Dunno mate, I tried my mate's Combro, which sits at the end of a barrel, but recently bought a 'proper' Chrony F-1 chronograph, and it's much better and - ultimately - more versatile (IMO), as I shoot powderburners, too.
Not to mention the fact you can measure the speed of elastic bands, catapult ammo and many more
Edit: Got a tip off here that 'Merlin Archery' are the cheapest for the F1 at around £85