Maruzen Type 96 Varying FPS after refurb

[Northman]

Hi Guys, 

I'm returning to the sport and have been in the process of servicing and upgrading my old Type 96 sniper rifle from a blunderbuss to something with a little accuracy.  The first step was to service the gun and install at PDI Hop-Up unit which is now done but the problem is one minute it will shoot 350-370 fps and the next it will be around 440fps,  which means I can't use it. 

I'm learning as I go on the tech front but everything seems to be working fine - Am I missing something obvious?

Please note

• I have not tested the rifle in the open yet - just chrono'd in the garage - (so could be the hop up?)
  
• The rifle is old and still has the spring from when I stopped playing 5-6 years ago.
  

All help is greatly appreciated!

 

[Immortal]

If I take a gun out of storage then first thing I do is:

Check/lubricate or replace the seals (or anything deemed perishable...)

Hardened, deformed or dry o-rings can give odd results.