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turtlemann14 · Member

in a pcp rifle the longer the barrel the more energy is able to be exerted on a projectile per psi correct?

is there a equation that determines the energy exerted on a projectile by the length of barrel?

something like
1000psi at projectile 0
750 psi at 25% barrel length
500 psi at 50% barrel length
and so on
which adds up to ____ pounds of force at 100% of barrel

i would figure that it would be a linear equation

or should i just ask how long are pcp barrels to attain good power output and accuracy

rsterne · Moderator

Unfortunately, it is no where near that simple.... A non-regulated PCP (ie most, like the Disco, Marauder, Hatsans, B-50/51).... have a bell-curve of velocity vs. tank pressure.... The valve is operating in a "self-regulating" mode where at the beginning (when the reservoir is full) the pellet gets a short, sharp blast of very high pressure air.... as the pressure drops, the valve opens longer and longer but the pressure goes down and down.... At the end of the shot string, the pellet is getting a longer but more gentle shove.... yet the velocity is very little different (maybe 4-5%) across the usuable shot string....

If you cut the barrel length in half.... you would find that at the beginning of the shot string the velocity wouldn't change much.... because most of the acceleration of the pellet is occuring in the first portion of the pellet travel.... However, at the end of the string, the velocity would be considerably less because the barrel is too short for that long, more leisurely, push to occur.... A nice bell curve in a long barrel would become a simple slope from high to low velocity with a short enough barrel....

Bob

turtlemann14 · Member

lets assume that the pressure is constantly topped off
how long of a barrel is too long (is it just the awkwardness that limits you?)

rsterne · Moderator

Depends entirely on the operating pressure, caliber, pellet weight, and a host of other variables.... Sorry, but there is not one "correct" answer....

Bob

Mark5043 · Silver Status Member

turtlemann14 · Member

i have a bad habit of asking those...

say 5000psi .45 say 100 grain

i am not really sure how to ask what i am trying to ask.

my thought is is that as long as you have enough pressure to overcome friction and accelerate the projectile though a long barrel(with enough volume to do so) that you can attain high fps with low pressure.

now say that i run 5k psi (constantly supplied) though a thirty foot barrel, that projectile should be moving substantially faster than one though a three inch barrel.

now i realize that a thirty foot barrel is not possible to carry but a barrel with gun attached that is say six feet long is(although it would be a little much)

i guess what i am really trying to ask is how much barrel length affects energy exerted on the projectile

packrat · Senior Member

Perhaps if you just posted exactly what sort of gun you're thinking of building, it would be easier to get answers. Question

turtlemann14 · Member

ruff idea is a long barrel bolt action pcp with a high caliber

(rifled if i can do it)

rsterne · Moderator

What you are trying to build is outside my experience.... so any answer I give would be a guess....

5000 psi is VERY high pressure.... You say constantly applied, so that means you have virtually unlimited power available per shot.... In order to achieve this, you basically have a valve that once open, stays open and drains the reservoir.... ie 1 shot per fill, with a huge (infinite) reservoir....

Under those conditions, the longer the barrel the faster you should be able to accelerate the projectile.... since you are continually adding more air at the breech end.... All the normal rules about a set amount of air expanding to accelerate the projectile pretty much go out the window....

Bob

turtlemann14 · Member

the reservoir wouldn't have to be infinite, just a very high flow regulator on a large reservoir

is there a written list of these rules?

rsterne · Moderator

No.... see my first post....

If you are using a regulator with a 5000 psi output, how high is the pressure in your "large reservoir".... What are you trying to build, a cannon mounted on a trailer?

With all due respect, this post is a waste of time.... If you ask sensible questions about real world situations I'll do my best to help.... A thirty foot barrel (or even 6 foot) running on 5000 psi isn't realistic....

Perhaps you should get some experience with existing PCP airguns to find out what is and is not realistic....

Bob

turtlemann14 · Member

what i am asking is completely realistic, if you have the money to do so.

guess i will just have to work out the math my self

fritz · Silver Status Member

Just build the gun already and see what it does.

Work from there.

turtlemann14 · Member

i am not a person to do that though, it waists money and time.

i would rather do research and math rather then building prototype after prototype.

one or two is good five or ten is not

SMP · Silver Status Member

Sorry to resurrect an old thread but this is a fairly simple equation.

Because air has mass it will expand at a fixed rate regardless of pressure. That rate is about 1640 ft/s in a frictionless column.

So the math is Newtonian physics. F=MxA

F is the force acting on your pellet. ie. 1000psi acting on a pellet of 0.1sq inches = 100pds/f
M is the mass of your pellet

solve for A

Then
D=VT + 1/2 ATsq
where d is displacement
v is starting velocity (0)
t is time
a is acceleration

solve for distance over time