreasing barrel length. This, in turn will change your velocity, but pressures aren't really the issue there.

Hope this helps, and good shooting!

Kevin Thomas
Click to expand...

Kevin from Sierra?

al
 
Nah, That'd be Kevin From Berger Bullets . . . formerly of Sierra Bullets.

Kevin
 
Travelor said:
According to QuickLoad, the maximum pressure (61,000 psi)is reached at 1.31" of bullet travel in my 6mmBR load (VARGET). At exit from the muzzle the pressure is 6257 psi. This does not mean taht thevelocities will be the same with a shorter barrel.

BTW, if you want to see a fault in the programming of QuickLoad, put a rediculously long barrel length and watch the velocity prediction: ie the above load with a 29" barrel is predicted to have a MV of 2919 fps, change the barrel length to 200" and the predicted MV is 3632 fps - NOT GOING TO HAPPEN.
Click to expand...

What is the pressure at 200"?..................Don
 
BrandonK said:
Hi all, I was wondering if or how barrel length effects pressure? I was given some loads that were for a 24" barrel that are 5k to10 k under SAAMI specs for pressure. If the barrel length increases does the pressure? Its a dasher Im talking about. Thanks:)
Click to expand...


Barrel length has nothing to do with increasing/decreasing pressure.

A given cartridge has no consciousness to alter its behavior based on the length of a barrel. It will attempt to do the same thing every time unless an outside force acts on it like:

Temp
Humidity
barometric pressure
change the brass
change the bullet type/weight
(probably a few more, but I can't think of anything else right now)

Now if a barrel is short, lets say stupid short like 5 inches; will this have an effect on things?

Sure, cause it's going to vent to atmosphere as soon as the bullet leaves the crown. But up until that moment the powder charge/pressure curve had no idea the barrel was any shorter.

Take a long barrel, apply the same logic. The pressure didn't increase any, all you did was provide for the pressure to continue pushing on the back of the bullet a bit longer. This will certainly influence velocity but it won't increase/decrease the pressure that given charge produced.

Make sense?
 
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Chad,

That's the answer to the question, but the good flip side of the coin is that with a longer tube, you can select a different, probably slower burning, powder & reach higher velocities with pressure equal to the shorter barrel.

John
 
John,

You sir are absolutely correct and if I sounded "absolute" then I must apologize.

I should have included powders in my list of things in the beginning. (see, I knew I was forgetting something)

In that case primers should be added as well I guess.

cheers,

Chad


PS:

To the OP. Now one thing I just considered is that you mentioned these hand loads were cooked up for a 24" tube. We know that barrels have a "sweet spot" with a given load (powder, bullet type, primer, seating depth, powder charge, etc)

We know that its common to see remarkable change in group size with very subtle changes in powder weight/volume and/or seating depth.

My personal explanation for this is we get lucky and find the "spot" where that barrel is motionless for that split second as the bullet exits the crown. It's much the same as fiddling with a barrel tuner. I'm certain of it cause what happens when you change ANYTHING with a barrel tuner? You have to start all over again.

What I mean by this is, if your loads are "tuned" for a specific 24" barrel then its tough to say if they will work as good in your rifle. Only way to know is try it.
 
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Chad,

I wasn't criticising you, just musing on other possibilities.

My apologies if it came over that way.

John
 
NesikaChad said:
Barrel length has nothing to do with increasing/decreasing pressure.

A given cartridge has no consciousness to alter its behavior based on the length of a barrel. It will attempt to do the same thing every time unless an outside force acts on it like:

Temp
Humidity
barometric pressure
change the brass
change the bullet type/weight
(probably a few more, but I can't think of anything else right now)

Now if a barrel is short, lets say stupid short like 5 inches; will this have an effect on things?

Sure, cause it's going to vent to atmosphere as soon as the bullet leaves the crown. But up until that moment the powder charge/pressure curve had no idea the barrel was any shorter.

Take a long barrel, apply the same logic. The pressure didn't increase any, all you did was provide for the pressure to continue pushing on the back of the bullet a bit longer. This will certainly influence velocity but it won't increase/decrease the pressure that given charge produced.

Make sense?
Click to expand...

There is a barrel length dependent factor, however minor, that the bullet is "conscious" of, and that is the air in the barrel. In traveling down the barrel, the bullet compresses the air, accelerates its mass and overcomes the drag of the air against the bore, all of which influence the force on the front of the bullet. That force is larger in a longer barrel, and it increases the pressure behind the bullet.

Cheers,
Keith
 
mks said:
There is a barrel length dependent factor, however minor, that the bullet is "conscious" of, and that is the air in the barrel. In traveling down the barrel, the bullet compresses the air, accelerates its mass and overcomes the drag of the air against the bore, all of which influence the force on the front of the bullet. That force is larger in a longer barrel, and it increases the pressure behind the bullet.

Cheers,
Keith
Click to expand...


Keith,

This may not be true. :) Speculate with me here....... there are those that propose that the "puff" or blast of gas which escapes around the bullet prior to startup may well be just enough to get the column moving. It wouldn't take much to make the "air column in front of the bullet" a non-issue.

I'm not arguing, just proposing an alternative view.

al
 
alinwa said:
Keith,

This may not be true. :) Speculate with me here....... there are those that propose that the "puff" or blast of gas which escapes around the bullet prior to startup may well be just enough to get the column moving. It wouldn't take much to make the "air column in front of the bullet" a non-issue.

I'm not arguing, just proposing an alternative view.

al
Click to expand...
The escaped gases would indeed have momentum that would tend to pull the bullet forward. But I doubt the column of air would be moving fast enough to cancel the effect of the bullet having to push the air out of the barrel. To cancel it, the gases would need to be moving in the bore at least at the average speed of the bullet. But the cross sectional area of the annular orifice through which the gases must pass is about 40 times smaller than the bore (say with a 0.003" total clearance between a 6 mm bullet and the case neck), a severe restriction. The velocity in the restriction would need to be 40 times higher than the average speed of the bullet, say Mach 60 for a mean bullet speed of Mach 1.5, not counting the effects of compressibility. That's too fast to be reasonably expected. Further, the effect of the escaped gases ends soon after bullet moves forward a fraction of an inch, closing off the flow. So the escaped gases reduce the effect a bit, but probably not by much.

Cheers,
Keith
 
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