This is an inexpensive (I got one for free when my brother got a RadarChron. He hadn't even used this one yet.) In theory, it should be fool-proof because you can only use it when you shoot thru it and it shouldn't be affected by outside influences.
This thing is inconsistent, difficult to use, and requires some pretty regular squeegie action when every third ball thru it breaks...no matter how well-aligned it is on your barrel. I don't think everthing BE makes is bad, but this blows.