I've been having good luck with the Leopold Mark4(China)Scope ,mounted on my Hatsan 135 .25 cal.Think airscopes sells them,they are tuff and have one on my 350 Panther.
I personally shot a Beeman Kodiak (Webley Patriot) .25 that was converted to a gas ram by Slade. The thing was a beast. If you think it's a big bad gun with the mainspring, it's a holy terror with the gas strut. It feels like a firearm and it shoots like one too The recoil is long, slow and hard on the stock Patriot whereas the rammed version everything happens so fast. You might need a mouthpiece when shooting one of these to prevent concussions