Thinking cap on... aka three beers later.
Watched the test video again and found the beam sensor is spot on, pretty pin point to trigger w/ a head or shoulder shot shown on the toothpaste target. This was tested w/ moving target at 24" from sensor. Sensor range is 2 meters/ 6'-6". Been placing the reflector at around 5-6' w/ bait at 3' anticipating a crossing from the basement access to kitchen where the Mouser is set up. Pretty stoked about the first kill but the last three have been hits w/o fatal. May have to place the reflector or bait in closer. Without a cam I'm just guessing.
There should be plenty of pressure even w/ the inner sleeves of the tube connectors. When I disconnect the quick disconnect coupler at the tank there is still ample pressure in the 5' of 1/4" hose for a stinging shot. If restriction proves an issue later I could add a long 1/4" nipple between the solenoid valve and hose coupler for a 'surge tank'. Otherwise pressure is holding and no leaky.
Measured the barrel bore w/ transfer punch at 5/16" w/ a good sliding fit. Focus on projectiles and aim next.
@ RevRico: Great idea there. I dunno about projectile design enough but it's a 5/16" sliding fit, weight unknown although when I tried to weigh the eraser tip and cig filter/ nail combo they came in at less than 1 gram on my electronic postal scale. Maybe something w/ a medieval killing tip FTW.
@ Crackers: The nail appeared to tumble in after hitting something, it lodged right in line of sight of the barrel. Barrel centerline is 3/4" off the floor, nail lodged in the beer box at 2-1/4", reflector mounted on short side of 2X4 would be 1-3/4" at best. Maybe the barrel needs leveled for longer range tho. Maybe duct taped to the floor too, I didn't check for kick but wouldn't be surprised. The board was taped to the bench during testing.
@ oldopelguy: I likes the ideas. Plenty of pencils tip leftovers too... eat lead motherberkeleyer!
Maybe this is projectile answer:
Not crazy about steel ball shot bouncing around in the kitchen tho. Windows, range and microwave door. E36 M3, the eraser tips bounced everywhere in bench testing. Need to wear face shield in testing.
Starting on the bucket roller trap for the garage tonight... after a few more beers.
The things that keep me amused.