Gentlemen,
as a whole group, the military snipers are not using the .300. There are some spec ops groups using the .300, but not the average sniper section attached to an infantry unit. I CAN GUARUNTEE THIS. We are issued either an M-24, Modified M-14 and some units are getting the new Semi auto .223. The .300 is not even used at the Army Sniper School.
I know that some units use the .300. I have been in those armories, however, I will not discuss who is using them.
As faar as the vap[or trail, or Trace as we call it is concerned, with a good spotting scope, you can watch trace on as small as a .223, maybe a little smaller too. I have see .223. What you need to do to watch this is as a spotter lay as close to the shooter as possible, non firing side generally, focus your spotting scope for about 3/4 of the way down range, and watch. You will see what looks like heat mirage, only going horiaontal insead of vertical. You will know it when you see it.