Higher pressure in shorter barrel??

[41 Colt]

Looking through my Lyman blackpowder reloading manual, I found pressures listed for the shorter barrels to be HIGHER than those of a longer barrel.

Example:

80gr Goex FFFg
.535" 230gr RB
.015" Lubed Patch
CCI #11 Cap
Twist 1-48"

24" Barrel PSI = 13,300psi
28" Barrel PSI = 10,700psi

WTF? I always thought a shorter barrel meant LOWER pressure. Is it because it is blackpowder?

 

[Andy]

Is that muzzle pressure?

 

[41 Colt]

No.

 

[ultimate_monkey]

is this consistent in all the data? the only thing that i can think of , is that the shorter barrel they tested had a slightly tighter bore then the longer barrel.. but thats out the window if its consistent threw out

 

[Boomer]

It might have to do with the lag time the bullet is in the bore. It takes longer for the bullet to exit the longer barrel, so pressure has more time to build.

 

[41 Colt]

is this consistent in all the data? the only thing that i can think of , is that the shorter barrel they tested had a slightly tighter bore then the longer barrel.. but thats out the window if its consistent threw out

It seems to be consistent for .50 & .54 cal round balls where loads are listed for both barrel lengths but not so much conicals or sabots. Bore dimensions are given and are exactly the same for the 24" and 28" barrels. I forgot to mention velocity is 32fps faster on the SHORTER barrel too.

It might have to do with the lag time the bullet is in the bore. It takes longer for the bullet to exit the longer barrel, so pressure has more time to build.

That is backwards from what I am pointing out. The shorter barrel has MORE pressure which goes against what I believed about pressure in a barrel.

 

[Boomer]

It seems to be consistent for .50 & .54 cal round balls where loads are listed for both barrel lengths but not so much conicals or sabots. Bore dimensions are given and are exactly the same for the 24" and 28" barrels. I forgot to mention velocity is 32fps faster on the SHORTER barrel too.



That is backwards from what I am pointing out. The shorter barrel has MORE pressure which goes against what I believed about pressure in a barrel.

In which case, perhaps the full pressure has been achieved prior to the bullet reaching 24", and pressure has bled off a tad by 28". Other than that, or a typo, I'm unable to account for it

 

[Andy]

There isn't a huge difference in those two pressures. The difference could be accounted for simply by the fact that the testing took place on different days with different guns.

 

[bigedp51]

What method does Lyman list or type firearm used to gather the data, a actual firearm or pressure test barrels?

 

[41 Colt]

Standard black powder barrels were modified to accept a pressure transducer (piezo) and were connected to a peak pressure meter.

From the manual.

perhaps the full pressure has been achieved prior to the bullet reaching 24", and pressure has bled off a tad by 28"

They used a peak pressure meter, but I was thinking the same thing myself.

The difference could be accounted for simply by the fact that the testing took place on different days with different guns.

True, but the fact that it's consistent with both .50 & .54cal is what gets me.

 

[Suputin]

It might have to do with the lag time the bullet is in the bore. It takes longer for the bullet to exit the longer barrel, so pressure has more time to build.

Clearly you are unfamiliar with the Gas Laws. As the bullet moves down the bore, the volume of the space behind the bullet grows dramatically. Under those conditions, the pressure MUST drop. Pressure CANNOT increase as volume increases.

Maximum pressure will be reached while the bullet is very close to the chamber because the volume of the chamber will be the smallest.

 

[Suputin]

There isn't a huge difference in those two pressures. The difference could be accounted for simply by the fact that the testing took place on different days with different guns.

THIS is likely the problem.

Lets consider what occurs inside the barrel when the powder is ignited. As the powder burns and pressure begins to increase, the bullet and especially the burning powder and the resulting propellant gases have no way of "knowing" how much barrel is in front of the bullet yet to be travelled. As the bullet moves down the barrel, the volume of the space behind the bullet increases dramatically which results in pressures dropping. Thus barrel length can have no effect on peak chamber pressure. Up to a point this will be true except if the barrel is crazy short which would cause the bullet to exit before peak pressure is reached.

Do not confuse peak chamber pressure with maximum muzzle velocity. The two are not the same thing.

 

[silverfoxdj]

Good explaination...