John, I use a Deben Hawke pro 600 rangefinder - 6x mag and solar powered so you can leave it in the sunlight to recharge, had no problems with it and would recommend it.
Chrono I now have is a Skan Pro 1 Series 3, easy to use and can use indoors under flourescent lighting and gives accurate results, I have had an Alpha Shooting Chrony which gave accurate results but needs to be used outdoors in daylight unless you get their lighting rig.
Air Arms EV2 MK4 + Hawke Tac 30 10 x 42
Weihrauch HW100 FSB .22 + Hawke Tac 30 10 x 42
2013 Air Arms HFT Team Member