I have a Combro Chronoscope and it works fine. I also have one of those cheap Chinese ones that also fixes to the end of the barrel. It works just as well and matches the Combro velocities. I think that I paid about 30 bucks for it. It is USB rechargeable and has infrared sensors and will work in the dark. The cons are, it reads in metric only and the instructions are in Chinese. They both work as well indoors as out but the Comro, like others, doesn't like fluorescent lighting.