The one you have with you. I have 3 of those cheap units that I pinch hold on the end of my barrels or suppressors to just check speed sometimes and they work great. I very rarely need to do more than just confirm a speed. I run strings on unregulated guns (I only have 2) but once I do that I only need to check their speed once. All of my regs are very consistent so I gave up wasting my time with strings unless tuning and changing something, including ammo. I have the new FX D1 radar unit on my FX which I do run all the time because a few fps at distance actually matters, and I'll slow down myself a little bit to get the spread down to just about nothing. The problem is usually me. If I wasn't shooting long range though, I would have just stuck with the cheapies. Not the cool answer everybody wants to hear, but they work well enough, and I have one at all my shooting spots, along with a cheapie laser range finder.