Shimming a NPSS nitro piston as you would a spring to bring up fps

[ccfree]

I'm going to start a little experiment adding 1/32, 1/16 and 1/8 shim stock behind the piston. (For added preload)
I was wondering if any one had tried this.
My hope is to add fps, just to flatten out trajectory with out adding sonic boom.
I'm also hoping I won't crash my seal before I get to about 900 fps.
I was also wondering if you added a small volume to the compression chamber area could you do the same thing.

[bambazonke01]

I was looking at the parts diagram of my Benjamin trail and they show an "Adjustment" disk in the rear  which my Benjamin Trail doesn't have.
As my rifle is an awfully tight shooter I've never bothered to find out why.

I think that when you discharge the rifle with a pellet in the breach the piston actually bounces on a compressed slug of air before the pellet is on its way.  As the air runs out the barrel the piston then settles into it position/
. Firing an Air gun without a pellet can damage the piston or spring as there is no shock absorber of air to cushion the impact.
"Hold into the Wind"
Jim

[RedFeather]

My bet it you will see reduced performance if you increase the compression chamber area.  I base this on the Cardews' theory that deeply seating pellets reduce veolocity (it does) because it lengthens the transfer port, providing more area for the combustion and escaping volume/gasses, lowering pressure.  If you are expanding the compression tube (call it the combustion chamber) but shortening the stroke, even though you increase the gas ram pressure, it might be hard to find that right amount of additional space.  If something as small as a little seating depth can shave fps, imagine what it might do to shorten the piston stroke, even a little.  That's a lot of surface area.

[SMOKEY]

Hi ccfree
 I have done just that with a rem.npss 22cal i used dimes 1st 1 then 2 then 3. dimes are the same size as the shim that comes in the benji trail.and there was only a very marginal difference i have a crono and tested each time with cphp 14.3 and 15.9 jsb. i later found out that most gas rams ( there are some exceptions) have similar starting force and ending force. and that force is linear. for instance starting force is like 150 lbs and ending force is 170 lbs so at half stroke you would have 160 lbs therefore that little bit of shimming barely changes force. i felt the difference in crono #s were just because taking the piston in and out and the slightly diffrent amounts of lube in diffrent places.let me know if you come up with any thing. i am in the process of adding aftermarket rams to a npss looking for more power  best of luck

[supertech77]

My bet it you will see reduced performance if you increase the compression chamber area.  I base this on the Cardews' theory that deeply seating pellets reduce veolocity (it does) because it lengthens the transfer port, providing more area for the combustion and escaping volume/gasses, lowering pressure.  If you are expanding the compression tube (call it the combustion chamber) but shortening the stroke, even though you increase the gas ram pressure, it might be hard to find that right amount of additional space.  If something as small as a little seating depth can shave fps, imagine what it might do to shorten the piston stroke, even a little.  That's a lot of surface area.

   x2 a decrease is my bet too.

[dukemeister]

Check this topic, of course you can do the tests and report the results you get.
http://www.gatewaytoairguns.org/GTA/index.php/topic,24250.msg219234.html#msg219234

[VAFarmer]

IMO, all shimming does with a CORRECT length spring is increase recoil, and make it harder on the cocking mechanisms, harder to cock, and harder to release the sear.

You really are not gaining anything by compressing the air faster (unless it is EXPONENTIALLY faster), but you do runt the risk of the piston bottoming out near the transfer port more intensely.  this may be what is referred to as piston slam.

NOW, a worthwhile effort IMO, is to shave off some spring, or piston sometimes.   Sounds counterintuitive, I know, but hear me out.   May or may not be proven, but.....if you can increase the volume of the tube, by taking 1/8th or lets say you have a quarter of an inch, there is a good amount of increased volume of air, that can be pushed at relatively the same speed.
I liken it to air compressors for 2 reasons....1.  You are still working with air power and 2. I'm not a smart man by any means.
But, if I am trying to use an impact wrench to loosen nuts, and I am given my choice between 2 compressors to use with the same horsepower pumps, but one is a 5 gallon capacity, and one is a 25 gallon, Im gonna choose the 25.  Why?   Because at the higher volume, I can maintain the amount of psi required to operate the wrench at that force level for longer.

Probably doesnt make sense......hopefully someone will explain it better than me.

God bless,

Farmer

[Scotchmo]

Shimming can make a small difference. Assuming that you have enough travel in the gas spring to take up another 1/8 inch of preload, I'll make some assumptions and do the math. Industrial gas springs typically have less than a 30% increase in force throughout their total travel. An NPSS looks to have about a 3 inch stroke:

(1/8 inch)/3 inches x 30% = 1.25% increase in spring energy

It hardly seems worth the effort to get a 1% increase in power.

[ccfree]

Hi; Everybody Thank you for input.
To Bambazonke01 I  don't know about the adjustment disc but it does make one think that this should work
To Redfeather I didn't do a proof read before posting the line should have read "I was also wondering if you added a small volume of silicone to the compression chamber area could you do the same thing.
To smokey I'll let you know if I find anything worth saying
To Supertech77  see Redfeather comment thanks
To Duke meister once I've gone over and looked I'll comment
To VaFarmer I think your right that increasing the stroke would increase the pressure If I'm following you
To Scotchmo I think your right, I'll check the math and or tech data with a manufacturer

Beyond all that I have placed a 1/32 , 1/ 16 brass shim in and have  seen no change.

[ccfree]

To Dukemeister I took a look and it would appear I'm going to finish this and and the hypnosis will be changing.
The next thought would be if a small amount of silicone was added to the compression chamber would this merely cause the bounce (compressed slug of air) at an earlier moment  and if so would that cause the pressure wave to be longer? And what effect would that have on the breaking loose of the pellet from the bore?

[Mike 4888blues]

 I changed my crossman.22 optimist to a nitro piston and just ordered the parts from the benji trail
 the gas ram end I sanded all the paint off and molly lubed, the end receiver part it goes into was sanded  inside so it would move freely and not simply be jammed, but the little spacer you mention i don't believe  is for adding power , but simply  to keep the gas ram free moving if it wants to twist
 just like  washers we add to main springs and just why we heat and flatten the spring ends  so they dont get hung up.  and yes  I know we do add washers on springers  to add pre load if  we want to as well.

The other thing to remember, if you don't mind  is pellet volosity, I am pretty sure after 900 fps,  is when the pellets begin  to become unstable and lose there flight pattern  , I am not totally up on that but I remember hearing that!

[ccfree]

Hey Mike488blues As it turns out the little spacers that I'm adding are in fact not for adding power but just to give me something to do. I live at 3200' elevation and the sound barrier tends to be about 1200 fps with  .22 cphp's.

[ccfree]

Well I finished the spacer test 1/32, 1/16, and 1/8 no real change,, that couldn't be attributed to reassembly.  As to trying spacers beyond 1/8 of an inch I might try that after I'm not have this much fun with it.

[Mike 4888blues]

  Yeah  Its fun learn stuff,
 I learned about the spacers  or  washers  when I first learned  to  tune the B 3 gene had post  on and info, that's when it started making sense. smoothing the spring end with a grinder, adding washers  so the spring can be able to freely move  and twist and not get hung up. plus adding pre load,
 I learned that the nitro pistons body housing from the benji xl was to long to use in regular springers   but at least i have it on hand  and learned something
I also learned  what rifles I don't care for I am picky, lol  quickly to return a rifle when i get one I can tell its not  for me. or its too problematic

 Love  this hobby I have the hastan mod 95  coming monday, and iam already thinking hard  what other hastan  mod i want lol

[ccfree]

Okay here's what I'm going to try. I'm going to place a 1/32" plastic washer in the front of the compression chamber see if that has any effect. Then if the results show improvement in fps I will add more till I bump the main seal.

[bambazonke01]

One of the problems with the Trail XL is the violent recoil which is caused by the gas spring discharging and throwing the accuracy off.  This recoil make holding the rifle critical.
  What if ,instead of all those washers, you could add some sort of shock buffer to dampen the discharge recoil of the gas piston?
It could be another short spring or maybe some stiff closed cell foam. Maybe a rubber hosepipe or flat faucet washer. What ideas do you have?
It doesn't have to be much to make a big improvement difference in accuracy.
 The rubber shock absorber could possibly be at the other end up inside the piston.
Ideas breed ideas.
Jim

[ccfree]

 I agree that the recoil is a problem with these types of guns. And I will try some rubber and or other possible volume robbers. I'm presently working with a NPSS .22 cal. Recoil is not my first concern as I'm getting a 1" group at 33 yards off the bench and a 2" group offhand at 33 yards. I do better than this but this is what I can do with regularly.
   

[ccfree]

I just realized that I should thank and give credit to Gene our host.
After reading various articles that he had  written and or posted by others I did these things to the gun.
1.Changed the spring out to bring the trigger pull down to about 1 1/2". The one I used came out of a vcr.
2.Changed the seal that was sent with the gun as it was damaged. By the way I went though three seals before it didn't damage the one in it now.
3.Put thousand rounds through the gun before I expect much improvement.
4.Put a knee rest / offhand raiser on it.