Thread: Fast twist barrels

Originally Posted by Micky Duck

not necessarily....once all the powder has burnt,a longer barrel slows the projectile down...WHY 22lr is hard to keep subsonic with really short barrel....
definately a longer barrel will be faster with large amount of powder to burn....7mm mag etc...but a 223 only NEEDS something like 16-18" to have complete burn....

Not quite that assumes that all energy from burnt powder is instantly passed on to the projectile but realistically it has intertia and take time to accelerate similar to dropping a bullet from a high building. It doesn't instantly reach terminal velocity. Just because all the powder is burnt doesn't mean acceleration stops. The bullet will still accelerate until the force from the pressure of the xplosion is equal and opposite to the drag from the barrel and air. If you want to see this go run a few loads though quickload etc. Complete burn is not the same as max velocity. Ie why you will see 7mm08 and 6.5 creedmoors with 30 inch barrels etc for target.

Originally Posted by Stocky

Not quite that assumes that all energy from burnt powder is instantly passed on to the projectile but realistically it has intertia and take time to accelerate similar to dropping a bullet from a high building. It doesn't instantly reach terminal velocity. Just because all the powder is burnt doesn't mean acceleration stops. The bullet will still accelerate until the force from the pressure of the xplosion is equal and opposite to the drag from the barrel and air. If you want to see this go run a few loads though quickload etc. Complete burn is not the same as max velocity. Ie why you will see 7mm08 and 6.5 creedmoors with 30 inch barrels etc for target.

Spot on. The pressure curve keeps driving the bullet faster until it runs out of puff. Area under the pressure curve over time vs resistance. Combustion doesn't drop to zero the moment combustion finishes.