Why would my NPSS drop 100 fps?

[Fisherdude]

I checked  my NPSS when it was new and got about 740 fps with CPHPs and I was fine with that.  Today, I checked it and it was only about 640 fps.  I did recently do a lube tune and changed out the seal with another from Crosman.  The old seal was not really bad but had a few slices from burrs and it was a very loose fit.  I did not put the sticky oil back in but used moly around the seal in a dry chamber. as I would have done for a springer.

Any thoughts?
Should a dry chamber with molly on the seal yield a lower fps than an oiled cylinder?
Is there a way to test the nitrogen piston?

[stacycfi]

Needs to break in again and loosen up? 
Maybe,  How many shots thru it with new seal?

[Fisherdude]

About 100 rounds on the new seal.  I did notice a sudden drop in elevation a couple of hundred rounds before I put in the new seal.

[stacycfi]

Wow I'm fairly new to springers.  I have had a couple of struts go out on cars.
They still had a little power but not enough to keep the hatch up.
Maybe you are right the piston might have lost some pressure.  I have seen a few
posts about the pistons loosing pressure.

[Norm_m]

Sounds like a bad seal! My NPSS went about 800 shots and my Chrony # went from the mid 700's to the mid 500's. Put a new seal and did a tune and was back up to the mid 700's and the NPSS was so smooth it was like a whole different gun.

[bwana]

It sounds like the seal is too tight. I had the same thing happen with my mendoza when I changed the seal, fps dropped big time. After 200 shots it came back up, but not to what it was shooting before the seal change. It does shoot smooth and accurate so I'm happy with the lower power.

[stacycfi]

If you lost a little power before you put the new seal it might be the piston.
Is there anyway to test the amount of force a spring or piston is producing?
If so, maybe someone with the same piston could test theirs and you could
compare the two.



Maybe like this put in place of the spring removal tool.
This one is a bit pricey tho. At $200

[RedFeather]

Probably the seal but are you testing outside?  Same temps?  They will drop a bit in very cold weather. 

[stacycfi]

I am curious,  do the gas spring rifles have lubes that could be slower to
react in cold weather?  Do the springers loose fps in colder weather?

I know that the pressure of nitrogen does not drop when it is cooler.
The airliners inflate their tires with nitrogen so when at altitude the tires
on the jets do not deflate.  Same with the landing gear struts. 

Just wondering why fps would drop in cold weather. 

I am new to springers and am trying to expand my knowledge base.

[Progun]

If the rifle became noticeably easier to cock then your ram/strut is probably failing. If cocking effort is just as stiff as it ever was then it's more likely your piston seal that isn't sealing well which is resulting in lower velocity.

[stacycfi]

That is a good indicator.  

I remember a video PA made using a bathroom scale to test the
cocking effort of a rifle he was reviewing.  He placed  a board across the scale and put the
muzzle of the barrel on the board then pressed with the stock to get the pounds needed to
cock the rifle.

Good idea to get a baseline on a new rifle that way if you think you have a
spring/gas issue you can double check on the scale.

My Titan GP peaked at 34lbs with this method BTW

[Don C.]

Just checked my Rem NPSS .177 using the bathroom scale method and got 33/34 lbs cocking force also, FWIW.

[Norm_m]

Reread your post and have a couple of questions.
Out of curiosity you said you recently changed the seal on your NPSS, so why was that? When you did change the seal did you hone the chamber? Did you sand w/400 grit paper the remainder of the tube? Did you sand, file or hone all the edges of all the holes and slots in the tube? Other than the lost of velocity did you lose any accuracy, or notice anything else? I'm just trying to think of why you are having any issues after you changed the seal and these question may help.
Also I saw where you said your NPSS was a .177 and you were getting 740 fps when new and lost 100 fps shortly after you replaced the seal. I not sure about anyone else but my NPSS is a .22 and I got 748 to 769 when it was new and about the same after I replaced the seal. To me a .177 with 740 fps seems low to begin with. Maybe I am just lucky with my .22!
Norm

[Duber]

Maybe give the breech seal a test with tissue paper to make sure there is no leaking there.



Regards,
Duber