Friday, August 03, 2007

How barrel length affects velocity in a pneumatic airgun

by B.B. Pelletier

How barrel length affects velocity in a CO2 gun

I didn't intend for this to become a series, but the comments to the first post about CO2 guns requested this post. Okay, I offered it at the end of that first post, so I guess I wanted to do this one, as well.

I was really surprised by the results of that CO2 barrel length report - they weren't what I expected at all. When I thought about it, the reason I said that I thought a 24" barrel on the Crosman 2250 would increase velocity by 100 f.p.s. is because most of my experience has been with air, where there really is a dramatic difference like that! Today, I will make the case that increasing barrel length in a pneumatic gun readily increases velocity.

Airguns of the past had longer barrels!
My first piece of evidence comes from the past. All the big bore airguns of old had long barrels, and the reason they did was for velocity. Those old guns (and I am including the few rifles among the guns) operated on air pressures of 500 to perhaps as much as 900 psi, though I suspect that last pressure to be a bit high. Most of them were comfortable with 600-700 psi. We know this for a fact because Dennis Quackenbush and Tom Gaylord did a study of vintage pump designs and determined their limits. They demonstrated that a single-stage hand pump can generate up to 1,275 psi if the person doing the pumping is heavy enough and the pump piston is small enough. They also found a practical limit for the old pumps, which actually delivered around 800 psi, give or take. Their study is published in Airgun Revue 4.

The ONLY way to generate higher velocity with pressures that low is with a longer barrel. Indeed, when we measure the length of vintage big bore airguns, they have barrels that range in lengths from 28 to 33 inches. There are guns with longer barrels, but there aren't many with shorter barrels (except for handguns and guns made for children). The locks on the old guns held the valves open much longer than contemporary airgun valves. An examination of the vintage valves reveals huge air passageways! Everything was designed to move large volumes of air. It takes a long barrel to take advantage of that.


The long barrel on this Shembor repeating air rifle is typical of all vintage big bores.


If you're interested in how these old guns operate, check out the DVD Antique Bigbore Airguns - From Novelty to Necessity.

A more elegant proof
Proof No. 2 is a remarkable air rifle that has stunned the world! The USFT rifle produced by Mac-1 Airgun delivers 55 shots of a 10.6-grain Beeman Kodiak pellet traveling at 900 f.p.s., and it does it on a starting air pressure of just 1600 psi! When the string is completed, the gun will still have 1100-1200 psi remaining, so all that work is done on as little as 400 psi of air. The air reservoir of the rifle is HUGE! Although the air PRESSURE is relatively low, the VOLUME is very great. Obviously, when a given volume of air is made smaller without air loss, the pressure increases. The USFT rifle reverses what most modern PCPs do. It uses a larger reservoir to lower the pressure of the air. Same amount of air - bigger volume.


USFT rifle from Mac-1 Airgun does more with less air pressure than any modern air rifle.


The ONLY way to use this lower-pressure air effectively is with a longer barrel. When the pellet leaves the barrel, the push provided by the air stops. The USFT rifle uses a 25" Weihrauch barrel, which is the longest one they make. If they offered a 28" barrel, I have no doubt it would be incorporated into the USFT - not to increase the velocity, but to lower the air pressure needed to achieve 900 f.p.s. By the way, this rifle starts at $1,650 and goes up rapidly past $2,000 (and Mac-1 is backordered for several months from the demand). Top competitors are switching over to this purpose-built field target rifle, and they all agree that 900 f.p.s. is the ideal velocity, when accuracy is on the line. This gun just took four of the top five places at the U.S. Nationals, including first place.

The final proof
The final proof is so positive that there is no way to refute it. The .22 caliber AirForce Talon SS gets 820-850 f.p.s. muzzle velocity with a Crosman Premier pellet. It has a 12" Lothar Walther barrel. When you substitute a 24" Lothar Walther .22 caliber barrel, that SAME GUN jumps to 1,000 f.p.s or a little more. That's pretty amazing, but it doesn't end there. Because the Premier is going too fast for good accuracy in the longer barrel, loading a 20.5-grain Beeman Kodiak will slow down the speed to 920-950 f.p.s., but the ENERGY increases from about 32 to over 40 foot-pounds. And, a 28-grain Eun Jin will take the energy up to 45 foot-pounds with a velocity of 850 f.p.s.! The basic rifle with its shorter barrel was getting about 23 foot pounds from the Premier!

You cannot keep extending the barrel forever. There is a turnaround at some point, and it will relate to the caliber of the pellet. A smaller pellet equals a shorter optimum barrel length. But air being so much thinner than CO2, this turnaround point will be much farther from the breech than what we saw in the CO2 test.

To the best of my knowledge, this has never been thoroughly tested. If it has, I don't know where to find the results.

33 Comments:

At August 03, 2007 7:13 AM, Anonymous Anonymous said...

thanks for addressing this,...now i feel comfortable doing my barrel mod.

 
At August 03, 2007 8:11 AM, Anonymous scott298 said...

B.B.--Scott298--you puled a fast one on us today-set the alarm for 5am just so I could reas the assurasy test on the gamo extreme-oh wel there's always tomorrow (please)--Scott298

 
At August 03, 2007 8:42 AM, Blogger B.B. Pelletier said...

Scott298,

What happens is readers ask for things that I then promise them. This morning was from that.

The accuracy test will be next week, but not Monday or Tuesday, I think.

B.B.

 
At August 03, 2007 9:07 AM, Anonymous Anonymous said...

BB

Variable Power PcPs

I own a Condor, and was shocked to find that when i turn the power down, after 10 shots or so its back to full power, yet the dial stays where i put it (full power was verifed by a chronograph). I called Daystate about their Airwolf, undeniably one of the finest techy airguns forsale, and asked them about their 40ft-lbs version. She told me that while it also had a power dial, after a number of shots, it would return to full power, just as my COndor does. She didnt however have an explanation as to why....Over to you BB :-)

 
At August 03, 2007 9:44 AM, Blogger B.B. Pelletier said...

You have discovered the performance/pressure curve.

Daystate is a fine company, but no amount of money in the world can get past physics.

The problem is the power adjustment feature. Without that, the valve can be optimized to one hammer spring tension and balanced to give a good performance curve (one that's relatively flat, start to finish). Whe you change the hammer spring tension without tuning the valve each time, you are asking for the impossible. A performance curve that is variable with the hammer strike, yet consistant begining to end. It cannot happen that way.

I'll tell you what I think is great is the fact that Daystate both knows that and is willing to be honest about it. I think that speaks well for the integrity of the company.

B.B.

 
At August 03, 2007 9:54 AM, Anonymous Anonymous said...

Power curve

regarding Daystates honesty, i absolutely agree. I was thinking about buying their 40ft-lbs gun on the basis that i could turn it down to 12ft-lbs for 90% of the time, and just turn it up for the odd bit of fun. Due to their honesty, they have not sold me something i would have regretted, which to them seemed more important than losing a $2500 sale

 
At August 03, 2007 9:56 AM, Anonymous jmz said...

BERETTA Px4 Storm anyone?

Has anyone seen it? (by seen it, I mean actually touched it)... tested it? heard from a friend about it?... anything...

 
At August 03, 2007 11:17 AM, Blogger B.B. Pelletier said...

jmz,

It's not available in the U.S. yet. Should be here soon.

B.B.

 
At August 03, 2007 1:14 PM, Anonymous Vince said...

BB - just got an RM2003 (refurb from Umarex). You're right about that dual trigger - it looks strange but works quite well.

The barrel insert arrangement looks pretty solid, although I'm shooting the .22 until the oil works out of it.

The rear sight on that thing is terrific - a tremendous range of adjustment and very solid adjuster mechanisms. It looks to be identical to the one on the RM2000 you tested, except that it has a peep apeture instead of a fiberoptic iron sight. I'm hoping I can find this sight as an aftermarket accessory.

 
At August 03, 2007 1:18 PM, Anonymous Vince said...

Just found them! It seems that Pyramydair carries them as replacement parts for the RM600 - in both the open-blade and peep version.

 
At August 03, 2007 4:13 PM, Blogger B.B. Pelletier said...

Vince,

Isn't that rifle well made? I haven't tried the RM 2003 with its changable barrels, but I have seen the peep sight. Best on the market for my money.

B.B.

 
At August 03, 2007 5:36 PM, Anonymous Vince said...

I've shot it some more, and the power seems to be a bit disappointing - as you noted when you shot the 2000, the factory REALLY exaggerated the velocity claims. I'm getting in the mid-upper 600's with Gamo Match pellets, which it seems to like... although I still have some inconsistency due to oil combustion. Still, considering how hard it is to cock (reminds me of a Powerline 1000), I would have hoped for better. My wife can shoot a Shadow for as long as she likes, she can barely cock this thing once.

That said, the rifle has quite a few neat features - on mine, the front of the piston is painted red, so the oil hole doubles as a "cocked" indicator. When the gun is cocked, you see red in the hole... when it is not, you see silver.

The firing cycle and trigger are notably better than a Gamo breakbarrel, but otherwise (like during the cocking cycle) the Gamo seems to work more smoothly. And the woodwork and metalwork on the Mendoza seems (to me, anyway) to be a notch below the Spanish guns.

I'm still in the "evaluation phase" with it. Hopefully, when the velocity gets more consistent it'll start shooting more respectably.

 
At August 03, 2007 6:17 PM, Anonymous .22 multi-shot said...

Vince, I really liked the RM2000, but I returned it because of the velocity. I was only getting 500fps with JSB Exact Jumbos (15.8gr) and 640fps with the Mendoza solid skirt pellets (13.8gr). That is way below the advertised 850fps and not even as good as my RWS 850 AirMagnum!

I didn't clean the barrel though. I remember BB said the Mendoza has the hardest barrel to clean and velocities improved afterward.

.22 multi-shot

 
At August 03, 2007 7:14 PM, Blogger makka said...

Bragging!
Not true.
I was just surprised that they were like that with the rifle resting direct. I was genuinely curious to hear what you had to say about it.
It doesn't require that much skill when the rifle is pointing at it all on it's own!

I'll get on and do some bragging if I manage to halve them on my hand!

 
At August 03, 2007 7:54 PM, Anonymous Anonymous said...

I'm considering acquisition of a 4500psi bulk tank to feed my 2800psi(max) Daystate Harrier.

Do I need a regulator, or can I simply (and safely) rely on carefully cracking the valve open while watching a pressure gauge?

Plus, how about a Daystate blog? Pyramyd doesn't sell them, but I'm sure a lot of us are interested.

 
At August 04, 2007 2:52 AM, Anonymous Anonymous said...

I love this blog but i would go to that to. I have an air wolf as of a month ago. Its having some custom work done so its not around for testing and use at the moment. I dont think Pyramyd would do a Daystate t blog for the same reason. They don't sell them.

Like my Wolf more than my theoben rapid. The rapid is as accurate. It has shot to shot consistancy that is about as good as the electric regulator (both have about 5 fps). I like the electric trigger but the theoben is close. I hade my theoben ready for action for $2000 (scope, 4 mags, 2 tanks, fill adapter, and a theoben scope mount). i am not done with the Wolf after and have spent twice that. A rapid mk2 is 1600 for just the gun and the Daystate AirWolf is 2200 for the gun alone. The theoben i have is set up with a small scope for hunting in the feild. The daystate i have is set up for hunting BUT its for longer range hunting (it does have 13 foot pounds on the rapid) If you ask me, the theoben is a deal. I find its ok to only spend 200 on a small low power scope. If you want 32x power you need to pay. Go tp pomona-airguns. com and they do a comparison between the two. See what they say.

i dont use with my AirWolfs power adjustment(much). I have it set for kodiaks at about 930 and thats what works. Somthing about that AirWolf. I may need a Theoben MFR next. I would pick a theoben over any daystate except the AirWolf. The mk3s dont have the power i like. Its all just my opinion.

If you have a daystate. PLEASE tell me everything. i an happy to answer any guestions on my guns.

-sumo

 
At August 04, 2007 3:02 AM, Anonymous Anonymous said...

I have a carbon fiber tank and you dont need a regulator. I find it to be easy to not over fill. If you do the gun wont explode (well it may). The gun should be able to take the full 4500. Dont try it, you do over fill it wont be by much unless.. If you are sevearly ADD you should get a regulator LOL. If you do over fill you can dry fire the gun down to a low pressure.

-sumo

 
At August 04, 2007 3:03 AM, Anonymous Anonymous said...

You can always get a regulator later if you are having trouble.

-sumo

 
At August 04, 2007 6:29 AM, Anonymous Anonymous said...

Off topic and first timer so please excuse any mistakes.

I was checking out the MAC-1 site and they say " Never try to pump the gun if it's not cocked." Has something to do with valve alignment.

Is this specific to the Benjamin/Sheridans MAC-1 tunes, all Benjamin/Sheridans, or all pump ups?

If it is good for all pump guns can you explain why in a little more detail?

Thanks JMY

 
At August 04, 2007 8:26 AM, Blogger B.B. Pelletier said...

4500 tank,

No, you watch the pressure gauge on the tank as you fill. It indicates how much air is in the fill line and therefore how much is going into the airgun.

The 4500 psi tank I have is very easy to use. The valve works very slowly.

I have mentioned Daystates numerous times in this blog, but I've never done a dedicated report. The difficulty for me is that while I own a Daystate Harrier, it is a single shot and today Daystate makes a lot of repeaters.

I personally cannot abide a repeater unless it's the only way the gun comes! Too many problems. I want to load each pellet indifidually. But the trend today is repeaters.

So if I do the report, it will have to be a single shot.

Okay?

B.B.

 
At August 04, 2007 8:39 AM, Blogger B.B. Pelletier said...

JMY,

It must be specific to their tunes because everybody pumps these models before cocking, to my knowledge. At least that's what I have always done, and my friends, too.

B.B.

 
At August 04, 2007 10:21 AM, Anonymous Anonymous said...

Daystate does offer a single shot loading tray that replaces the magazine. Operationally with it looks to be identical to the single shot Harrier configuration

 
At August 04, 2007 2:45 PM, Anonymous Anonymous said...

BB,

I would like a report on a single shot gun like the huntsman or harrier. Thanks.

-sumo

 
At August 04, 2007 3:24 PM, Anonymous Anonymous said...

sumo,
Did your airwolf come supplied with the single shot loading tray?

 
At August 04, 2007 5:43 PM, Anonymous Anonymous said...

The airwolf i got from pamona airguns came with a single shot tray. It loads the pellet better than i can! Perfect.

-sumo

 
At August 04, 2007 5:44 PM, Anonymous Anonymous said...

I was trying to say:

Pomana-airguns.com

-sumo

 
At August 04, 2007 11:35 PM, Anonymous Anonymous said...

BB,
Please do Daystate blog us. Development history, harrier vintage rifles, as well as the current electronic wonder guns are all of interest here.

 
At August 05, 2007 9:03 AM, Blogger B.B. Pelletier said...

I will do a Daystate blog series. I have used their guns as much as anyone, and I got started over a decade ago. Also, I have talked with the developers of the electronic trigger and from their conversations maybe I can shed some light on how that gun works.

Blog begins this coming week.

B.B.

 
At August 05, 2007 1:02 PM, Anonymous Anonymous said...

Thanks BB!

-sumo

 
At August 05, 2007 9:10 PM, Anonymous Anonymous said...

BB

Could you point me in the right direction for clearing stuck pellets from a barrel. Amazing how well C02 can wedge them in! Is there a penetrating oil for lead:) I see a Dewey cleaning rod in my future what else would you recommend?

Thanks,

Ray

 
At August 06, 2007 6:32 AM, Blogger B.B. Pelletier said...

Ray,

There is no magic and no solution to make it easier. The trick is to not get pellets stuck in the first place. I'm betting you're shooting a CO2 gun and you let the gas get too low?

B.B.

 
At August 10, 2007 9:40 PM, Anonymous Anonymous said...

BB,

Can you please do a detailed and comprehensive post specific on the USFT? It is a very interesting rifle it deserves a class of its own. From the pricing, it looks like I may never own onw in this lifetime but it does not hurt to read about and learn that such a fine rifle exists and enjoyed by others who love the hobby.

Thanks.

Dave

 
At August 11, 2007 7:36 AM, Blogger B.B. Pelletier said...

Dave,

A comprehensive test on the USFT? I will try. I understand that Tom Gaylord has recently purchased one, so I'll ask him to do the report.

B.B.

 

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