Surefire M300v with KM1A lens on a BCM Keymod adapter: 2.9 oz
Surefire M300 mount alone (no light): 0.8 oz
Surefire M920v mount alone (no light): 1.7 oz
Haley Strategic Thorntail M620 Light mount alone: 1.0 oz
Surefire M920v with KM3 lens, Thorntail Keymod Adapter: 6.4 oz
Surefire M920v with KM3 lens, stock Surefire Mount: 7.4 oz
Surefire M300v with KM1A lens & Larue LT752 Mount: 4.1 oz
Surefire M300 with KE1F lens & BCM Keymod Adapter: 3.4 oz
You can probably extrapolate some of the mount weights from the data here. I used my postal scale downstairs, made sure to remove the batteries, etc.
And I found a Haley Strategic mount for the Surefire M952v that I needed for my AR15A4, so winning. Go down in the basement, weigh some lights, find the needed mount!
Somewhere around here, I have a Thorntail keymod adapter that's not attached to any lights. If I find it, I'll make sure to get the weight.
Now, to put rifles back together!