Re: Chrono 1st attempt
My Beta master Shooting Chrony has an 18' cord and I usually have it stretched as far as I can in front of the muzzle. Most of the time it works pretty good.
I don't have many big cannons...the .264WinMag is my heaviest and it throws a pretty good shockwave (about like the 7mmRemMag). Every now and then I'll get a fail when shooting that rifle with the chrony set about 15' from the muzzle.
Most of my errors though are from getting offlline with one of the sensors or just a light reflection if it's a sunny day with the sun low instead of directly overhead so the screens shield the sensors.
Your 1180 readings from the 7mm could've been caused by either the muzzleblast or stray reflections.
I saw a really neat idea for sky screens a couple weeks back that I might try someday. One guy at the range had a white plastic 10-12gal barrel with the ends cut out. He sets the chrony inside that barrel. This gives him a nice wraparound screen for the sensors that shields them from stray reflections even when the sun is at really odd angles. He doesn't have a stand for it, so it's ground-level only but I'm sure a person could rig up a stand to hold it up at bench level too.
I've draped translucent plastic grocery bags over the top of my screens before to make a larger shield area but it's kind of a rinky-dink setup compared to that barrel.
Another tip that works for funky lighting conditions is to color the bullet with a black magic marker to give better contrast against the sky/screens.