Ertogrul has joined us from Cyprus. He, like so many, have wanted to learn what makes a PCP tick.

There are some basics that are pretty universal, and may be of interest to readers. I won't be getting into any "How To" discussion. So, please keep questions to learning BASIC theory. No port sizes, no secret fairy dust stuff.

Right then. The thing that makes an AIRgun possible, is the fact that air can be compressed. Dan mentioned he thought coffee and beer were proof that God loves us. haha. Add in the fact that "Air" can be compressed, and act as a spring. By contrast ... take "Oil" (which cannot be compressed). The fact that oil cannot be compressed, is the basic fact that makes "hydraulics" possible. Put oil in a pipe ... put pressure on one end of the oil, it moves away from you, and the oil on the other end of the pipe moves the same amount. Wholah! ... Hydraulics. Given a chance to release the pressure applied, you just get one small "spssst", and no more pressure. Put "Air" into a pipe ... compress one end ... and the pressure just rises within the pipe. The air is compressing. When it has a chance, it will SPRING back - VIOLENTLY. Wholah!! ... Pneumatics.

The two basic airgun types are: 1) Spring guns. 2) Pneumatic - or PCP. Spring guns make their compressed air during the firing cycle. They first make it (with a spring driven piston in a cyclinder) ... and then they release it behind the pellet .. all during a single firing cycle. They are limited to the vol. of air they can make (while in a scale that a man can comfortably use). The PCP gun stores it's air pressure, in an internal reservoir. The firing cycle "taps" into that stored air, and meters out the amount used during one firing cycle.

Most PCP guns are of the "knock open valve" design. Envision a wine bottle. Envision the cork in the neck of the bottle. Now, in your mind ... take that cork, and put it "inside" the bottle ... stuck in the neck backwards. Now, since the cork is bigger than the neck ... it wedges into the neck (backwards from normal) ... and it seals anything inside from being able to come out. That's the basic configuration of the PCP gun. The firing mechanism (the hammer and trigger assemblies) ... are for the purpose of momentarily "punching" that cork back inside the bottle - to let out a pulse of air. They drive the cork (firing valve's head) back into the bottle (reservoir) ... and a violent pulse of air escapes ... AND THEN ... the remaining air inside the bottle (reservoir) drives the cork back into the neck (valve seat) ... and reseals the reservoir. Everything is at rest again ... the firing cycle is over.

Now, that violent burst of air that has been released during the firing cycle, must find it's way into the breech of the barrel, and build pressure behind the pellet. The pellet has been sitting there, in the barrel's breech. Once the burst of air finds it's way (through the transfer port) to the pellet ... it starts to build up pressure behind the pellet. At a certain point, as the pressure rises, there will be enough force applied to overcome the: a). weight of the pellet. b). tightness of the pellet in the breech. c). resistance of the rifling against the nose of the pellet. At the point where the breech PSI is sufficient to overcome the inertia of the pellet in the breech ... the pellet will slip into the bore, and start it's way down the barrel.

While the pellet starts down the bore ... the PSI behind the pellet will start to drop. This is because there's now more space for the air to occupy (the length of the barrel's bore). While the pressure is dropping, the pellet is streaking down the bore. So; it's resistance to movement is highly reduced ... and takes far less PSI to drive it. So, the fact that the pellet is moving, even though the bore psi is dropping, means the pellet is easier to push ... and keeps accelerating.

Now ; the breech PSI is dropping, and (back in the reservoir) the remaining compressed air in the reservoir is pushing back against the cork that's just been rudely driven into the reservoir space. That means the remaining reservoir psi slows, stops, and then forces the cork to reverse direction. It shoves the cork back into the neck (firing valve head back into the valve seat), and seals the reservoir. This happens about the time the pellet is nearing the muzzle of the barrel.

The pellet bursts from the muzzle, and the pressure which was behind the pellet, is released to the atmosphere ... BANG!!!!!!! The airpressure can spring back ... it washes all around the pellet. Around, and passes the pellet. This is why it's important that the base of the pellet, and the crown of the barrel - are symetrical. That released pressure, which had been pushing the pellet ... it now passing it in a rush. Imagine, if you carry your keys on your belt (like I do) .. and you are rushing thru a tight space. The darned key ring hooks on anything around. You either rip off your key ring, or bend up whatever you caught. Well ... that's the air bursting thru the space between the pellet base, and the muzzle crown. They better both be smooth and uniform ... or ... something is gonna throw off that blast of air ... and slap the base of the pellet with a rough air flow. It will slap the pellet with a force it has to compensate for.

OK ... that's a quick thumbnail of how a PCP fires. With some other details tossed in that seemed appropriate. Hope you enjoyed - and maybe learned something.

Best,
Gary

I understand all that. And I assume the port is much bigger, or the spring holds it opens a lot longer or a combination of both in the difference of a .25 vs .87. I don't need the details.

My question, is, you mentioned, the bullet exits about the time the valve closes, does this affect accuracy much, if it's before? And does it take longer for the bigger bore? And since the bigger guns have a much larger mass would it be, not as much effect or more since it's more spring and air? How do you tell if it leaves first? You open up another can of worms Dory

After you work with it alot, you can "hear" the difference between a "clean, crisp shot" and one that .... is different than that. How do you really know, that the light goes out in the refridgerator, when you close the door? hahah.

The timing is about the same no matter the caliber. Bigger bore, bigger chamber, bigger valves, bigger ports, bigger hammer, bigger springs, more mass to the gun, more mass to the slug. The function is pretty relative. I suspect things happen at about the same time, in guns of the same velocity, regardless of caliber. The air is the constant.

At any rate, in my guns, the systems all operate without detering from the accuracy.

And Dory, you can only ask 14 more, rapid fire questions. I've gotten "keyboard burn" before, ya know. haha.

Gary

I guess when Jim gets that high speed laser trap set up. Take picture just as it leaves barrel and see the barrel, not move. maybe. I sure can't tell by listening.

I type so slow, I usually have another question before I finish the first one. To make thread longer, I guess I'll ask one at a time. Wouldn't want to slow you down in shop with finger burn. Dory

That's OK ... I'm just pullin' yer chain.

You want the most brief sounding muzzle "BANG". If there's a huge "whoooooooooshhhshhh" after it, you are probably porting more air than the barrel can use. I have it down pretty good ... so you aren't going to experience that with my guns. That's one of the things I'm aware of, when I "function fire" the mechanisms ... and then when I "range test" the rifles. If there were wasted air, I'd tune it out .... (by being good at it ... to save your fingers too ... haha).

I'm not sure what you are saying about photographing the barrel at the moment the bullet leaves, and not seeing the barrel move. Sure it's gonna move. "For every action, there's an equal and opposite reaction". There's alot of energy going down range ... the gun is gonna go the opposite direction.

If you are moving toward the old "lock time" discussion .... I'll just say up front ... I'm a poor sport about it. I think it's all a bunch of crap when it comes to anything we can do. I can't shoot well enough to tell the difference ... and some say I shoot pretty well. Maybe some people can tell a huge difference in a few nano-seconds. I can't. I think having proper ammo, a repeatable bench grip, a clean trigger pull, a good follow through, a proper aim, and no flinch ... is far more important than sawpping out some steel component for titanium.



Gary

I can hear sometimes the first click on the trigger, but it's just a bang to me. I just thought you had figured out a way to tell, port closing, bullet leaving.

The Hoss is a really loud gun .45 short barrel. The .87 not as loud as you would think it would be with one of the longest barrels. Did you have to tinker with the springs, with them? See one question. Of, course you know what the next one will be. Dory

The smaller the caliber, the higher the pitch of the blast. It's sharper, and more annoying. The huge bore of the 87 caliber is more mellow. It's a lower frequency.

The steps of the firing cycle are intuitive common sence steps (knowing what's taking place). Once the slug slips from the breech, the peak of the breech pressure has been broken. The slug also - at around 1,000 feet per second, isn't gonna take much time to travel 20 inches or so. Therefore; the slug is exiting the muzzle, at as the firing valve is closing. The factor I call a "clean shot" is balance of those things, so air isn't wasted.

No ... I don't have the little split screen 100,000 frames per second side by side of the valve and the muzzle.

Also, in a short barrel PCP ... sometimes you have to run them overly "hot" in order to get the desired velocity from the short barrel. It's fuel wasteful. Like afterburners. Yet; it gets the slug out there faster from a smaller package. Makes a gun that's not nearly as enjoyable to shoot - because it's wild and raw sounding. The Hoss is in the middle. It's running hot, but not so hot as to be annoying.

Gary

Did you say you sometimes hear the first click on the trigger???? Ah HA!!!!! Then you HAVE shot your Barnes gun??? Gottcha.

thanks for all information about the airguns. I understant all things but they are just theory. I learn theory now. but I must have practical. so I will go Gary's shop, Then, I will help to complete his customers guns and I have practical with him.

Thanks Gary
Ertogrul

love it ,,, love it ,, but , couldnt you have the port close before the slug leaves the barrel ,, no woosh , silent airgunning , but i guess if the post clossed too soon , the pellet would never leave the barel. compramises , lifes full of them . but a really good description of a lot of the magic that happens when you squeese a trigger . normally never think about the different parts of the overall sound.

Once the firing valve has opened, the expelled air is acting to push the pellet. That air is going to be vented, along with the pellet. Can't put it back into the reservoir.

Glad you enjoyed reading it.

Gary

Very interesting topic...

Well, I have really started listening my airguns recently; and only recently I have started hearing some difference between shots. Namely, there is some definite degree of difference between each shot of the one fill of the resevoir. I mean, first shot sounds bit different than second, and second sounds bit different than third... That sound of the number is clearest in my big bore (.469), though sightly smaller bore (.25'') makes that difference too (only not so clear way and only between shots of longer sequence).


Big bore has it like: First shot is shortest, sharp and very powerful, second one is rich and probably the fullest; yet third is the longest and quite surely the mellowest.. Sometimes I shoot four shots, but not too often because it drives the pressure in reservoir quite low; that fourth "BAAANG" is truly the long-one, to th point that I think it wastes some air (which is hopefully only air which it wastes).

Good descriptions.

What you are sensing, is the fact that your hammer system is always the same, but the back pressure in the reservoir, acting on the firing valve, is always changing. As the sound gets more mellow, and longer ... your hammer is driving the firing valve open harder, farther, and the cycle is lasting longer, as the remaining air in the reservoir cannot fight the valve head closed as quickly. More air, of a lower psi, is escaping with each hit of the hammer. Once you get below the ability of the remaining reservoir pressure to fight the valve closed again, it will dump the remaining air pressure in the reservoir.

Gary

Pretty good basic description there, though of course it neglects the one small question about how you got that cork into the bottle to begin with. I've pushed a few in myself over the years; just never on purpose, and rarely in one piece. : ) It's got me to wondering though.

PCP airguns have been built in enormous sizes in the past, in the form of "dynamite guns". Some of them had bore sizes in the eight-inch-plus range, and were used in the days before insensitive high explosives like TNT were developed. The modern-day form is the (far less dangerous) "pumpkin chucker" cannon used for entertainment. But...

I wonder if anyone's ever made a giant "springer" before?

Hi Rotor.

Regarding getting the cork inside the bottle ... "Use your mind" ... hahah.

I think there was also an air cannon, named after the volcano that buried Pompei (Vechevious) ... I can't spell either. Anyway ... it was a WWI shipboard mounted "steam driven" air cannon IIRC. I don't know the details ... might be the closest to a huge spring gun. I think a pure PCP is so much more logical on a huge scale.

Yeah ... the pumpkin canons are wild. Love it.

Gary

I've been searching facts concerning dynamite guns.

What could have been their operating pressure like?

I found out that vesuvius could shoot one and half nautical mile, but zalinski dynamite gun, a pneumatic coastal defence weapon could have fired a 15” shell weighing 966lbs over 4,500 yards. That's something!



Picture is submarine holland firing its pneumatic gun. Look close... projectile is visible too.



Edited 1 time(s). Last edit at 04/05/2009 06:25PM by sepeteus.

I think you'll find that the Dynamite guns were bought not because the technology for insensitive exploding shells was not there (I've read that though) but because the air guns were more accurate. Dynamite guns were installed to protect San Fransico and they had to meet accuracy requirements that the powder burning guns of the day could not. They were large guns and operated off off low pressure like the pumpkin guns do, just more volume. The Vesuvius had a rather uneventful career but did participate in at least one coastal bombardment. The problem with the guns were that they did not traverse, the entire ship had to be steered to aim.

BTW,

One thing I didnt catch in all this way cool discussion.... the valve return spring....

So, assuming a full gas tank (or at least enough to actuate a valve return...) what function does the spring serve? To affect/influence the timing of the valve? To allow for fuel economy, better performance for a specific pellet, sonic signature reduction.... and I suppose a lot of other reasons.

So assuming all things equal, which is better, a heavier hammer spring or a lighter return spring? can see the arguments for both. Both will keep the valve open longer.

Ah .... the valve return spring.

The one part of the gun that's over rated. It holds the valve closed, when the reservoir is empty, so you can start the fill. In a knock open vlave system, it does nothing else. Ruins all the "tuning lure" you've ever read ... doesn't it!

Think about it. With a "knock open valve" design, during the firing cycle; the unseated valve head is being held open by the mass of the hammer laying against the valve stem AND the massive spring that is driving that large hammer mass. Now ... get a mind's eye picture of the whimpy little valve return spring ... and tell me how it's gonna push back the huge mass of the hammer, and the strong hammer spring. Hint ... it isn't.

The force that fights the valve closed, against the hammer mass, and the hammer spring .... is ...... wait for it ...... the residual pressure within the reservoir (acting on the valve head). That's a heck of a force.

Most of the time ... that valve return spring is simply riding along.

Gary

Yeah, I hijacked this thread and I'm not letting it go until you meet all my demands! Let's see, I want...uhm...let me get back to you on that one...

Didn't I just read somewhere here that the function of the valve return spring is basically to keep the valve assembly from falling into the reservoir in the even that all the air pressure is discharged? Compared to the force of the high-pressure air pushing the valve closed from the reservoir side, any spring that would fit in there is pretty small potatos by comparison.

It's the same reason they don't have key locks on the inside of a passenger jets door. They don't worry that a crazed passenger could somehow yank the door open and jump out or whatever, it's just not possible. When the plane is pressurized, no amount of yanking on the door handle would ever be able to open it, since it fits as a plug from the inside. In fact, even an exceedingly small remaining pressure differential after landing can make it impossible to get the door open until the inside and outside pressures are equallized. Same issue with submarines but the plug fits the other way, since keeping the inside in and the outside out is a bit more... critical.

And here's one reason that they used compressed air for dynamite guns. When the stuff gets too hot or too old (or both) it sweats nitroglycerine, which crystallizes as it dries. It's devilishly sensitive, even slight friction can detonate it once it's decomposed to a state like this. This example was found by a friend of mine at an old mining site in AZ. I think they set it off with gunfire from a distance before they left, seeing as it was basically a bomb waiting to go off at the slightest touch.

I thought that was deer balogna. again.

Gary

I did too! I scrolled down and was expecting a hunting story!

-- Jim

Yes, far as I know, follow-through is very important for accuracy, and not just in airguns/firearms.

But I have one question of my own: Is a moderator's primary purpose to quiet the gun, or does it also "moderate" that burst of air around the pellet? I keep thinking, if you could "strip away" that air, so nothing comes out with the pellet that can disturb its flight, that should make a difference in accuracy.

Or am I just blowing air...? (I know, second question... )

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Built in shrouds can alter flight - usually ONLY if something strikes the pellet. They must alter the air blast ... but I've never had a properly made unit degrade accuracy. Some of the best groups I've ever shot were from shrouded guns.

There are ways of stripping away the muzzle blast - with holes in the barrel, behind the muzzle. However; it's not needed. Myself, and my customers, have demonstrated world class accuracy soooo many times that it's not debatable.

Practice is what's lacking in 100% of shooters who want to improve the accuracy of their guns.

Gary

Gary,

Practice makes perfect, but, from what you said, it seems the shroud does help, when the shooter does his or her part. And also like you said, at least it doesn't hurt, so it sounds like a shroud would still be a valuable option. Me, I like the bull-barrel look, especially in the brass and stainless combo (not that brass and blue doesn't also look good).

Just goes to show how important all those little detail things are, when it comes to craftsmanship....

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