Disco down on power?

[C.ALLS]

When I first got the gun it was slinging the JSB 18.1 grain at 790-800fps and was good for 10 shots. It was Chronyed within the first 100 shots, fast forward around 800 pellets and it's now shooting them (JSB 18.1) at 720-740fps for 17 shots. The mods I've done where 2 barrel bands and an extended bolt probe. All these things were done before it was shot when new. Is it normal for the gun to "settle" and loose a little speed? What should I look for or what have you guys seen that would cause the drop in velocity?

Next question is if I wanted more power (840-880fps with JSB 18.1) what would be the best approach? I have the preload adjuster and a stiffer spring on order from Disco-r-us and a spring guide from ebay. I was hoping not to have to open the exhaust port and barrel port because once I take material out I can't put it back and my barrel is nice an accurate so I would hate to mess it up.

[Ribbonstone]

Probably had that mainspring cranked up a good bit to get it to  790-800fps (+25 foot pounds).  Going to guess that the spring has let down a little. Could be some other things going on (cool weather thcikening the striker's lube, valve stem seal compressing a bit, etc), but I'll stick with spring let down.