Wednesday, January 25, 2006

Do all spring-piston airguns diesel?

by B.B. Pelletier

This topic was suggested by a flurry of questions about dieseling that we received just after the new year began. I thought I'd take some time to explain what we believe goes on in the spring-piston powerplant when it fires.

This book is a standard airgun reference. It's hard to find. However, you might find one at an airgun show.

The standard reference
For airgun operation, especially spring-piston guns, The Airgun from Trigger to Target by G.V. and G.M. Cardew is the source of most of what is known. The Cardews did extensive research and experimentation in this field, and they published the complete results, including their test designs. So, if someone doubts what they say, he knows where to begin if he wants to prove them wrong. To date, very few have taken up the challenge.

Four phases of spring-piston guns
The four "phases" refers to how much velocity is generated and also how that velocity is obtained. The phases are:

  • Blowpipe
  • Popgun
  • Combustion
  • Detonation

  • Blowpipe
    This is the weakest of all spring-piston guns. The powerplant doesn't generate much compressed air, and the loose-fitting projectile doesn't make good use of what there is. The Marksman 2000K pistol is a modern example of a blowpipe gun, though there have been numerous other exapmles throughout history. The "Gat" pistols are another example, as is the vintage Quackenbush line of airguns (not the big bores made today by Dennis Quackenbush). The blowpipe is really just a mechanical device that emulates a common pea-shooter or blowgun.

  • Popgun
    The popgun phase begins when there is more compressed air, but it doesn't generate enough heat for combustion. The pellet fits the bore tightly and makes maximum use of the air pressure. Though the name sounds degrading, all the spring-piston target rifles and some of the pistols are actually popguns. The FWB 300 rifle generates velocities of less than 550 f.p.s. in .177. Since there is no combustion (I will address this in a moment), the popgun is usually the most well-regulated of all spring-piston guns, not varying in velocity by more than a few feet per second. The IZH 53M is a good example of a modern popgun pistol, while the IZH 61 is an example of a popgun rifle.

  • Combustion!
    This is the phase that almost all adult sporting airguns are in. When the piston compresses the air, the heat it generates is so high that it ignites any small droplets of lubricants that may be present in the compression chamber. What the Cardews proved by their testing is that all powerful spring-piston guns burn fuel to generate their power. The power that's added by combustion depends on the amount and combustion quality of fuel available. This is where some badly-tuned spring guns shoot at all different velocities. Ten years ago, I was advised by the importer of some Chinese air rifles to liberally lubricate the compression chamber with corn oil. He said the wax in the oil would leave deposits on the walls of the compression chamber and make the piston seal fit tighter, raising velocity. In fact, the corn oil was a very good fuel! The treated guns jumped by 100 f.p.s., or so. [I bet I've just started a race to the kitchen to "borrow" the Wesson Oil!]

  • The Cardews maintain that all powerful spring-piston airguns are in the combustion phase. They also say that isn't a bad thing. As long as the combustion is controlled and small, it benefits us all. In other words - all guns diesel. But when it gets out of hand, we move up to the next and final phase.

  • Detonation!!!
    This is the phase you DON'T want to be in! An abundance of fuel in the compression chamber no longer burns - it explodes! You get much higher velocities out the muzzle - and broken mainsprings, swollen compression chambers and guns that sometimes actually re-cock themselves at the other end! All combustion-phase airguns are capable of detonating, so the shooter has to severely limit any fuel-like substance that is introduced into the compression chamber.

  • When you hear a loud BANG! and sometimes see a bright light coming from the muzzle (flames in the barrel!), you have a detonation. Since all combustion-phase guns can detonate, the thing to do is to reduce the number of detonations to as few as possible. Just having smoke in the barrel is not a detonation, but the byproduct of a normal combustion. BB guns are the biggest combustors of all, but almost all pellet rifles and many powerful pistols are also combustors.

    You'll have to get the Cardew book to read about their experiments. It's also a good reference book for anyone who wants to know more about airguns.


    At January 25, 2006 3:38 PM, Blogger turtle said...

    Wow never would have thought it. Ihave seen what looked like smoke coming from an old spring pistol I have...always thought it was just water vapor from the compression. very wierd science today BB.

    At January 25, 2006 4:18 PM, Anonymous B.B. Pelletier said...


    Next time, smell the smoke. You can tell it's from oil or grease combustion! Chinese springers sometimes smell like bacon fryng.


    At January 25, 2006 7:02 PM, Blogger kd5byb said...

    Good evening B.B.!@

    Question for you on CO2 powered guns, like the Crosman NightStalker. Once the seal on an Airsource cartridge is peirced (assuming they have a seal like a powerlet), will the gas leak out over time? How long for it to leak out? I'm impressed by the NightStalker, but know that I won't consumer a whole Airsource cartridge in one sitting (Have an 2 month old son that keeps me busy - I get maybe 30 minutes once a week for airgunning) and worry that when I shoot the next time, the gas will have all leaked out.


    At January 25, 2006 7:24 PM, Anonymous B.B. Pelletier said...


    Good question! On the Nightstalker, which has no shuit-off valve for the AirSource cartridge, if you follow the recommendation of putting a drop of Pellgunoil on the tip of the AirSource cartridge before you insert it, the CO2 may have leaked out when your child has graduated from college, but probably not. I have CO2 guns that have had powerlets installed for years and they still shoot first time, every time. I once bought a Crosman pistol that had gas from more than 20 years earlier still in it.

    If you were talking about a Crosman 1077 with the AirSource adapter, there is a shutoff valve on the adaptor, which means you can shut off the gas and remove the whole thing from your gun. That's a way parents can keep children safe around their airguns, 'cause when the "go juice" isn't in there, nothing bad can happen.


    At January 25, 2006 9:48 PM, Anonymous SamV. said...

    Hi B.B.

    A little off topic here, hope you don't mind...

    I recently bought my 11 year old son a 22SG (from Pyramid, of course!). Compared to my 30 year old Crosman 1400, it has no power. It literally bounces pellets off an aluminum soda can at 25 yards, and isn't consistently accurate with any pellets that I have tried (FTS, Crosman Premiers, RWS superdomes, Daisy precision max wadcutters,or Benjamin diabolos). I was under the impression that the 22SG is supposed to be a great little hunting/plinking rifle at 10-30 yards. Am I missing something here? It dosesn't seem to be leaking, but I think that a 5 year old child could put ten pumps in it! I'm not sure it could knock a sparrow down at 15 yards. Is there something that I could oil or tweak that might help? I bought him a .177 760 commemorative edition that was totally gutless, and sent it back to Pyramid and exchanged for the 22SG. Needless to say, he is once again disappointed, as am I. Any other suggestions short of returning this gun, too? The restocking fees are beginning to add up. I wonder if a Benjamin 392 wouldn't have been a better choice, but I was a little afraid of his ability to pump it. I usually shoot a .22 RWS 52 myself.



    At January 25, 2006 10:38 PM, Anonymous Anonymous said...

    Pyramyd Air would not charge you restocking fees on a damaged item...

    At January 25, 2006 10:43 PM, Anonymous D. SMEDLEY said...


    At January 27, 2006 1:33 AM, Blogger D.B. said...

    Wasn't there a Daisy toy rifle based upon the entry level BB gun that made a loud pop when fired and also smoked via dieseling when oil was added to the oil fill hole?

    At January 27, 2006 8:03 AM, Anonymous B.B. Pelletier said...

    D, Smedley,

    Your RWS only detonates when there is too much oil in the compression chamber. If your gun is detonating, stop putting oil in that often. Go to a drop every 5,000 shots and wait a day before firing. RWS says in the manual not to use too much oil, and a drop every 1,500 shots seems to be too much in your gun. They used to recommend a drop every 3,000 shots.

    In my Beeman R1 that did a similar thing, I stopped oiling altogether and waited until the piston seal squeaked. That took a long time. Grease from the mainspring migrates up past the piston seal to add fuel to the compression chamber, so it may not be necesary to oil your gun at all.

    The rest of the time, it simply combusts the oil/fuel. Diana didn't design it to do that, but the physics of the spring-piston powerplant make it happen. I recommend you get the book listed above and see what the Cardews did to test these guns.


    At January 27, 2006 8:06 AM, Anonymous B.B. Pelletier said...


    Yes, Daisy made several popguns that smoked. Neal Punchard's book, Daisy Air Rifles and BB Guns, shows several of them


    At January 27, 2006 9:36 AM, Anonymous B.B. Pelletier said...


    Your 22SG isn't developing full power. at 25 yards is should drill several aluminum cans in a row.

    Flip it over and apply oil to the felt washer that rides in the pump mechanism. You'll see it though the pump slot. That should restore the power.

    I was probably the one who recommended the 22SG rto you, so I want to hear how this turns out. At 20 yards, all shots shopud hit a quarter.


    At January 28, 2006 9:39 AM, Anonymous Anonymous said...


    Great topic! I have been looking into the springers' mechanics since last summer and find them pretty amazing.

    Thanks for the recommended reading as well.


    At February 05, 2006 8:01 PM, Anonymous Brit visitor said...

    BB, I've had severe dieselling in a rifle just as you described, including the muzzle-flash... it was the first Gamo CFX Royal (.22) that I got, and even after several hundred pellets it put out clouds of smoke - clouds, not mere wisps. Accuracy was... well, interesting... and consistency notable by its absence. Got a replacement - and it's great for accuracy, even with the UK (non Firearm Certificate) maximum power limitation; this afternoon it was repeatedly hitting the same level on a small-sized aluminium fizzy-drink can from a measured 75 yards... a performance which isn't much bettered by my FX2000 (.177).
    I reckon a good Gamo rifle must be the equal of any around, though perhaps Gamo's quality control isn't quite as rigorous as some of the English and German brands.
    One point this web-journal might care to look into, is the fact that most of the other quality rifles have choked barrels, whereas the Gamo CFX (and Hunter 440, perhaps all) do not... now, given that in a springer the pellet has reached maximum velocity by the first twelve inches, and the barrels on many springers are longer than that, do Gamo effectively gain in velocity through not choking theirs?

    At February 06, 2006 7:44 AM, Anonymous B.B. Pelletier said...

    Brit visitor,

    No, I don't think Gamo gets more velocity from their rifle by not choking the barrel, but you have to remember, most spring gun barrel chokes are the accident of swaging in the dovetails for the front sight. They are not true chokes.

    Weihrauch is a good example. They don't choke their spring gun barrels, but the dovetail they put in does introduce a small amount of constriction at the muzzle. And their barrels are pretty accurate.

    Gamo now uses a plastic sight mount that seems to be held on by a single screw. So there is no need to swage the barrel - hence they don't. Would they get more accuracy if they did? I think so.


    At February 06, 2006 7:40 PM, Anonymous Brit visitor said...

    Thanks, BB.... so is that enhancement of accuracy down to aerodynamics - the choked end tidying-up whatever irregularities might remain in the sides of the pellet after its trip through the rifled barrel?

    At February 06, 2006 7:45 PM, Anonymous B.B. Pelletier said...

    Brit visitor,

    That's the general consensus.


    At February 17, 2006 10:56 PM, Anonymous Brit visitor said...

    Guess that makes it all the more important to use top-quality pellets and handle them with care... my CFX and 440 both have a liking for H&N FTT, and these last few days both rifles have continued to show very good long-range accuracy, in adverse weather conditions...
    For readers thinking of getting one, the CFXr is noticeably more sensitive about how it's handled - that is, as lightly and smoothly as possible - than is the 440. If anyone didn't realise that, they could end up (mistakenly) disappointed with their CFX.

    At May 18, 2009 8:30 AM, Anonymous said...

    I am beginning to tire of pellet rifles since I can get a .22LR for $100-300 that shoots way better and can shoot .22S.

    But I still like to shoot my Gamo CFX in the front yard (Legal but its actually louder than .22's!) How do I make sure it doesn't diesel off and piss off the neighbors?

    At May 19, 2009 6:04 AM, Blogger B.B. Pelletier said...


    Your CF-X is detonating if it sounds louder than a .22 rimfire. It is probably either over-oiled or you may be shooting the wrong caliber pellets in it. CF-Xs don't detonate, as a rule.

    Tell me what, if anything, you have done to this rifle4.


    At May 07, 2010 5:38 PM, Anonymous Rich said...

    Interesting discussion on muzzle termination- I have a turkish springer, 1100 fps, that I have mounted a Leapers scope that I got from Pyramid (your tech service is great, btw. I was having problems keeping the dovetail mounts from walking off the slots, and they hooked my up with these double screw, HD mounts..). Here's my question: I ended up cutting off the molded front sight, figuring it was an unecessary appendage at this point. After I started cutting it off, I noticed that the actual steel barrel ends well behind the sight barrel extension. Does the (plastic) barrel extension I removed have any affect on the flight characteristics, or is it simply there to provide a stable platform for the sight, and protect the barrel end from damage?


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