Does it damage a PCP to dry fire it??

MrDellski said:
There’s a lot of mixed info here, but it’s actually pretty simple.

On a PCP the firing cycle is controlled by air pressure, not the pellet. The hammer hits the valve, it opens against the regulated air pressure, releases air, then closes again. Whether there’s a pellet there or not makes very little difference to what’s going on mechanically.

So dry firing with air in the rifle is absolutely fine, and it’s literally how many manufacturers tell you to degas them. It won’t harm the regulator either.

The only time it’s slightly less ideal is when the rifle is very low or empty on air, as the valve isn’t being cushioned by pressure anymore and can open a bit more sharply. But that’s only really a concern if you’re repeatedly doing it for no reason.

I’ve dry fired mine plenty over the years, as most people probably have when a mag runs empty, and never had any issues. I just avoid sitting there snapping it completely empty for no reason and leave it at that 👍
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Thanks for the explanation, that really helped me understand it better .

I did worry when I dry fired mine the other day.
 
1260engineer said:
Personally i would not be dry firing a pcp to empty it . coz as the pressure drops exhaust valve will be banging off the stop . and more so if the pre load on the spring has been turned up .
dry fire with a full charge you should be ok though.
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Good point. What about the FTP900? You can't go lower than say 50 - 70 bar before it vents the remaining pressure.
 
Dear me...

Is it really that hard to understand???

When you fire you PCP in normal use, the valve is opened by the hammer. When the valve closes, the pressure on BOTH sides of the valve, is about equal, the 90 bar or whatever it started out as, [before some was let out into the barrel] on one side of the valve, and whatever the pressure is trapped behind the pellet, on the other side of the valve.

The pressure difference will not be much, and therefore, the valve will settle gently back into its seat.

You fire it with no pellet, and the valve will slam shut with the huge pressure on one side, and nothing on the other... This will do it a world of good [not]

As for comments about "the makers tell you to do this to let the air out" Perhaps they do, as the accountants [that control production] forced them to omit a sensible way of letting the air out, to save costs.

It is a bit like car axles and gearboxes, where you cannot change the oil, as some clown decided it was cheaper not to fit a drain plug..

Anyone care to tell me that was a good idea???
 
I get what’s being said, but the “valve slamming shut” idea isn’t really how a PCP behaves in practice.

The valve is closing against its spring and whatever pressure remains in the system at that moment. The presence of a pellet doesn’t create some equalised cushion on the other side of the valve, the air behind the pellet is already expanding down the barrel and dropping rapidly in pressure. There isn’t a sustained opposing force there keeping things “gentle”.

In reality, the dominant force in the whole cycle is still the regulated air pressure and valve spring, not whether a pellet is present or not. That’s why rifles can be safely dry fired with air in them, and why manufacturers routinely tell you to do exactly that to degas.

As pressure drops very low, yes the valve can open a bit more freely and you’ll get slightly harsher behaviour, but that’s just a function of low pressure, not the absence of a pellet. Same would happen at the tail end of a normal shot string.

On the FTP900 point, that’s actually a good example… it physically won’t let you run it down past a certain pressure, which is more about protecting consistency and regulator behaviour than avoiding dry firing damage.

End of the day, occasional dry firing with air in the rifle is normal use. Repeatedly snapping it completely empty for no reason is just unnecessary wear, but that’s about it 🙂
 
MrDellski said:
On the FTP900 point, that’s actually a good example… it physically won’t let you run it down past a certain pressure, which is more about protecting consistency and regulator behaviour than avoiding dry firing damage.
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From my own experience it is not only the FTP900 design that will dump all the air below a certain pressure, pretty much all pre-charged guns that use a knock open valve will reach a point where the mass of the hammer and the hammer spring pressure will exceed the combined force of the EV spring and air pressure left in the reservoir bearing on the HP side of the valve and will unseat the valve sufficiently for the residual air to drain.

After all that is why you normally need to cock the hammer to fill from empty.
 
rockdrill said:
From my own experience it is not only the FTP900 design that will dump all the air below a certain pressure, pretty much all pre-charged guns that use a knock open valve will reach a point where the mass of the hammer and the hammer spring pressure will exceed the combined force of the EV spring and air pressure left in the reservoir bearing on the HP side of the valve and will unseat the valve sufficiently for the residual air to drain.

After all that is why you normally need to cock the hammer to fill from empty.
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Yes mate, that’s just the point where hammer energy starts to exceed what little closing force is left from the valve spring and remaining air pressure.

Once you get that low, the valve can’t properly shut anymore so it effectively just bleeds the rest off. I just used the FTP as an example as it was mentioned... just where the balance of forces tips the other way.

Same reason you need the hammer held back to fill from empty... otherwise the valve can’t stay seated against incoming pressure.
 
MrDellski said:
I get what’s being said, but the “valve slamming shut” idea isn’t really how a PCP behaves in practice.

The valve is closing against its spring and whatever pressure remains in the system at that moment. The presence of a pellet doesn’t create some equalised cushion on the other side of the valve, the air behind the pellet is already expanding down the barrel and dropping rapidly in pressure. There isn’t a sustained opposing force there keeping things “gentle”.

In reality, the dominant force in the whole cycle is still the regulated air pressure and valve spring, not whether a pellet is present or not. That’s why rifles can be safely dry fired with air in them, and why manufacturers routinely tell you to do exactly that to degas.

As pressure drops very low, yes the valve can open a bit more freely and you’ll get slightly harsher behaviour, but that’s just a function of low pressure, not the absence of a pellet. Same would happen at the tail end of a normal shot string.

On the FTP900 point, that’s actually a good example… it physically won’t let you run it down past a certain pressure, which is more about protecting consistency and regulator behaviour than avoiding dry firing damage.

End of the day, occasional dry firing with air in the rifle is normal use. Repeatedly snapping it completely empty for no reason is just unnecessary wear, but that’s about it 🙂
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Yes, but it does!!! The idea that there is hardly any pressure behind the pellet [and hence the back of the valve] is just wrong..

If it WERE the case, then when you fired the thing, you would just get a whoosh of air from the muzzle, [like you would from an air line] and not the very loud crack you get in practice as all that high pressure corked up air escapes from behind the pellet..
 
Oldtime said:
Yes, but it does!!! The idea that there is hardly any pressure behind the pellet [and hence the back of the valve] is just wrong..

If it WERE the case, then when you fired the thing, you would just get a whoosh of air from the muzzle, [like you would from an air line] and not the very loud crack you get in practice as all that high pressure corked up air escapes from behind the pellet..
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The air behind the pellet absolutely does start at high pressure, but the key point is how quickly it drops.

As soon as the pellet starts moving, the volume behind it increases very rapidly, so the pressure falls off almost instantly as the air expands down the barrel. It’s not a static “high pressure on both sides” situation, it’s a rapidly changing one.

The crack that we hear isn’t because high pressure is being held behind the pellet for any meaningful length of time, it’s simply the rapid release and expansion of compressed air. Same reason you get a loud report from an air blast even without a projectile.

From a valve point of view, the dominant forces are still the reservoir/reg pressure and the valve spring. The pressure on the barrel side decays too quickly to act as any kind of sustained cushion.

That’s why, in practice, the presence or absence of a pellet doesn’t materially change the stresses on the valve during the cycle.
 
@MrDellski is absolutely correct, I think.

It's also probably worth considering that very often the valve is still open long after the pellet has left the barrel - this most often happens when the reg pressure is too low and the valve doesn't snap home quickly enough, wasting air. It'll also happen on high-power PCPs; the valve is open against high pressure (>130 bar) regulated air, and the pellet is long since gone. Doesn't seem to cause any problem.
 
Exactly that 💯

The pressure behind the pellet drops almost instantly once it starts moving, and in many cases the valve is still open after the pellet has already left the barrel anyway.

So mechanically, pellet or no pellet doesn’t really change what the valve is doing 👍
 
The full closed mechanism state is
1. EV spring tends to close the valve. (These are often conical so as to allow more depression without getting spring bound)
2. Bottle pressure tends to close the valve.
3. When settled and NOT cocked, the hammer spring tends to open the valve.

No. 3 can exceed No1. So low bottle pressure can allow a leak- not a fault. My S400 does this. So to fill some guns, you have to remove No3 by cocking the gun.

So when firing with no pellet, initially, the EV valve is pushed open so fast and hard, there can be little difference in what the spring sees then. Closing, with less back pressure because no pellet present, will be rather different and firmer. A lot may depend on the overshoot, dwell and subsequent back recoil of the hammer spring- that should not change much, and will tend to "soften" the closure of the EValve.
I can imagine that an unregulated gun at high bottle pressure will be more stressed on the seat closure than at low bottle pressure, as the pellet presence is part of the tuning equations. A regulated gun will have an easier life, with limited plenum volume, lower initial pressure and a slower refill rate.

So there's a lot going on there!

Having said all that, I have had one EV spring shatter (Not by dry firing) and part of the spring managed to get into the seat area, necessitating a valve and seat regrind to repolish it. I could not obtain a conical valve spring, but the normal straight spring replacement in the same wire size and turns number has performed faultlessly for at least 18 months.
 
Another fact that a lot of people don't seem to grasp if the fact that HPA looses a lot of force when it has to travel through small pipework and apertures, otherwise our air rifle valves would require ridicules force to open instead of just a small lightweight hammer.
 
Weihrauch don't recommend dry firing. Now when the mag runs out and the user makes one last shot is probably okay. I suspect Weihrauch don't want you to sit and keep doing it.

"Avoid unnecessary dry firing (firing without a pellet) in order to prevent wear or breaking of parts."
 
There’s definitely a lot going on in the cycle 😬, but in simple terms the system is governed by pressure and spring forces, not the pellet 🙂

In practice, dry firing with air in the rifle isn’t an issue. Manufacturers only caution against it to avoid unnecessary wear if someone sits there repeatedly doing it.

Occasional dry fire or empty mag shot is just normal use.

My take on it anyway 🤪
 
Oldtime said:
It is a bit like car axles and gearboxes, where you cannot change the oil, as some clown decided it was cheaper not to fit a drain plug..
Anyone care to tell me that was a good idea???
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And are those axles and gearboxes routinely failing in huge numbers?
 
Andy from Sandy said:
Weihrauch don't recommend dry firing. Now when the mag runs out and the user makes one last shot is probably okay. I suspect Weihrauch don't want you to sit and keep doing it.

"Avoid unnecessary dry firing (firing without a pellet) in order to prevent wear or breaking of parts."
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Just decock the thing then!
 
I'll add one item to this pool of knowledge, this one based on personal experience, and I'm not what you would call a newbie.

It's about firing a rifle with no air in it.

It seems this is acceptable in some circles, so long as it is not done to excess, where excess is not defined.

Don't try it with a FX Verminator II. It jams the firing valve and will cost you several £10 notes to get it fixed. I know. I did it and I don't intend to do it again.
 
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