FWIW, if I were to make the investment in a rifle for a junior shooter, it would come with the condition that regular and serious practice would be part of the deal. I've seen too many kids that felt they "needed" their own rifle only to see their interest wane and the rifle become an expensive dust collector.
I'm guessing when you're talking Hammerli, you're looking at the AR-20. We have a couple of those in our program and while they are attractive from price standpoint, we steer parents away from those. They look good from far...but they are far from good. Plus Umarex (parent company of Hammerli) has not been easy to work with for parts and support. Ironically they support their Walther brand better. FWB's are your better bet. They're much better built and supportable into forever.
The Crosman Challenger is match proven (it pretty much rules sporter air rifle all the way up to the national level) and if the owner is handy around high pressure air, it can be user serviceable (since you're on your own for many things it seems). It's power adjustable, does NOT have a removable cylinder and is not regulated.
Here's a German document on the stand and spring.
https://www.schiettechniek.nl/system/files/pdf/de_verende_schietsteun.pdf Google can do the translation for you.
