Burna Boyz get the option to turn their burnas into power weapons, which means that since it is unusual, it would be
AP 3. They cannot turn their power weapon into a power axe magically by adjusting their nozzles.
I believe that there is no shenanigans as this seems pretty cut and dry, but is there any other ambiguity with this that I may need to know about before I start using them in 6th ed?