If you put BBs inside of a raw chicken and then put it in the microwave, will it explode?

No, it won't explode. Microwaves ironically enough don't use microwaves to cook food. They use radio waves that causes water molecules to spin very fast creating friction which cooks your food. The BB's will ionize because of the radio waves but they will not cause the chicken to explode or anything that would resemble exploding. And contrary to the belief that microwaves cook from the inside out, that is not entirely true. Only some of the radio waves what you are cooking to actually cook the center but most only reach just below the surface and the outside will cook a lot faster than the inside.