It reminds me of the first day of my buddy who now owns his own smithing business. On his first day smithing for the 1st shop he ever worked at about 20 years he shot himself using something similar. The shop he worked for had a long test fire tube that looked like yours but 20’ long and you stick your arm in to fire the gun. With his first shot the bullet ricocheted and came back sticking in his arm. He headed to outpatient on the first day of work to get the bullet removed and stitches. He was back at work the next day repairing firearms...but never test fired down that tube again.