The standard USCG line throwing rifles are converted 1917 Remington and 03 Springfield rifles. The barrel is a shortened smoothbore with an underslung reel. There are 3 different wieght projectiles consisting of a rod, line lanyard and two plastic and one heavy bronze tip. The rods slip down the bore conected to the underslung line. The cartridge used is a standard blank as used for grenade launchers. I recall many stories of black powder muzzleloading longarms discharged by accident or haste in combat with the charging rod the projectile. The safest option for a dedicated line throwing rifle is to locate a Greener Harpoon rifle. These were imported by the old Harrington & Richardson company in the 70's. It is a splendid piece ( it is the weapon used in JAWS) built on a Martini action. Last time I saw one, it had a pricetag of $800. There are two low tech options; A. making up a monkeyfist knot and practising, and B. Archery can be set up for fishing with a light reel. I doubt if archery would be viable. Conditions that merit any line throwing device mean throwing a handline is difficult due to distance and/or high winds. You must be extremely carefull discharging such a weapon. If everything works, there are tremendous recoils associated with the heavier rods. The bronze projectile must be fired with the rifle but braced on the deck. I watched a shipmate shatter his collarbone and dislocate the rotator cup firing offhand.