Muzzle Velocity Question

[Missed_dinner]

To my understanding, the longer a barrel is, the higher the muzzle velocity, right?

So am I correct in saying that just like different calibres, the action is set differently in order to change the fps to keep them sub 12ft/lb. The same applies to barrel lengths to match the muzzle velocities???

and if the the two barrel lengths have exactly the same set up then the longer one must have a higher muzzle velocity even if it is only fractional?

[CuppaT]

Everything has it's optimum.... longer barrels are generally more efficient, but there is a limit which if crossed the barrel would be less efficient.
So yes, different barrels lengths will need different amounts of air. But barrel length is more than this, it is about being short enough to use in a hide, or long enough to give a better hold. It's different strokes for different folks. Just buy and use what you're comfortable with.

Example: a .22LR is generally most efficient with a 16" barrel, however a .17HMR is generally most efficient with a 20" barrel. Not an example of air rifles but it does go to show that there is more to this than worth worrying about.

[bighit]

some info here
http://www.pyramydair.com/blog/2005/...irgun-barrels/

[philipk5]

That assumption is as accurate as saying red cars are faster than blue cars!

It is a very complicated subject that I know enough about it to say I know nothing about it.

Looking forward to others comments.

[Turnup]

To my understanding, the longer a barrel is, the higher the muzzle velocity, right?

So am I correct in saying that just like different calibres, the action is set differently in order to change the fps to keep them sub 12ft/lb. The same applies to barrel lengths to match the muzzle velocities???

and if the the two barrel lengths have exactly the same set up then the longer one must have a higher muzzle velocity even if it is only fractional?

There is an optimum barrel length. Take an extreme (I often find that extreme examples aid the understanding) Really really short barrel, say 5mm, then the great majority of the air will be vented after the pellet has left the barrel, so low efficiency, low MV. A really really long barrel, say 5 meters, and the pellet will have a significant mass or air in front of it, reducing acceleration, and after some maximum velocity is reached, friction against the barrel will make it slow down.

For a given action and barrel, then there will be a length which delivers the maximum MV, shorter or longer will result in lower MV. I think that similar reasoning would apply to BP and nitro powered firearms, but the actual lengths are probably longer, and of course the burn rate of the propellant is also a factor (slower burn rate needs a longer barrel for best efficiency)

[robinghewitt]

There used to be a thing on long barrelled springers that you could increase the power by sawing the end off a bit.

The only possible explanation is that the pellet was actually slowing down towards the end rather than speeding up.

In old fashioned cannon the optimum barrel length was 25 calibres, (probably need a bit more in an airgun, I just put that in for fun)

[rhyslightnin]

There used to be a thing on long barrelled springers that you could increase the power by sawing the end off a bit.

The only possible explanation is that the pellet was actually slowing down towards the end rather than speeding up.

In old fashioned cannon the optimum barrel length was 25 calibres, (probably need a bit more in an airgun, I just put that in for fun)

This extra mv through shortening a barrel could be due to the choke being removed preventing the pellet from being decelerated by the narrowing of the barrel more so than the effect of the barrel being shorter.
I could be wrong but i think its more likely than the barrel being more efficient shortened.
Rhys

[angrybear]

There needs to be enough length for the available blast of air to accelerate the pellet up to the required velocity, once it's reached that velocity job done, so a pellet with more mass/area ratio requires a longer barrel because it will accelerate slower, and if FAC going for max power then a longer barrel allows for longer acceleration.

Equally once at max velocity, for the available air blast, any further barrel length is detrimental as it will only cause friction & slow the pellet.

So I think the answer to all 3 questions is;

yes, up to a point, but not necessarily so.

[Benelli B76]

Longer barrels definitely make a difference on PCP and C02 performance, but springers are different and barrels that are too long can tend to slow down the pellet. John Bowkett did some tests with a spring rifle and published his findings in one of the magazines.

Baz

[hareng]

This extra mv through shortening a barrel could be due to the choke being removed

If a barrels been shortened properly it would retain the choke, too easy to do just a 3 minute job for proper job money.

Springer barrels substantially more than 8" for peak power.
Pneumatics generally peak 17 1/4" to 20 1/2" .177 at sub 12ft/lb, minimal velocity increase there after.

Comparisons need to use the same barrel.

[lensman57]

To my understanding, the longer a barrel is, the higher the muzzle velocity, right?

So am I correct in saying that just like different calibres, the action is set differently in order to change the fps to keep them sub 12ft/lb. The same applies to barrel lengths to match the muzzle velocities???

and if the the two barrel lengths have exactly the same set up then the longer one must have a higher muzzle velocity even if it is only fractional?

It is a rather complicated affair. When I changed my KT barrel of my HW 100 to the the longer 40cm one the power went up from 11.6 ft.lbs to nearly 13 at the same reg pressure and hammer preload settings using the jsb 8.44s. I then readjusted the reg pressure and the hammer preload to bring the power down to legal limit 11.7 ft.lbs using JSB heavies. Then when I tested the power with the 8.44s the MV was 795 ft/s which was a little close to the limit of 800 ft/s.

But when I adjusted the MV for 785 ft/s with the 8.44s the Heavies only did about the same output which was surprising. The gun had also lost power with other heavy pellets that I use but had gained greatly in shot count. So for a certain power, reg pressure /hammer preload there is an optimal length beyond which the gun does not benefit from a longer barrel and if anything may indeed loose power as with mine and accuracy too.

A.G