From your description, I'd assume you have some sort of "unofficially shortened" arm made up, long ago, by outsiders, for some utilitarian purpose.

It cannot be a "proper" or "standard" or "normal" school gun, since they used actual carbine barrels, not cutoffs, and M1899-marked receivers. Supposedly there are no PCR's (which were converted at Manila) in the US.

It is absolutely not a BoOF rifle.

You sound quite knowledgeable, so I almost hate to ask, but you are measuring the barrel length in the bore, right?