The barrel sounds short for that model, I think they were about 26" but I'm not sure. If the barrel has been reblued I would look closely for ether screw holes that have been plugged or a dovetail that has had an insert installed then filed and polished to the barrel's contour.

At 21" I suspect the barrel has been shortened. If done by a professional the end of the barrel will look nice, clean and even. If done by a someone not a professional the same crown may look rough and uneven.

 

My 1st thought was the barrel was shortened. If so...extremely professional.
However, the magazine is also shorter than my 46b. In addition, there is no slot for loading cartridges. The rod and spring have to be removed and cartridges dropped into the magazine.
I'm new to this forum and if I can figure out how to send photo, I will!

 

Shortening magazine tube(s) may have been done also. If that barrel was 26" the cutting to 21" is a full 5" lopped off, this likely left the tube too long.

I meant to mention also, the receiver may have been drilled for a receiver sight and/or the barrel has provisions for a rear iron sight. If the barrel has been set up for a rear sight then the barrel has very likely been cut, no provision then probably not set up for a front sight.

 

 

My 46b is 26".
The 46B-B is actually 18".
It's tapped for rear sight but no indication there was ever one there.
I also see the slots in the wood where a barrel band would have been, but no marks on the barrel there either.
The wood is immaculate and it fires beautifully.
Who knows??? Maybe someone had it modified for a youngster.

 

It looks cut and crowned to me. The list "why" can be long.

 

The 21" barrel without front sight base suggests it might have been a special order or possibly a training rifle. Mossberg produced so many variations that some configurations are poorly documented. Have you checked the Mossberg collector forums?

 

I removed the stock for a deep cleaning and some adjustments...lo and behold there was the cartridge feed slot in the magazine all taped over!!
This rifle was expertly cut down to a carbine. I have a vintage mossberg (weaver) scope to mount. It came with a nice red dot, but I'm just too old school to use it on a septuagenarian 46B-B
So...until I see another one, I will claim to have the only Mossberg 46B-B carbine in the world. lol