For the Love of Carbines

[rsterne]

As a result of some questions via PMs from various members.... and in trying to answer their questions as accurately as possible.... I have "discovered" some things that may be of interest to anyone comtemplating building a Carbine.... Well to be fair, I knew them already, intuitively, but now I can put some numbers on them.... I was able to "do the numbers" primarily because of a spreadsheet from Lloyd which should perhaps be called "The PCP from Click to Bang" (with apologies to the Cardew brothers' books "The Airgun from Trigger to Muzzle/Target").... It is becoming invaluable to me, many thanks, Lloyd....

One of the most frequently asked questions is "how long should I make the barrel".... The answer, from a performance point of view, is inevitably "as long as possible".... With the help of Lloyd's spreadsheet, I can take a PCP (or CO2, maybe a pumper) rifle of known performance, substitute a shorter (or longer) barrel length, and determine quite accurately what would happen if you change the barrel length.... The answer depends on many factors, but the starting point is the air pressure and volume, velocity, air usage, and barrel length.... Knowing those, I play with the other variables in the program until I get the numbers agreeing with the baseline rifle.... Then by simply changing the barrel length (and nothing else) the spreadsheet predicts a new velocity and energy for the new length....

I'll give you an example, starting with my .25 cal Disco.... It has a 135 cc air reservoir, filled to 2100 psi, and returns 11 shots of 45 FPE (25.4 gr. @ 893 fps avg.) from it's 24" barrel.... Here is what the spreadsheet tells me for different barrel lengths.... It won't be perfect, but this should be a very close approximation....

24" - 45.0 FPE
22" - 43.3 FPE (3.7% loss)
20" - 41.5 FPE (7.8% loss)
19" - 40.5 FPE (10% loss)
18" - 39.4 FPE (12.4% loss)
16" - 37.1 FPE (17.5% loss)
15" - 35.8 FPE (20.4% loss)
14" - 34.5 FPE (23.4% loss)
12" - 31.4 FPE (30.2% loss)

I highlighted some of the lengths to demonstrate the trend.... The first 5" you cut off the barrel loses 10%.... The next 4" loses another 10%.... and the next 3" loses yet another 10%.... At half the barrel length, you are down over 30% in FPE.... Incidently, adding 6" to the barrel length (ie a 30" barrel) adds about 10% to the energy.... Now if you start with a gun that has a lot less power, the losses are less, not only in absolute numbers, but in percentages as well.... Let's start with a gun with less than half the power in .22 cal.... 20 FPE (18.1 gr. @ 706 fps) from a 24" barrel.... Here are the results....

24" - 20 FPE
17" - 17.9 FPE (10.3% loss)
12" - 15.7 FPE (21.3% loss)

This time it takes about 7" off the barrel to lose 10% in energy.... and half the barrel length drops the energy just over 21%.... compared to a 30% loss in the more powerful gun.... There are a lot of variables, particularly in a PCP, that can sway these results.... Higher pressures and very short valve timing will generally produce less losses with short barrels.... Lower pressures and long valve timing will not work well in short barrels, as eventually it will become a "dump shot" where the valve is still open when the pellet has left the barrel, simply wasting air.... This shows why Crosman took the approach they did with the PRod.... high pressures and limited valve timing because of the short barrel.... Otherwise you waste a lot of air, especially if you are trying for big FPE....

One other thing I discovered when running these numbers through Lloyd's spreadsheet.... The number of shots apparently stays the same, even though the FPE is less.... That means that the efficiency is dropping by the same amount as the FPE numbers.... I guess I also knew this intuitively, but seeing the numbers really brought it home.... Using the first example, cutting the barrel in half not only dropped the power by 30%.... it reduced the efficiency by the same percentage.... You are using the same amount of air to produce the same number of shots, but at 30% less FPE.... 

I hope this information is useful to any of you contemplating building a Carbine.... The lesson is, cut that barrel with care.... or the gun will likely not live up to your expectations.... Here is a specific example, starting with my .25 Disco.... Let's use a 19" barrel to limit the loss to 10%.... Let's shorten the air reservoir by 5" as well.... That reduces it from 135 cc to 100 cc instead of 135.... The resulting Carbine should produce about 8 shots at 40.5 FPE.... Cut two more inches off the barrel (17") and air reservoir (85 cc).... and the spreadsheet predicts 7 shots at 38.3 FPE.... 

Bob 

[bradyman1]

Great info Bob. Thanks for the explanation. I have a question. Now what happens if we go to Co2? In particular the 22XX series. I know my self as well as several others I have seen have questioned what barrel length is the best trade off of velocity and compactness for the custom shop 2400 carbines. Can you run the numbers on that gun using the barrel lengths offered by Crosman in .22. Crosman offers a 24", 18", and a 14.6" version of the .22 barrel.

[run and gun]

i'm firm on the 14"barrel for the 2240.very very effective. some good points you make here.thanks bob.i love the carbine approach on this gun.

[rsterne]

CO2 is difficult, because the numbers change with the temperature.... I've seen the article about the optimum barrel length being about 20-21" for CO2, but that hasn't been my experience.... or Crosmans.... They list the 8" barrel 2240 at 460 fps.... the 14" barrel 2250 at 530 fps.... the 18" barrel 2250 XE at 550 fps.... and the 24" barrel 2260 at 600 fps.... I've found those numbers to be quite accurate at about 70*F (except for the 24", 580ish is more like it).... I've played with the spreadsheet, and it also predicts ever increasing velocities with barrel length, right up to 30".... The change in velocity is very small at the longer lengths, and greater with carbine lengths, just as you would expect.... It is very large at pistol lengths, going from 8" to 10" gains about 30 fps.... The more you mod it for power, the more important a longer barrel becomes.... Here are the spreadsheet predictions, starting from 580 fps on a 24" barrel (ie a stock 2260).... Note that you lose twice as much velocity with a 10" barrel as you do with a 14.5" barrel....  

24" - 580 fps - 10.7 FPE
18" - 557 fps - 9.9 FPE (9% loss)
14.5" -  535 fps - 9.1 FPE (15% loss)
10" - 489 fps - 7.6 FPE (29% loss)
8" - 459 fps - 6.7 FPE (37% loss)

I played around with the spreadsheet to get it to agree with some known data points (mine and Crosmans).... In no way did it predict a "magic" barrel length shorter than 24".... I suppose that for a given combination of a tight pellet and low temperature there might be such a thing, but I've never seen it.... Incidently, by the time I got to the above set of numbers, the 2240 was experiencing a "dump shot" which means the pellet is leaving the barrel while the valve is still open.... Ever wonder why a 2240 is so loud, and why you can often see liquid CO2 coming out, especially if you are shooting uphill?.... With a 10" barrel, the valve is closing just as the pellet leave the muzzle.... After that, increasing barrel length sees expansion of the CO2 in the barrel after the valve closes.... The gun gets quieter and more efficient as a result....

Modding the gun to increase power and CO2 consumption means even a longer barrel for best performance.... Sorry, but the same rules apply as for a PCP.... especially if you mod your 22XX....

Bob

[Bimota]

Thanks great info.

[run and gun]

the main reason i chose the 14"barrel the extra fps and fpe.this one the internals  have been done up by Roy@ MA.i guess i wanted all the power with that barrel length.it shoots really strong providing the temp is above 50*f.i only get 18 full power shots per powerlets.using cphp's.but that's fine with me. your right, lots to consider when building a carbine..

[bradyman1]

Thanks Bob. I have the 14.6". For the compactness of it the numbers seem to tell me I made the right choice. If I decide to mod the valve that 18" would probably be the choice.

[rsterne]

I would agree with that assessment.... My Grouse gun is a 20 FPE 2260 based PCP Carbine and I have the 14.6" barrel on it.... If I was going for more power, I would use a longer barrel....

Bob

[rkr]

I can more or less confirm your results based on my AR6 and Drozd experiments. The gun needs to be tuned for a certain barrel length or you'll be wasting air. Shortening the barrel for a well tuned PCP will always waste air. When shortening the barrel, you'll loose only a little fps if you down tune the gun while gaining lots of shots.

[glassman98]

Bob,
Good reading. Could this also be used for springers/gas pistons guns?  Craig

[0351_Vet]

Bob,

Good stuff here. Thanks for taking the time to present this helpful info.
BTW, Can you post a picture of your 2250/14" Carbine. That would be cool.

Thanks

[lloyd-ss]

I can more or less confirm your results based on my AR6 and Drozd experiments. The gun needs to be tuned for a certain barrel length or you'll be wasting air. Shortening the barrel for a well tuned PCP will always waste air. When shortening the barrel, you'll loose only a little fps if you down tune the gun while gaining lots of shots.

Good observation (I added the bold font for emphasis) and one of the tradeoffs Bob is talking about.  Another way to think about it is that if your PCP is "High Power" for its caliber, you will loose more velocity from chopping the barrel than would a lower powered PCP of the same caliber. 
Bob's numbers quantify those losses so you know what the velocity sacrifice will be.
Lloyd

[rsterne]

I agree that tuning the gun down is a good idea for a Carbine.... Your statement is correct comparing barrel lengths AFTER the detuning.... The detuned Carbine won't "lose only a little fps" compared to the full power rifle you started with, however....

This data is not valid for spring / gas piston guns because they don't rely on the expansion of a gas, but rather on its compression, which produces heat and may even produce slight combustion of the lubricant film on the cylinder wall.... They are a completely different breed, and can often work extremely well with barrels under 12" long....

Here is a photo of my 2260 based PCP Carbine with 14.6" barrel.... 13XX grip and modded skeleton stock.... 4 x 32 AO Leapers scope....



I made all the aluminum parts, including the repeater breech.... The gun gets 16 shots (2 clips) at 20 FPE (18 gr. JSBs at 700 fps) per 2000 psi fill, refilling at 1100 psi.... all shots within 5% of peak....

Bob

[0351_Vet]

That's "Bitchin" Bob, thanks for sharing. Also, that is .25 Correct?

[rsterne]

No, my Grouse gun is a .22 cal (18 gr. JSBs)....

Bob

[0351_Vet]

Bob, So those numbers were on .22? (Not .25) Thanks...Okay, I'm an idiot, trying to leaarn.

[Nate the airgunner]

I agree that tuning the gun down is a good idea for a Carbine.... Your statement is correct comparing barrel lengths AFTER the detuning.... The detuned Carbine won't "lose only a little fps" compared to the full power rifle you started with, however....

This data is not valid for spring / gas piston guns because they don't rely on the expansion of a gas, but rather on its compression, which produces heat and may even produce slight combustion of the lubricant film on the cylinder wall.... They are a completely different breed, and can often work extremely well with barrels under 12" long....

Here is a photo of my 2260 based PCP Carbine with 14.6" barrel.... 13XX grip and modded skeleton stock.... 4 x 32 AO Leapers scope....



I made all the aluminum parts, including the repeater breech.... The gun gets 16 shots (2 clips) at 20 FPE (18 gr. JSBs at 700 fps) per 2000 psi fill, refilling at 1100 psi.... all shots within 5% of peak....

Bob

Nice gun.

[bobster]

Great stuff Bob!

I have a question relating to velocity and muzzle brakes.

I assume that the longer barrel increases velocity due to allowing greater expansion of the gases behind the pellet.  I have seen this in .22 rimfire rifles/pistols.  A .22 CB long round (700ish fps) will sound quieter than a cap gun in a long barreled rifle.  I assume this is due to full expansion of the propellant gases.  The same round in a pistol is very loud.  I assume this is due to the gases expanding still as they leave the barrel.  In other words the pistol is allowing a big cloud of expanding gas immediately after the bullet leaves the barrel vs on a long barreled rifle the gases have already expanded and come out at a lower pressure.  It would be interesting to make a photo of the gas cloud of a rifle vs. pistol with the CB long round.

So my question is, what effect would a muzzle brake (specifically a baffled LDC) make on velocity, if any?  It seems to me all the gases have done their expanding within the barrel and imparted whatever velocity they would on the pellet by the time it leaves the barrel.  The brake would simply catch the expanding gases (the cloud) and not add any propellant force to the pellet.  Also, any chance it would slow down the pellet?  Thanks for the great info.  Love these type of posts.

[MustangMike]

I think its been tested before and you loose about 10fps give or take a few with a ldc like the tko ldc's.. but don't hold me to that its been awhile since I remember reading that

[melvin]

Good info.  Thanks Bob.