Rich, I personally don't trust them at all as they were used.
Infact the three times I have had to use them the reading are way off. Waste of time for me free standing.
At best, they need to be in a Jig of some sort. I believe, the Bfta are supposed to fit some sort of Rod to them, which you line up your muzzle with. So consistency should then be better.
Not holding breath though
I was at the chrono when some chap went 806 with 8.4 on first shot. His muzzle was very close to the chrono as no dought he was trying not to shoot it!
He loaded again but this time I made him move his muzzle back about 6 inches. His reading fell by 25 fps!
Caldwell and infared would have worked better!!