There was a French 58mm torpedo projector (a form of trench mortar with the bulk of the projectile being larger than the barrel - only the tail was fitted into the tube). Possibly the French sold them on to the US Army?
... I believe Centruion's theory is likely correct. We do know the French spigot mortor, the "58T" was in large scale use from January '15 onward. the weapon was simple, but effective. It fired winged bomb containing 6 kilos of HE. ( the 58 refered to the diamiter of the tube the bomb's tail rod fit into.)
Examples of the mortor were supplied to Russiam Rumania, and Serbia. There is every reason to believe, therefore, in as much as the Americans used artillerie primarily of French manufacture, that this little device found it's way into the AEF also.
... I believe Centruion's theory is likely correct. We do know the French spigot mortor, the "58T" was in large scale use from January '15 onward. the weapon was simple, but effective. It fired winged bomb containing 6 kilos of HE. ( the 58 refered to the diamiter of the tube the bomb's tail rod fit into.) Examples of the mortor were supplied to Russiam Rumania, and Serbia. There is every reason to believe, therefore, in as much as the Americans used artillerie primarily of French manufacture, that this little device found it's way into the AEF also.
One minor correction the 58T was not a true spigot mortar like the German Granatenwerfer 15 .The tail of the 56T fitted in the barrel rather than over a spigot. A spigot has no true barrel of its own and the tail of the missile itself acts as the barrel. I've attached 3 digrams showing how a spigot operates. Spigot mortars have the advantage that they can be manufactured by companies with no experience of gun founding as there is no barrel or complex breech mechanism to be made. The German 'granatenwerfer' (grenade thrower) spigot mortar used throughout the First World was actually manufactured by a toy making company. The Blacker Bombard and the famous PIAT anti tank weapon of WW2 are good examples of spigit 'mortars' used in WW2