A new spring is only £10 for a standard one, £20 for a posh one, so its not a big deal if the life of the spring is shortened a little.
It doesn't damage the spring permanently.
A spring kept under tension will lose power over time.
Three days IIRC, according to a recent article in Airgun World.
But as soon as it's re-cocked it's business as usual.
Born To Be Mild.
It wouldn't be a problem to leave it cocked for a long period of time,i've heard that it can actually do the spring some good to have a little bit of compression on it sometimes,any decent modern spring gun wouldn't find this a problem.
here comes another another one:
what fps should i expect with jsb exacts??
You should get high 700's. Impossible to say exactly as depends on the FPE of the gun. Most springers improve after the first couple of tin have been shot and the internals bed in.
that gun advertises 951 fps but with lighweight pellets as weihrauch told me via email. Many reviewers say that after 1000 shots you should expect high 800's and very low 900's if you are lucky