What happens when you chop the barrel on a PCP ?

[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.
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]

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

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.

[Markez]

Thanks Lloyd It is a stock Mrod but my big idea of getting a LW barrel is now on hold because I just got in from testing a new
pellet(Barracuda Match 5.53 mm 21.3 gr.) and the gun really liked them. The very first 25 shots came in at 1/4"(3 5 shot groups)
and 3/8"(2 5 shot groups) at 25 yards. Not sure if I can improve on that with a new $200 barrel. I still need to do some 50 yard
testing with this pellet. I was getting frustrated cuz every pellet I tryed shot like &^^&. I think I have close to $150 in pellets that don't
work in my gun but now I am feeling better about the gun now. My best groups before were 1/2" at 25 yards and 1.5" at 50 yards
which is ok but not great.

By the way that is some kind of hammer you got there.

Mark

[Jay]

Man I love having you around Lloyd!!! About half of what you do make's me have to sit an scratch my head to get a firm grip on, but the info is just great. I save all of your's an Bob's stuff(wish the best to him).

[lloyd-ss]

Hey all,
Glad some of you are enjoying this thread.  I must admit that it's not everybody's cup of tea. Well, seeing that Markez had been so careful about recording the shot data from his MRod, I went ahead and put his data into my PCP program.

Stock MRod
This graph shows a .22 MRod with stock 20" barrel and 215cc tank, shooting 14.3 gn at 839 fps, and
using approx 23 psi per shot. This particular shot was made at 1650 psi.
                   Stock MRod


What I think this graph really shows is that this MRod was shooting at a fairly low power for its real capability, and that much of the barrel length wasn't really adding to the velocity.   But the extra length does give room for the air to expand and drop the noise signature.  You can see that cutting a few inches off the barrel wouldn't cost too many fpe.  But if you chopped the barrel and then wanted to up the power level, you might have some problems.

The steepness of the lines in the graphs  is an indication of how hard you are pushing the set-up (barrel length) that you have.  The MRod velocity is practically flat for the last few inches, and for the .25 cal shots at 65fpe, the velocity is still climbing as the pellet leaves the barrel.  Extra length will give you more velocity without needing anymore air. 
Lloyd

[MustangMike]

lloyd ur are the man when it comes to this to stuff.. i wish i understood this stuff as well as you do, but i guess that will come in time after messing around with pcp's

[rkr]

Lloyd,

Since you like doing these charts, would mind showing me what would happen if I were to swap the 18.5" barrel on my Dynamax to a 15" one?

Here's shot string data with .22 21.3g baracudas:


bar    200   197   195   192   189   187   184   181   178   175   173   170   167   164   161   158   155   152   149   146   143   140   137   134   131   128   126   124   122   120
fps    795   809   803   808   812   816   816   816   817   819   820   820   824   824   824   815   820   822   821   822   816   812   810   807   804   801   796   789   788   779

[goodland]

Thank you Lloyd great work and very interesting stuff.
I love math too

[lloyd-ss]

Lloyd,

Since you like doing these charts, would mind showing me what would happen if I were to swap the 18.5" barrel on my Dynamax to a 15" one?

Here's shot string data with .22 21.3g baracudas:

..................

Need one more piece to the puzzle... the tank volume.  Or if you don't know what it is, you can measure the inside diameter and internal length of the air tube and give me those figures.  Or a good estimate of the inside dimensions.
Lloyd

[lloyd-ss]

Thank you Lloyd great work and very interesting stuff.
I love math too

Cool.  The graphs show only 3 of the outputs from the excel spreadsheet that does the calculations.  I really have about 30 different inputs and outputs from the spreadsheet, including various efficiencies and what-ifs, such as: what will happen if I use a heavier pellet, or a tighter pellet.  Do you know how many g's a pellet feels? I might post some more of the output data.
Lloyd

[TimmyMac1]

On the Stainless Steel Barrels Benchmark makes there is an enormous amount of frictional drag compared to a conventional barrel which means the Barrel time is longer.
The reason for the drag is the Barrels are tapered for the entire 30" length and pushing a pellet thru it is a very different feel. The Barrels are tighter to start with then they taper down .0007" (7 tenths) and the taper is totally gradual.
I fitted a gun with a 24" LW barrel in .22 an it shot harder than the 29" Benchmark. I shortened the Benchmark a few inches and it sped up a little. I suspect the duration of barrel time relative to valve time to be lowerring the pressure at the muzzle to the point where the last few inches are not accelerating and are even losing a little speed.
You've done some really neat work. I've often said the quiet guns that shoot hard are efficient and the Muzzle Pressure (noise)is low. Accuracy and Efficiency almost always hold hands.
15 fps per inch has always been the figure when you were talking cost per inch but as you have discovered it is not always the case. Exceptions to the rule do exist and the Benchmark Taper Lapped AirRifle Barrel that took the worlds BR win plus 3rd & 7th has its own particular charachteristics that work awesome.
When they tested the Guns for Power at the Worlds with the Benchmark Barrles they had the 5.52 Baracuda Match pellet to use as the test pellet. The pellet was mismarked apparently (More like 5.56)'cause Our guns normally shot them at 15 fpe and the ones the Worlds Staff had shot about 200 fps. We were allowed to shoot 20 fpe with a Pellet our guns put out at a paltry 2 fpe.
If we had tuned for their pellet we would have probably been shooting the 13.4's at almost 1000 fps.

[lloyd-ss]

Tim,
Thanks for the comments and the insight on the tapered Benchmark barrel.  Seven tenths taper is significant in a small caliber, and I find it interesting that that amount of taper is less than the manufacturing tolerance of many mass produced barrels.  Really good target barrels seem to be on a totally different level from commercial.
I had to read your comments about the varying sizes (5.52 vs 5.56) and velocities a couple of times to grasp what you were saying.  Then I realized how disastrous that could have been, at least temporarily, until you were able to tune the gun back for the proper size pellet. 

I have done my best to reproduce the effect of real-world variables with my program, and pellet drag is one of them, both the initial break-away force at the breech, and the drag in the barrel.  Would you estimate that the drag might increase by one  to five pounds from it's initial value in the Benchmark barrel?  With a little more information, I am wondering if my program would accurately predict those velocity changes?  Knowing the actual pellet drag values in pounds would be needed.   I have seen the phenomenon of a velocity increase when shortening a barrel, but only with low velocity airguns in small calibers and long barrels, and my program can reproduce that, with a fixed drag value, but a linearly increasing drag seems to be more appropriate in some situations.   
The larger calibers generally significantly overpower the bullet drag unless it is very significant.  There are so many variables that affect airgun power, and as you have pointed out, barrel length is only one of them.
Lloyd

[Markez]

Great chart Lloyd. It's cool how the chart matches up with the shot strings that I have recorded at that psi.

Mark

[rkr]

Lloyd,

Since you like doing these charts, would mind showing me what would happen if I were to swap the 18.5" barrel on my Dynamax to a 15" one?

Here's shot string data with .22 21.3g baracudas:

..................

Need one more piece to the puzzle... the tank volume.  Or if you don't know what it is, you can measure the inside diameter and internal length of the air tube and give me those figures.  Or a good estimate of the inside dimensions.
Lloyd

154cc according to BSA's export catalogue.

[TimmyMac1]

Tim,
Thanks ... Really good target barrels seem to be on a totally different level from commercial.
I had to read your comments about the varying sizes (5.52 vs 5.56) and velocities a couple of times to grasp what you were saying.  Then I realized how disastrous that could have been, at least temporarily, until you were able to tune the gun back for the proper size pellet. 

I have done my best to reproduce the effect of real-world variables with my program, and pellet drag is one of them, both the initial break-away force at the breech, and the drag in the barrel.  Would you estimate that the drag might increase by one  to five pounds from it's initial value in the Benchmark barrel?

With a little more information, I am wondering if my program would accurately predict those velocity changes?  Knowing the actual pellet drag values in pounds would be needed.   I have seen the phenomenon of a velocity increase when shortening a barrel, but only with low velocity airguns in small calibers and long barrels, and my program can reproduce that, with a fixed drag value, but a linearly increasing drag seems to be more appropriate in some situations.   
The larger calibers generally significantly overpower the bullet drag unless it is very significant.  There are so many variables that affect airgun power, and as you have pointed out, barrel length is only one of them.
Lloyd

It is hard to quantify the value as I don't do that sort of thing but I'd quess the combination of smaller bore to start with and squeezing the pellet 7/10s amount to three to four times the normal resistance. Instead of leade and start forces added the choke transition and general friction it doesn't come close to how much your pushing on a Rod.

With an oversize pellet it is getting forced down to a smaller caliber and there is no room for the mass. I tried to explain the phenomena to the Sanctioning body telling them the Success of the BR SS Benchmark would foster growth in that direction and the use of the Baracuda match should be reconsidered in light of what went on this year. I suggested the test pellet should be something more inline with what we are going to actually use to make sure these sort of irregularities would converge again.
I suggested that if the opportunity to turn her up legally was dangled in front of all using the BR SS barrel someone was bound to take advantage of the situation and be totally within the letewr of the Rules albeit not the spirit of the rules. I did not turn my gun up as I had not tested it at any power level other than 20. ft lbs. so I wasn't going to change a thing as this was the worlds. The point I was making was if they use the same pellet in Australia 2015 they could be creating grief for themselves, because someone will be ready to take advantage of the anomoly.

The reaction was swift and abrubt as they explained to me nobody cheated and it is outrageous to suggest they did.

This is what we get when we communicate with a common language but it is Brit/American and their can be misinterpretations. I was trying to explain the physics of the situation and they took it entirely somewhere else. NeverMind!

A perfect example of too much barrel. 1986 I did the Painball development work for Benjamin/Sheridan B4 they got bought by Crosman. They had 22" painball rifle with standard output F valves. It shot 190 fps on a Warm day which is 100+ fps shy of the target.

I cut the barrel and tube off to 13" and the gun shot 300 fps. They had already done the hipe and the gun was going to be long so they kept sending me more guns to turn into 22" single cartridge guns. I kept dual CO2 modifying them and sending them back with 13". It worked awesome and the customers were chomping at the bit. The factory threw up their hands and I offerred to buy the entire 200 guns. They took my money and then reniged on the deal and returned my money a month later.

The point was the gun would not work with a 22" but that is what they had told the consumer they would deliver.

The funds had been earmarked at a certain rate of return that was not going to happen if I had not come up with the Pump Action Innovation to save my butt. Neccesity can be a mother sometimes.

Getting the valve volume to match the barrels needs is how we make accuracy as far as I can tell.

On PCP's the valve volume available needs to be throttled at some choke point so the Duration of the valve is aided by the port choke stahl that provides a pressure wave/signal for the valve to shut.

WE use dynamic flow choked at a Specific point(in US FT it is the Tranfer port restriction creating the choke and in a LD or Crosman we use the Bolt Probe end which we tune for barrel length). Just like a Wing that loses lift and drag as airspeed goes thru transitional Mach(.82-1.0+), porting will choke when the velocity of the air hits mach. By trimming the port ID or changing the Probe OD we can influence the timing.

When you get it right it is amazing and the gun shoots great.

There is a certain sound that is crisp when the gun fires and it sounds like it is right even. When a gun goes blaugh! vs Crack it is dead obvious which is more efficient.

[lloyd-ss]

Spysir, did you actually cut the barrel off an inch at a time?  I love to see the real numbers.  It is starting to drop off faster at 14", and will drop even faster as it gets shorter.
Lloyd

[lloyd-ss]

rkr,
This is for you and your Dynamax and the numbers you provided. Looks like shortening the barrel isn't going to cause a problem.  I ran the numbers at both full tank and low tank to see if there was going to be much difference.  See what you think.
Lloyd