Velocity Question

[campcreekfarm]

I realize that most of the ammo manufacturers rate their velocity based on tests out of 28-30 inch barrells.  As a general rule of thumb about how much velocity is lost per inch of barrell length ? 

[alan73]

 probably cant put a general rule on velocity loss its going to vary once you reach the length where most of the powder is burnt there will be very little gained in velocity by increasing the barrel length.
per example a 24 inch barrel and a 28 inch barrel may only have a difference of 25fps in velocity.
and an 18 inch and 22 inch could have as much as 100fps second difference there both only 4 inches difference but the loss gets alot greater in shorter barrels.

[campcreekfarm]

probably cant put a general rule on velocity loss its going to vary once you reach the length where most of the powder is burnt there will be very little gained in velocity by increasing the barrel length.
per example a 24 inch barrel and a 28 inch barrel may only have a difference of 25fps in velocity.
and an 18 inch and 22 inch could have as much as 100fps second difference there both only 4 inches difference but the loss gets alot greater in shorter barrels.

Isnt the powder burned practically within the first inch(instantly)?

[allaboutshooting]

I realize that most of the ammo manufacturers rate their velocity based on tests out of 28-30 inch barrells.  As a general rule of thumb about how much velocity is lost per inch of barrell length ?

Muzzle velocity is normally measured at 3' from a 30" barrel. Velocity can be influenced both by barrel length and by degree of constriction of the bore. There have been several studies published in which actual loads were chronographed from a standard 30" barrel and then as one inch increments were cut off that barrel. If I can find that on line, I'll be happy to post a link.

Thanks,
Clark

[campcreekfarm]

I would appreciate that very much. 

[paboxcall]

I would appreciate that very much.

I'd like to see that test data too Clark.  It's always been my understanding, and it may be factually incorrect, that powder burn and pressure rise peaked around 20", and anything over 20" of barrel length actually resulted in reduced velocity due to drag and friction loss.

[BandedSpur]

I would appreciate that very much.

I'd like to see that test data too Clark.  It's always been my understanding, and it may be factually incorrect, that powder burn and pressure rise peaked around 20", and anything over 20" of barrel length actually resulted in reduced velocity due to drag and friction loss.

Not true. That is what the mfgs of short barreled shotguns would have you believe. You will actually lose around 10 fps/inch of barrel less than 30". But Clark is correct, some of the velocity is gained back when using tightly constricted turkey chokes. The lab I send handloads to for pressure/velocity testing normally shows about 50 fps faster velocities from their 30" test barrel compared to my 26" gun in which the loads were developed. That is a little apples to oranges since the same chronograph is not being used. There are a lot of variables, such as bore diameter, powder burn rate, payload, choke diameter, temp, etc. that affect velocity. But if you take two otherwise identical barrels with identical choke tubes, one 21" and the other 26", the 26" will shoot faster.

[teke]

Trick is finding the velocity vs. payload sweet spot.