Couple questions

[Flattitude]

Is there any info out there on how much increased FPS one could expect from a longer barrel.  I have a 1322 with stock barrel.  I am contemplating going to a 12 inch barrel from stock.  Will it make much of a difference if any?


Second question.  Playing with the chronograph today and looking over previous numbers from stock to current configuration.

I am wondering if my thinking is solid or if I did something wrong. 

Stock 4 pumps got me 344 FPS  with new valve and flat top piston 4 pumps is getting me to 331FPS.   Now I have a theory and either I am wrong and so is the gun or I am right and I am ok with that.   

My theory is the new valve has more space in it and therefore develops less pressure at the same amount of pumps.  Before I maxed at 513 at 12 pumps I now hit 531 at 12 and hit 595 at 25pumps.

It does seem easier at first to pump and then begins getting harder but not too terrible.  I have taken it to 30 pumps but never got more than the 595fps just had more air in the valve after firing, enough to pull a second shot of 300fps on average.

Just wondering if my goal of hitting 650 is plausible or if I should be happy and not chasing my tail.

[quickster47 †]

Here is a link to some barrel testing I did using a 1377 a couple of years ago.  Most of it is still relevant.

http://www.crosman-air-pistol-owners-forum.com/board/index.php/topic,2522.0.html

Carl

[WyoMan]

I've found that increasing barrel length gives somewhat predictable results in terms of fpe...not fps. In another words, if you increase the length 30% you get about a 10% increase in fpe (same gun, powerplant, etc)

So using the 3:1 ratio - your increase is 17% you should get about 6% more fpe...I don't know what pellets you're using but e.g. 531 fps with 14.3 gr would go to 547 fps.

You could be right about the larger valve volume...that would explain it. However, the stock set up is pretty good at the lower pumps. It starts flexing and losing compression the higher you pump so it's hard to say if your new valve has larger volume...which leads into your last question...Yes...you can make 650 fps with a large enough valve and a long enough barrel. Assuming you don't mind pumping that is.

[Flattitude]

I've found that increasing barrel length gives somewhat predictable results in terms of fpe...not fps. In another words, if you increase the length 30% you get about a 10% increase in fpe (same gun, powerplant, etc)

So using the 3:1 ratio - your increase is 17% you should get about 6% more fpe...I don't know what pellets you're using but e.g. 531 fps with 14.3 gr would go to 547 fps.

You could be right about the larger valve volume...that would explain it. However, the stock set up is pretty good at the lower pumps. It starts flexing and losing compression the higher you pump so it's hard to say if your new valve has larger volume...which leads into your last question...Yes...you can make 650 fps with a large enough valve and a long enough barrel. Assuming you don't mind pumping that is.

I know the valve was modified at least that is what I paid for,  i don't know how much volume was added but I know the fps is more predictable and stable with the new hardware.  I was hoping for 650 but would settle for 600 since either with a 14.3 gr pellet is fine for dropping tree rats with properly placed shots.  I don't want to go any longer than a 12 inch barrel  due to packing the gun in and out for hunting.  I don't mind pumping although I need to pin the valve before i make these high pump sets a regular occurrence.

[Flattitude]

I also have an alchemy boss buddy port and am going to add the lighter valve spring using the trigger spring tomorrow.  I am curious to see the results if any from the port alone. 

[Flattitude]

Ok new issue and I am about to be frustrated.   

Went through the power plant build step by step.   Followed instructions to a T.  Put everything back together and am having an issue.  Pumps 1-2 are useless.  Wont even generate enough power to dump a pellet.  Pump 3 -9 is fine and showing decent increase over stock.   Pump 10 is where the issue starts.   At this point I pull the trigger and hear the hammer hit the valve but it won't release at all.  I have to remove the back cap and manually hit the hammer with a dowel up against a wall.  Since there is a pellet already loaded this is a huge issue that I don't like.   

Anyone know what is going on.  Did the valve stem bend?  I inspected it and it looks fine.  Stock spring is in the valve as well. 

[WyoMan]

Do you have an overtravel screw for the trigger? If so, that can be a source for hammer drag if it is adjusted to tight (the sear will not drop enough)

If no overtravel screw, then look at your hammer for signs of wear...esp. the top. You can have a bur in the tube dragging on the hammer. Also check your hammer friction...does the hammer slide forward when the tube is raised a couple inches like this:


It's probably not going to be the valve stem but if you ever have the valve apart again...here's a simple check...Put a thin coat of lube on the end of the stem and put the exhaust valve in while holding the valve verticle. The exhaust valve should slide to the bottom smoothly and slowly - by just it's own weight...hth

[1377x]

Stock trigger and hammer? Hammer pin seated correctly?
Please give fill details about your set up.
Here's a good read for you
http://airgununiverse.net/13xxvalve.html

[Flattitude]

Wyo I will try them tomorrow.  Hammer does slide freely, but i will try the valve slide test in the morning after my oldest son's honor roll ceremony. 

Stock trigger and hammer? Hammer pin seated correctly?
Please give fill details about your set up.
Here's a good read for you
http://airgununiverse.net/13xxvalve.html

Setup is as follows.

1322 Stock barrel, hammer, trigger with the exception of a lighter trigger spring from alchemy
Flat top valve and piston setup from alchemy as well.  Valve has been modified with 2 threads removed  Valve has also been worked per the Pumper power thread in the technical section.  Stock hammer spring, nirvana trigger mod, 2289 fore grip and a mellon extended probe bolt.  Boss Buddy transfer port and reinforced pivot pin.

Other than that it is how it came.

[quickster47 †]

Sounds like it could possibly be a case of Valve Lock

Be aware that air pressure can become so high the hammer/striker cannot knock open the valve.  Meaning the force of the hammer hitting the valve stem will not open the valve because the pressure has become too high.

How does an impact pneumatic valve work?

The most common valve is the impact type or knock-open valve, and that's the one that has a problem with over-pressurization.  When a hammer strikes the end of the valve stem of an impact valve, it forces it to momentarily lift the valve face off the valve seat.  When that happens, air can flow through or past the valve stem and out into the breech of the airgun.

The valve face is held against the valve seat by a return spring.  Also, any air pressure inside the reservoir where the valve face and seat are located pushes against the back of the valve face, forcing it against the valve seal.  These two forces (the return spring and air pressure) are what keep the valve closed.

The hammer has to strike the valve stem with enough force to unseat the valve momentarily, allowing air to flow from the reservoir.  The weight of the hammer and the strength of the spring that pushes it have been calculated to open the valve when the pressure inside is at its maximum.  For most multi-pump guns made today, the valve allows all the stored air to be released.  That's easy because their reservoirs are very small.  But, precharged pneumatic reservoirs are larger and only a portion of air is released.  The next time the valve opens, the pressure inside (pushing against the valve face) is slightly lower, so the valve remains open slightly longer.  A little longer flow of air at lower pressure is released, giving the same velocity to the pellet.  This is always easier to control when the barrel is longer, so long-barreled rifles are generally the most consistent, though a valve can be tuned for any barrel length.

What happens when a pneumatic is over-pressurized?
When the air pressure inside the reservoir is higher than the design of the action can accommodate, the hammer cannot open the valve as far as it should, so less air escapes.

When you put air into an airgun, it is nothing like putting gasoline into the tank of a car.  Even then, more gas doesn't make the car go faster, does it? What a pneumatic gun needs is air FLOW, and that happens only when the valve remains open as long as it was designed to.

Carl

[Flattitude]

Thanks Carl.  What I had been noticing is after 12 or 14 pumps there would be enough air left to take a second shot without pumping and put out about 250fps on the .22 pellet.  I had wondered if lightening up the spring in the valve would help it dump more and placed the replaced trigger spring into the valve as it is thinner and lighter.  It was light enough that I would have to cock the bolt to get the valve to close and hold air.  That is when I started having the issue.  I have since replaced the stock spring and it appears to be working ok.  I was going to add a pre load to the hammer instead of using the stronger spring I have been using as a hammer spring.