increased barrel length vs speed, linear graph?

[rumbleUU]

For example: if a pcp shoots 10 joules with a 12 inch barrel, same everything except length of the barrel.
Is there a rule between the speed vs the length of the barrel?

Will a 24" barrel shoot the same pellet with twice the energy(20 joules)?

Im not after a perfect mathematic formula,, as I know its more complicated, just a general rule if there is one.

[FuzzyGrub]

I think most of those formulas were based on pumpers and dump valves.  Back a good number of years ago, I swapped out the 12" prod barrel with a 20" mrod barrel, without touching the tune.  Only increased 1fpe. 

I think the more inefficient your current tune is, the more you will gain with a longer barrel.   

[rumbleUU]

been reading some and it seems it varies great, in some cases it will slow down the pellet with longer barrel, depending on transferport, hammerweight and hammerspring/tension.... it probably more complicated to just use a rule of thumb..

I think its easier to predict if you are aiming for max power on a good tune... maybee then a twice length will almost give twice the energy?

[Loren]

Here is a good place to start.

https://hardairmagazine.com/ham-columns/how-to-obtain-the-maximum-power-from-a-pcp-air-rifle/

Some other articles there too that may help.

[Motorhead]

sadly while there is a general rule of thumb for FPS per inch +/- .............  In PCP's where you have control of a valves dwell, port sizes etc,  A gun can be set up for a specific power by limiting valves output pressure & volume where adding more barrel won't do squat.  Same thing at the other end of spectrum where if a gun is set up and just is a heavy breather the power will go up with increases in barrel length.

Then you have the set up where pressure is lower if ports are large and a light hammer is used the gun can shoot fairly high velocity out of a short barrel .. where a set up with smaller ports would require more pressure and a longer barrel to equal the same speed.

MANY ways to skin the cat .. catch the mouse !      There is a trend that larger bore and higher power does require more barrel length ... but a whole lot of flexibility also exists all the way down to the smaller calibers.

[rumbleUU]

yeah, its a whole world knowing all, Ive stumbled across the FX Dreamline compact, cant figure out form the specs? but it seems that one is capable of 850 fps in .22,  Im sure that barrel cant be more then 13-14 inch?

how it that possible, must be wasting air bigtime

[Scotchmo]

For example: if a pcp shoots 10 joules with a 12 inch barrel, same everything except length of the barrel.
Is there a rule between the speed vs the length of the barrel?

Will a 24" barrel shoot the same pellet with twice the energy(20 joules)?

Im not after a perfect mathematic formula,, as I know its more complicated, just a general rule if there is one.

If you have a very heavy pellet, and flow through the smallest constriction stays subsonic, you will get an almost proportional FPE increase. The % velocity increase will be equal to the square root of long_barrel_length/short_barrel_length.

That assumes a large enough plenum and that you can increase the hammer strike so you get sufficient dwell in the valve.

In your example, the 10 joules at 12" would give 20 joules at 24". Velocity in the long barrel would be 1.41x the old velocity. sqrt(24/12)

However:
We are usually only concerned with pellet FPE and not the total system FPE. Some of the FPE goes into accelerating the air as well as the pellet. For a lighter pellet, the mass of the air is significant, so the air "steals" a significant portion of the FPE. So the velocity increase will never be as much as you might first think, and that is especially true for low mass pellets.

[Rallyshark]

Generally speaking mid to high power guns will see more of an increase in power with a longer barrel.  Really low power guns don't benefit as much from extra barrel length.  As Scott said, that doesn't mean power and flexibility can't be had with a shorter barrel.  Also, a gun shooting 30 fpe out of a 14" barrel likely won't be as efficient as a 30fpe gun with a 20" barrel.  This is all very general, of course.  There are valves/tuning methods that can get great efficiency out of a short barrel, but that will drop once the power starts getting higher in most cases. 

[Long_Gun_Dallas]

yeah, its a whole world knowing all, Ive stumbled across the FX Dreamline compact, cant figure out form the specs? but it seems that one is capable of 850 fps in .22,  Im sure that barrel cant be more then 13-14 inch?

how it that possible, must be wasting air bigtime

I have a crown with a 380mm  (15" barrel)  it shoots 18gr .22 pellets st 930fps.  Is also fairly quiet as well.  Not much wasted airto be honest.

I can take my 700mm (27" barrel) and using the same tune, only pick up 110fps.  I have multiple tunes and various adjusters and weights.  But that tune is optimized for the 15" barrel.

I also just finished a 7.6" barreled crosman 1322.  I was worried I would struggle to meet my goal of 6-650 fps with 18gr pellets, but it shoots them at 785fps!!  I can get it to shoot slightly faster, but it is an inefficient tune currently.

[rsterne]

For maximum power shots, FPE is proportional to barrel length.... Therefore a 10% increase in length gives about a 10% increase in FPE (note, NOT velocity, as that varies by the square root of FPE)…. This means that a 1" change on a 10" barrel will have twice the relative effect of a 1" change on a 20" barrel.... Put simply, shorter barrels gain more (and lose more) velocity per inch of length change....

Most of our guns, however, are not tuned for maximum power, and the valve closes long before the pellet gets to the muzzle.... This means that the FPE is no longer completely bound by the percentage proportional rule of thumb.... Longer barrels will not gain FPE in proportion to barrel length, and in fact if you go REALLY long, pellet friction can exceed any remaining force from air expansion, and the pellet may slow down again....

Bob

[oldpro]

10-12 fps per inch is what I find on AVERAGE but in no way a rule.  The tune it has before the change is critical unless like Bob stated were working with max output then its somewhat linear.

[rsterne]

Well not linear, but with maximum power builds the FPE is proportional to the barrel length.... The velocity graphed against barrel length is a parabola....

Bob

[subscriber]

Sebastian,

After the valve closes, every time you double the distance the pellet travels down the barrel, the force accelerating the pellet is halved.  So, inches near the muzzle add much less FPE, than inches near the breech:

If you pick a few "easy numbers" the pressure decay down the barrel become more apparent for low power tunes:

For a .177 caliber yielding 12 FPE from a 12" barrel, it means that the average force accelerating the pellet is 12 lb.  It is slightly more due to friction, but lets keep it simple.

If your valve is sipping from a 3000 PSI supply, the initial force accelerating the .177 pellet is 75 lb.  If that force starts at 75 lb and averages only 12 lb, then the valve is open for a very short time.  The force when the pellet exits will be less than 12 lb.  The decay curve can be calculated.  In any event, if the driving force at the muzzle of the first foot length is 6 lb, that is the peak force for the second foot length (after you double the barrel to 24").  It is easy then to see how the second 12" length will add much less FPE than the first.

If you repeat this exercise for a "wasteful tune" for a 12" barrel length, then the second 12" can extract the energy that would otherwise have been wasted.

If a 12" barrel .177 yields 30 FPE, then the average force acting on the pellet is 30 lb; or 1200 PSI.  As that would be over a third of the starting pressure, the decays curve would be "fat" with very usable pressure at the muzzle of the 12" barrel.  Stretch that barrel to 24", and the "muzzle pressure" of the first 12" becomes the "breech pressure" of the second "12".  Not as high as the first foot, but high enough to gain significant FPE from the second foot.

So, the answer to your question is; it all depends on the details.  The potential energy available for transfer to the pellet is proportional to both the pressure and the volume of the air released by the valve.  Then it is a matter of the expansion ratio that determines the efficiency (how much of that energy is transferred to the pellet):  A 30 ft.lb .22 will be more efficient than a 30 ft.lb .177, if both are hamstrung with a barrel as short as 12".  Lower the FPE to only 10 ft.lb and double the barrel length to 24" for both, and the smaller caliber may have the edge in efficiency due to its lower pellet to barrel friction.

[subscriber]

Just to add; incremental velocity added per inch depends on the average pressure applied over that inch.  The rate of change in pressure is higher near the breech than the muzzle, but the average pressure applied over each inch of travel, is much higher near the breech than the muzzle; unless the barrel is really short and the valve stays open unusually long.

[rumbleUU]

interesting stuff guys..... , I knew it would be more to it then    just simple EASY !

@ Long_Gun_Dallas: How efficient is your FX compact ? Sure it shoots the 18 grains @930fps, but how many CC/shoot?

I know it all depends on many thing, but lets take an example: Would a PP700 pistol gain from a longer barrel?

[Long_Gun_Dallas]

interesting stuff guys..... , I knew it would be more to it then    just simple EASY !

@ Long_Gun_Dallas: How efficient is your FX compact ? Sure it shoots the 18 grains @930fps, but how many CC/shoot?

I know it all depends on many thing, but lets take an example: Would a PP700 pistol gain from a longer barrel?

The shot count is too high for me to care to count, I can say that.  Based off how much air it uses, I'd say I should be able to get 5 magazines before fallig off the reg so, 90 shots in a 480cc tank.  Could be higher or lower.  Either way, far more than I need while hubting.  The gun is quiet, and precise.  I actually detuned the gun a bit, and it was tuned very well from the factory.

Also, I typically shoot at 670fps.  Shot count is thru the roof.  Hunting, it gets turned up.

As for a pp700, there would be some gain, but due to reduced plenum volume, I doubt you could squeeze a whole lot more out of it.  Try it and find out 🙂

[Ribbonstone]

HAven't run many good tests.

Rather than different barrels, seems the best way to test would be with the same barrel (minimalizing barrel-to-barrel variables).  So would have to start with a long one and cut it down in steps.

(Which kind of not worthy as you'd nearly always want to go back to a longer barrel.)

Not really wanting to do that with an expensive barrel just to see.....used cheap barrels (Crosmans).

BEst simple relationship was to the percentage of barrel length change to the velocity change.

So no set vel. gain per inch.   A 4" change in a 14" barrel is  worth more than a 4" change in a 24" barrel.

Never worked out linear....at best, would be more like a correlation than hard numbers.

Did show that inefficient (air hogs) gained or lost more than efficient rifles (even at the same power level)....makes sense to me, that "extra air" with a short barrel actually does help in a long barrel.

[subscriber]

Would a PP700 pistol gain from a longer barrel?

If a PP700 is tuned to shoot at 8 FPE from a 8.25" barrel, doubling the barrel length might add 10% FPE - probably less.  If it is tuned to shoot at 20 FPE, then doubling the barrel length would almost certainly increase the velocity and FPE significantly.

If it is a .22, then the less it will gain from a low power tune when you increase barrel length, because the larger bore already has a 150% larger expansion ratio than a .177.  The converse is that a .22 will gain 150% more FPE from a longer barrel than a .177 when using a full power tune.  Note that when you double FPE of a given projectile, you only increased the velocity by the square root of 2; as in 1.414, or 41.4% higher.

Looking at this another way, if you increased the PP700 barrel length from the stock 210 mm, then it would enable you to tune the pistol to proportionately higher FPE, without spoiling the efficiency. 

Another way of stating this is, pistols of a given caliber are more wasteful of air than rifles when operating at the same FPE.  This is because their muzzle pressure is higher when the pellet exits.  If the muzzle pressure corresponds to perhaps twice the value of the friction force at the muzzle, then a longer barrel would gain measurably more velocity. 

Someone on this forum determined experimentally that this practical muzzle pressure threshold was about 100 PSI for pellets, regardless of caliber.  For slugs, the lowest practical muzzle pressure is probably higher in proportion to their running friction, caused by the longer bore contact length.  Drop below that, and the projectile starts slowing down.  If the barrel is unchoked, it will gain more from a few extra inches than if it is choked.   This is even more so with slugs, as it would take more energy to swage down a slug diameter by 0.002" than a diabolo pellet.

So, keep increasing the barrel length until the muzzle pressure is about 100 PSI (for pellets).  If a 24" barrel corresponds with a muzzle pressure of 200 PSI, then to get to 100 PSI, the barrel length would need to double to 48" long.  Expanding the air until it is below 100 PSI, the gains are mostly theoretical, and the barrel becomes as impractical as Pinocchio's nose.

Remember that a long skinny barrel is a lot more flexible than a short stiff one.  Eventually barrel flex will make a given airgun pellet and tune fussy, and generally inaccurate.  What is the point of missing the bullseye (or rat's brain) with 50% more FPE, when a good hit at less power will get the job done more effectively?

[JungleShooter]

Would a PP700 pistol gain from a longer barrel?

If a PP700 is tuned to shoot at 8 FPE from a 8.25" barrel, doubling the barrel length might add 10% FPE - probably less.  If it is tuned to shoot at 20 FPE, then doubling the barrel length would almost certainly increase the velocity and FPE significantly.

Very interesting!
Now — I have the reverse question — seriously. I'm considering shortening the barrel and the airtube of a PP700. 😊


Caliber .22, but could modify to .177, whatever is better.

Shortening of barrel by 1 to 3 inches.

Desired power level = 8FPE

Any suggestions, any guess-timation of power with a shortened barrel? 🤔


Matthias

[Motorhead]

Would a PP700 pistol gain from a longer barrel?

If a PP700 is tuned to shoot at 8 FPE from a 8.25" barrel, doubling the barrel length might add 10% FPE - probably less.  If it is tuned to shoot at 20 FPE, then doubling the barrel length would almost certainly increase the velocity and FPE significantly.

Very interesting!
Now — I have the reverse question — seriously. I'm considering shortening the barrel and the airtube of a PP700. 😊


Caliber .22, but could modify to .177, whatever is better.

Shortening of barrel by 1 to 3 inches.

Desired power level = 8FPE

Any suggestions, any guess-timation of power with a shortened barrel? 🤔


Matthias

You can't be serious
Try it and let US know ...