[Mrgnjr]

A shotgun mutated into an axe has the following properties

Range
Benefit from MG Shotgun passive
Lack of benefit from Berserker Shell
Scales off Skill
Ignores Defense
Blocked by blade barrier (if it keeps range i'd suggest keeping this)

It seems like some of this is likely unintended

[Ranylyn]

The game states quite clearly that the weapon functions as normal (exception being mutating into bows.)

The only benefit it gets is being useable with axe skills - which scale with strength and are mitigated by defense, overriding individual weapon scaling properties - in exchange for no longer being useable for any gun skills.

Seems pretty par for the course. Nothing out of the ordinary.

[Soapy]

The only problem here is it not working with Berserker Shell.

[Neus]

It has been corrected to work with Berserker Shell. Other items mutated into axes should also be corrected in this respect.

Nothing else sounds like a bug though.