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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

What happens when you chop the barrel on a PCP ?

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lloyd-ss:
When you go to chop the barrel on your beloved PCP, you first want a good guess at how much velocity you are going to loose.  A good question, because once its chopped off, its not growing back.  And what else happens besides just loosing velocity?

I thought I'd show what might happen if I chopped the barrel on my disco double tube with its absurdly long 30" .25 cal barrel.  I think that folks generally agree that a longer barrel on a PCP will almost always give more power, just like heavier pellets almost always get more energy out of the air that is used.

What I have done is plotted the FPE of some shot strings out of the Disco double, in various states of tune in various points in its life.  The strings are taken from approximately 2200 psi down to 1600 psi, but not exactly. We are looking at the trends. 

One thing about plotting FPE instead of velocity is that the graphs of FPE look twice as bumpy as the same shot string with velocity graphed.  That is because FPE varies with the square of the velocity change, so for example, if the velocity drops by 3 %, the energy will drop by around 6%. So if you chop that barrel and loose 3% of velocity, you can see what the energy is going to do.

The reason I plotted energy is because it shows what is going on in the various states of power tune.  These strings were shot with different weight pellets, with the lighter pellets used in the lower power variations, so velocity isn't too meaningful.  Take a look at the trends, and in my next post I show what will happen if the barrel gets chopped.  And a lot of things happen.  I hope you like this.

Lloyd

Disco Double air tube, 310cc,  .25 cal, 30" barrel, various power tunes and pellet weights, strings are from approx 2200 psi to 1600 psi.

lloyd-ss:
So lets see what happens if you chop that 30" barrel back to something more reasonable?  First we'll take a look at what the effect would be when shooting at 65 fpe.  The example is 1900 psi as the reservoir pressure with 31.5 gn pellets.

Take a look at the graph; it shows the internal ballistics of the barrel.  There 3 curved lines of the graph represent:
1) The blue line at the top is the air pressure in the barrel. Its scale is on the right of the chart.
2) The red line is the FPE and it's scale is on the right edge.
3) The green line is the velocity of the pellet as it accelerates in the barrel and out the muzzle.

Shooting at this high power level, at the muzzle (30") the velocity is 965fps,
the energy is 65 fpe  and the air pressure that is behind the bullet when it reaches the muzzle is 591 psi.
An important point about this is that the air valve closes when the pellet is only 9" down the barrel at VC on the graph.  But if you look at the blue line you will see that the air pressure drops off very quickly, but still has 600 psi behind the pellet when it leaves the muzzle.  600 psi Will make a big boom !!

If you look at the vertical blue line at 14" of barrel length (if you chop the barrel to 14") you'll see that you lost about 165 fps, the 65fpe is down to 45fpe, and the muzzle psi is now a whopping 1250 psi.  That is a very big boom.

So by chopping the barrel from 30" all the way down to 14", you have lost a lot of power and velocity, and made the gun very loud.  But you can also see that each barrel length will produce different results.
Next time we'll see what happens when you chop the barrel of a low powered gun. Do you think it will have as much effect as in the high powered gun?
Regards,
Lloyd

This is a graph of a shot from the 16 shots at 65.6 fpe avg string.
 1900 psi, 31.5 gn, .25 cal 30" barrel , 965fps, 65fpe



This next graph shows a shot from the 44 shots at 43.5 fpe avg string, shot at 1900 psi.  In this shot you can see that the valve closes when the pellet is only about 4 inches down the barrel.  Most of the velocity is gained in the first 16" of barrel travel, so chopping the barrel at this power level won't show much of a power loss.
Also, look at how much lower the residual air pressure in the barrel is when the pellet leaves the muzzle.  That is what determines the loudness of a plain barrel.
This is a graph of a shot from the 44 shots at 43.5 fpe avg string.
1900 psi, 25.4 gn, .25 cal 30" barrel , 895fps, 45fpe

Note- The last two graphs are theoretical, with validation via empirical data collected from thousands of shots in numerous PCP configurations of many different calibers and barrel lengths.  The methodology takes into account multiple variables including xfer port volume, pellet fit in the breech, barrel drag, mass of the air column behind the pellet, tank volume, system efficiency, and numerous other factors.  The airflow through the valve is averaged as a crisp open and close scenario which simulates the actual abrupt rise and fall of flow in the valve.  All algorithms were developed by Lloyd-ss.

I know this is the real geeky math stuff, but I really enjoy it and I hope it can provide some ideas about barrel lenghts and other things about PCP performance.
Please feel free to make comments or ask any questions or if you want me to graph any other types of setups. :D
Lloyd

Markez:
WOW you must have read my mind, I was just tinking of ordering a LW barrel for my Mrod and I can get it machined
from Jim Gaska in 20" or 17" and was wondering which would be best for me. The LW barrels are going to be louder
than the factory barrel because they are larger diameter with less room in the shroud to scrub off air. If I get the 17"
then I have room for two more baffles in the shroud to compensate for the extra noise. I have the Mrod tuned for
2000 psi shooting down to 1300 psi with a .22 14.3 gr. and an average of 839 fps 22.3 fpe with a 30 shot string. I can
see from your 2nd graph that I would not loose allot even though my numbers are not even close to yours, about
half the fpe. I can always tune the gun up if I need to. That kind of math is way over my head and I appreciate the
time it took to do, Thanks you.

Mark

lloyd-ss:

--- Quote from: Markez on January 10, 2012, 12:52:22 PM ---WOW you must have read my mind, I was just tinking of ordering a LW barrel for my Mrod and I can get it machined
from Jim Gaska in 20" or 17" and was wondering which would be best for me. The LW barrels are going to be louder
than the factory barrel because they are larger diameter with less room in the shroud to scrub off air. If I get the 17"
then I have room for two more baffles in the shroud to compensate for the extra noise. I have the Mrod tuned for
2000 psi shooting down to 1300 psi with a .22 14.3 gr. and an average of 839 fps 22.3 fpe with a 30 shot string. I can
see from your 2nd graph that I would not loose allot even though my numbers are not even close to yours, about
half the fpe. I can always tune the gun up if I need to. That kind of math is way over my head and I appreciate the
time it took to do, Thanks you.

Mark

--- End quote ---
Hey Mark,   If you have the current barrel length and tank cc's  that you shot the 30 shot string with, I could run some numbers for you and see how it might change with both the 17 and  20" barrels.  I could probably also do a prediction of how the shot count might drop if you re-tuned the Mrod with the shorter barrel to bring the fpe back to the current 22.3fpe average.  The numbers you cited are for that 30 shot string, shooting from 2000psi down to 1300, correct?
Lloyd

Onebaddj:
Thats for all info lloyd! Little nerdy yes but very helpful. Ive only built one pcp and am planning a 2nd and this will help alot.

Along with your videos ive have learned a ton from you and the other members.

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