Hw80 losing fps

[Relum527]

After a recent overhaul, my old hw80 loses some velocity with each consecutive shot...any ideas what causes this ? I don't know what it was like before the overhaul as I didn't put it over the chrony.

[Unframed Dave]

Please be aware of forum rules when replying ro this thread.

Discussion regarding altering power is not allowed.

Dave

[Shed tuner]

assume it settles down after a while, ie it doesn't keep losing until the pellets fails to emerge ?

[Anothermiss]

Seal getting hot and expanding?

[Bellycrawler]

Dismantle & check the piston seal for damage also for a loose or tight fit. It might even be OK but now with improved performance there blow by past the breech seal

[tinbum]

This is very common after a rebuild, the seal needs lubing a little more

[TonyL]

It sounds to me like a very tight seal? So the tightness creates extra friction. The extra friction creates more heat leading to seal expansion. Even tighter seal causes even more friction and on and on we go......?

I'd be checking the seal fit and, if tight, size accordingly. I've never felt the need to size one, preferring to let them run in. But, depending on the scale of the power loss, sizing might be apt here?

Or....have we used a low grade, non-scragged spring and, as it goes through a few compression and expansion cycles, is scragging itself and settling????

[evert]

An old 25mm HW50s did this, and it was the old seal that had swelled up for some reason.
A seal change fixed the issue.

Interestingly, the tight seal was fit in an Osprey I converted to HW seals, and worked well after it was sized down to fit the tube.