" there supposedly was a Springfield Bushmaster carbine with an 18 inch bbl but it wouldn’t take a bayonet"

The Bushmaster's carbine was a real thing, maybe not officially sanctioned by an arsenal, but they did exist. My father was in the 158th Regimental Combat Team (Bushmasters), part of the Arizona national guard mobilized at the start of the war. He told me about the shortened Springfields and using them in patrols in the dense Panama jungles. Full length rifles were impossible to carry over the shoulder before they were cutoff. In his describing them, they were done locally, maybe in the canal zone machine/maintenance shops. Later research said they were crudely done locally (in the canal zone) and later destroyed. The article said they were used only in training, my father said patrols actively guarding the canal used them.
Because almost all were destroyed and the local work was crudely done it is hard to tell if any still exist, I guess crudely done is easily copied today.