Velocity vs barrel length

[Ganderite]

Now that I am shooting IDPA, I want to make sure my favourite 9mm load meets the 125 power factor in all my pistols.

Most have barels around 5 inches, but my Norinco 228 has a 4.2" barrel.


Today I shot several loads with 135 gr bullets in both, to check velocities.

A fast powder (231) gained about 30 fps in the longer barrel.

A medium speed powder (7625) gained 60 fps in the longer barrel.

 

[Levi Garrett]

I figure powder must burn up in the gun to effect velocity, Thats why the longer barrel and the slower powder worked to give more velocity.
Same for rifles, 35 to 50 fps for every inch of barrel, now thats a guess, to lazy to look it up..

 

[Ganderite]

I once had a 30" 308 that had to be cut back to 28", so I did it in 1" increments with a hacksaw, and chronogrphed it at each length. It was about 25 fps/inch. That number would get bigger, as the barrel got shorter, i think. Would not surprise me if it was 50 fps/in around 18 to 19 inches.

 

[Levi Garrett]

not unlike the seating depth of bullets, their is a rule , if the powder has done its job, then the bullet starts to slow, reason for the long barrel being slow in .308, now turn that into a 300 rum, and the longer barrel keeps on giving, because of the gas tank on the rum..

 

[BCRider]

You also need to look at the pressure build over the length of time the bullet is spending in the longer barrel and the peak pressure of the two powders. Unless you match the total pressure-time energy delivery you're comparing apples to oranges and your results are at best confusing..... which is obviously why you're asking.

Having said this a shorter barrel will be more likely to benifit from a faster burn where the pressure comes up faster so it's not still building or is not overly late to build during the bullet's time in the barrel.

 

[tjhaile]

The greater the powder charge for a given bullet, the more velocity can be gained with a longer barrel. For example, we can expect a .300 Winchester Magnum to benefit more than a .308 from going from an 18" barrel to a 26" barrel, although the .300 Magnum should produce higher velocities for any given barrel length.

Of the two powders tested, which gave the highest velocity? Did one powder give the highest velocity with both barrel lengths?