OK, I'm with you now- "Line Projector". Any chance it is a Harrington & Richardson?
The answer is probably not. Although the shell chambers, a shot shell functions differently than a metallic cartridge when fired. A metallic cartridge- the bullet is pushed straight out of the metal case. But a shot shell UNFOLDS.
Think about a fired shotshell for a moment- they are longer than an unfired, due to the mouth of the shell opening OUT. In a shotgun, there is a place for that to happen- that is why a 3 inch shell will chamber in a 2 3/4 inch gun, but is unsafe to fire- pressures jump WAY the hell up there, because the opened shell partially rstricts the bore. Your line thrower has no equivalent space for a shotshell to open- bored straight thru, I betcha.
COULD you load 45-70 shot rounds, and use them safely? Probably- the line thrower guns were not designed for that, but they take pretty stout pressures. Would it be wise? Not unless you have pirates climbing the taffrail, and you have no other weapons. Will a single .410 shell blow it up? Probably not. Could I ask you to move down to the far end of the range before you do that? Seeing stains on my range bag always makes me depressed.