Odd phenomenon with JTS airacuda standard

[Bladebum]

So, I witnessed something odd today. My uncle who owns the jts standard(as have I, my brother and my father. Very familiar with them)  was making claims by plugging two holes on the reciever with the gun out of the stock, his velocity would drop tremendously. The two holes are basically at the end of hammer spring bore, right where the valve protrudes. You can actually see the valve stem through one of the holes. I told him thats physically impossible as no air passes through or around that area, therefore that would not change a thing. His claim, is that by plugging those holes, there somehow keeps a sort of vacuum pressure in the hammer spring bore and does not allow the hammer spring to strike....I have never heard of this before, so I chronied it myself..gun chronied perfect with jts 18 grains at 905, like it should. I plug those 2 holes with my fingers, fire next shot and sure enough gun sounded significantly quieter and velocity was 550! I repeated the test multiple times and it was consistent,  also tested my fathers and same exact result.

This came about on accident, my uncle was adjusting the HS and chronying at the same time and was noticing big fluctuations before he realised his fingers had been blocking those ports.

So I ask,  can anyone explain this to me? In all the gun tuning I've done, ive never encountered this.

[nervoustrigger]

Yes, the rapid forward movement of the hammer compresses the air in front of it.  If the air cannot escape freely, it acts as a cushion (a damper), preventing the hammer from accelerating fully, and causes a weaker strike.

The same effect can occur behind the hammer, on account of forming a temporary vacuum.  In other words, the hammer needs venting both in front and behind it.

[Bladebum]

Yes, the rapid forward movement of the hammer compresses the air in front of it.  If the air cannot escape freely, it acts as a cushion (a damper), preventing the hammer from accelerating fully, and causes a weaker strike.

The same effect can occur behind the hammer, on account of forming a temporary vacuum.  In other words, the hammer needs venting both in front and behind it.

I mean I get the concept and all...but HOW is it making sealing pressure with no seal? As in, based on this same premise, we should be able to sloppy fit the piston in a spring airgun and no longer use piston seals. If there's no seal on the HS , then the air being compressed should be escaping to behind it as it travels forward correct? We put seals on pistons for this very reason, so this concept is blowing my mind.

The other side of this is, those holes are blocked pretty well by the stocks tight fit. Makes you wonder then, if that wood stock swells would it start causing erratic velocities.

[nervoustrigger]

Oh okay, well the hammer is still a close fit to the tube and moves very quickly.  Too quickly for the pressure to equalize on each side of it so it produces temporary zones of compression and rarefaction on either side of it.

In the example of a spring piston gun, a close-fitting piston having no plastic/elastomer seal will fire a pellet at a somewhat lower velocity on the average and with a lot of variance.  Or to say it differently, the absence of a seal will cause some modest loss of compression as well as poor consistency arising from how the piston cants and shifts in the tube unpredictably as it accelerates from one end to the other...sometimes sealing better, sometimes sealing worse. 

A failing piston seal produces very much the same results.

Regarding the potential for the stock to cause this problem, there were in fact reports of it early on...some users making mods to alleviate it.

[CraigH]

Fairly well known phenomena.   I machine slots (~ 5/8"L x 3/8" W) on 22xx builds in the area where the valve stem and the forward portion of the hammer (striker) are located.  (There are already small slots in the guns for the sear and hammer pin to allow air to escape.)

When firing the guns, air escaping from these machined slots in the sides of the gas tube will be felt.

[Bladebum]

Oh okay, well the hammer is still a close fit to the tube and moves very quickly.  Too quickly for the pressure to equalize on each side of it so it produces temporary zones of compression and rarefaction on either side of it.

In the example of a spring piston gun, a close-fitting piston having no plastic/elastomer seal will fire a pellet at a somewhat lower velocity on the average and with a lot of variance.  Or to say it differently, the absence of a seal will cause some modest loss of compression as well as poor consistency arising from how the piston cants and shifts in the tube unpredictably as it accelerates from one end to the other...sometimes sealing better, sometimes sealing worse. 

A failing piston seal produces very much the same results.

Regarding the potential for the stock to cause this problem, there were in fact reports of it early on...some users making mods to alleviate it.

Very interesting,  this is all new info to me regarding hammer bore venting.  My uncle has drilled a hole in his stock for the bottom hole and made a relief channel for the side vent so that should solve any future issue.

[Bladebum]

Fairly well known phenomena.   I machine slots (~ 5/8"L x 3/8" W) on 22xx builds in the area where the valve stem and the forward portion of the hammer (striker) are located.  (There are already small slots in the guns for the sear and hammer pin to allow air to escape.)

When firing the guns, air escaping from these machined slots in the sides of the gas tube will be felt.

Sounds like I am late to the party lol, learn something everyday

[Motorhead]

VERY REAL .... ANY resistance to the motion of the hammers travel toward the poppet will result in a softer strike.

This can be Sear drag ... Lubrication drag ... Pressure built up in front of, or vacuum behind the hammer.

[aluminumfetish]

Is this why "fluted" hammers pop up occasionally ? Wouldn't solving this issue in the hammer design negate all these issues ? Does a hammer have to be full diameter to function ? I recall seeing a crosman hammer cut into thirds similar to arrow fletching. I remember thinking it was overkill at the time, perhaps not overkill at all.

[nervoustrigger]

Yes, fluting can be the solution.  I presume the reason we don’t see it very often is because venting the tube is usually less  costly or nearly free just based on features that have to be machined anyway (e.g. cutout for trigger group, slot for cocking pin, etc.). 

Also, sometimes fluting is done to reduce friction or weight.

[Bladebum]

Is this why "fluted" hammers pop up occasionally ? Wouldn't solving this issue in the hammer design negate all these issues ? Does a hammer have to be full diameter to function ? I recall seeing a crosman hammer cut into thirds similar to arrow fletching. I remember thinking it was overkill at the time, perhaps not overkill at all.

Good point,  based on the fact this is an actual thing, im surprised hammers are still being made round.

[hguffman]

I'm wondering if this could be the reason why on TP setting 1, the lowest power, pellets are getting stuck in my barrel. I've tried 4 different weights and styles and with the psi at 3000, and the lowest setting it sounds like a pumper with 1 pump when fired.

I little just bought this, it definitely doesn't seem like it's functioning properly.

[CK]

I'm wondering if this could be the reason why on TP setting 1, the lowest power, pellets are getting stuck in my barrel. I've tried 4 different weights and styles and with the psi at 3000, and the lowest setting it sounds like a pumper with 1 pump when fired.

I little just bought this, it definitely doesn't seem like it's functioning properly.

It could be a bit of valve lock happening at 3000 psi, you could try to shoot it down to 2900-2800 on a higher TP setting and then dial it down to low and see what you get.

Also, where is the hammer spring adjusted? You may need to turn the adjuster in (need to pull out of the stock and loosen the locking grub screw first).

[hguffman]

I'm wondering if this could be the reason why on TP setting 1, the lowest power, pellets are getting stuck in my barrel. I've tried 4 different weights and styles and with the psi at 3000, and the lowest setting it sounds like a pumper with 1 pump when fired.

I little just bought this, it definitely doesn't seem like it's functioning properly.

It could be a bit of valve lock happening at 3000 psi, you could try to shoot it down to 2900-2800 on a higher TP setting and then dial it down to low and see what you get.

Also, where is the hammer spring adjusted? You may need to turn the adjuster in (need to pull out of the stock and loosen the locking grub screw first).

I haven't adjusted a thing but I did see a you tube video about doing so. The pellets also got stuck when the psi was at 2500. Just seems like it shouldn't come out of the box this way?

[anti-squirrel]

This is exactly the type of educational information that makes this forum such an invaluable resource. 

Anecdotal and generally worthless but interesting tidbit:  In some ways, it is very reminiscent of the internal bleed paths on HP air systems found on valve actuators and HP reducer valves.  I can recall setting a "bleed path" on a Marotta actuator (used for emergency blow valves on a sub) using a razor blade tapped ever so gently on the valve seat.  The air has to move to do stuff.  No air movement=no movement or slow movement.  Microns matter in HP Air.

[CK]

[/quote]
I haven't adjusted a thing but I did see a you tube video about doing so. The pellets also got stuck when the psi was at 2500. Just seems like it shouldn't come out of the box this way?
[/quote]

You could remove the barreled action from the stock and test fire it, that would tell you if the vent hole is being obstructed by the stock. If so, some quick chisel or Dremel work would fix it, or shimming the action.

When I got mine it shot Crosman Premier Hollow Points on low, it was really slow but I don't remember chronograph readings. Mine came from the factory with the hammer spring adjuster flush withe the receiver.

[Madd Hatter]

Why do you think people sell fluted hammers for crosman airguns. China isn't the best at designing things that work perfectly right away.

[hguffman]

You could remove the barreled action from the stock and test fire it, that would tell you if the vent hole is being obstructed by the stock. If so, some quick chisel or Dremel work would fix it, or shimming the action.

When I got mine it shot Crosman Premier Hollow Points on low, it was really slow but I don't remember chronograph readings. Mine came from the factory with the hammer spring adjuster flush withe the receiver.

So I just removed the stock and test fired and am getting the same thing. I went to try and adjust the hammer spring. Below is a picture of what I found. It is locked tight in this position, and I can't move it. Any ideas?

[CK]

See the locking grub screw on the left side of the receiver, near the back? Loosen that. Don't overtighten it or it will booger up the hammer spring adjuster threads.

[hguffman]

See the locking grub screw on the left side of the receiver, near the back? Loosen that. Don't overtighten it or it will booger up the hammer spring adjuster threads.

Well, it didn't fix the stuck pellet issue, but I did learn how to adjust the hammer spring. I can at least tell the JTS product people when they get back to me all the stuff I tried!