Re: How to build a LR gun, revisited.
I have checked my 35P's against Oehler's big multi-thousand dollar lab unit called the Model 83 and the velocities are bang-on.
There is also a more sophisticated model called the M43 that will do chamber pressures but it requires a laptop to operate. It can use acustic mic's (super sensitive microphones instead of skyscreens) that utilize the sound wave of the bullet to not only determine speed but the location that each bullet traveled relative to others (it shows your group and group size). The M43 will also work with standard screens. It is a great unit but not as userfriendly as the 35P - more for advanced reloaderholics.
You don't need the diffusers on overcast days, other than they give you a nice referance point for ensuring your bullets fly through the "zone". I believe the reason that diffusers were designed was because of a problem with a "glint" or hotspot on the nose of certain really polished bullets on bright sunny days that fooled the sensor. The diffuser makes for a bit of shade you might say. I had the predecessor of the 35P called the Model 33 back in the stone-ages and it did not originally have the diffusers. I recall glint problems with Sierra bullets with wonky numbers coming up.
Simple fact: Orange and black material streaking through your field of view is not good...