Yes, Virginia, there was a 1903 line thrower, and it still gets some use
Naval and merchant ships have used line-throwing rifles (and shotguns, as well as small cannon) for centuries to heave lines from ship-to-ship and ship-to-shore to greater distances than what could be done with a deck division guy and a slungshot. Currently, the Navy uses M14s and M16s with blank-firing adapters for this task, but this post is about the USCG and their slightly more elegant 1903s.
The old Revenue Cutter Service/Revenue Marine used Coston Shoulder Guns– a converted U.S. Springfield Trapdoor Model 1884 rifle in .45-70 (and the similar Winchester Model 1886 Line Throwing Guns, in a 14-5 inch smoothbore of the same caliber)– from the late 19th Century through, in some cases, WWII (and by some accounts, remained in armories for a couple generations longer).Don’t get me wrong, the .45-70 line thrower was always a good gun for its purpose, even if dated. Today the Bridger Shoulder Line Gun uses a single-shot H&R Handi Rifle for the same concept and it is very popular.
However, around the 1930s these began to be supplemented by a series of line throwing 1903s. These 30.06-caliber rifles were converted by having the barrel rifling and sights removed to produce a 24-inch smoothbore with the handguard wood shortened to match. Two-pounds of lead was placed in the butt under a modified padded butt plate. The line bucket is mounted under the abbreviated forend and, as noted by Brophy, these were used with three different projectile rods in light (13 ounces) heavy (15 ounces) and illuminated buoyant types.
They show up at auction from time to time, being replaced by M16s and shotguns years ago, and are very curious.
The serial number on the above Port Clinton gun, #1211224, makes it a Springfield Armory-manufactured receiver made in 1920, so the gun has very likely been in the Coast Guard’s stocks since Prohibition when a number of brand new BARs, 1911s and 1903s were transferred to help arm the cutters patrolling Rum Row against often well-armed bootleggers. As the service used the .45-70 single shot line thrower through WWII, this Springer was probably converted post-1945 using the old rope bucket from retired black powder guns.
And the last Coasties to use them probably haven’t been born yet.