Using Arduino relay board outside its intended voltage
Likely the contacts will weld the 1st time you use it.

2nd, the pull-in time of the relays will vary (nearly randomly), and this will degrade the sequencing of your coil gun.

3rd -- the EM pulse from the coil gun might cause the Arduino to malfunction. You'll have to shield it very well from the discharge -- the relays might not allow that.

Best to use semiconductors -- IGBTs would be well suited for this task, and can run 1000's of A