Not the best scope, but certainly can see what you are wanting to see at 400yds - bought from Menard's last month "Barska spotting scope" for $69.99
Seems like you would want a spotting scope for many reasons when shooting at that distance?
I have also used auto car alarm shock sensors to activate a brake light to indicate a hit at long range but I don't have any photos uploaded.
This is precisely what I was looking for!
I originally considered a motion sensor, or an audio sensor but car alarm is perfect.
I will run it to a flashing light and set it to "go off" for 10 seconds.
Thanks very much!
I'm surprised you can't hear a clang even with ear protection on.