While I always thought the two bulls were the same, I read this and wondered. So....I went out to my target stash and got an NRA official B-8 and an official FBI bull (LE Targets FBI-IP 1) and a caliper. Following are the diameters of the 10 rings and the nine ring (the bull image):
FBI:
10 Ring - 3.355" (8.84 square inches)
9 Ring - 5.455" (23.37 square inches)
NRA B-8:
10 Ring - 3.35" (8.81 square inches)
9 Ring - 5.55" (24.19 square inches)
So, the FBI's 10 ring is a TINY bit wider and its overall bullseye image diameter is a bit smaller than the B8. Sounds like a wash to me on that. The FBI's eight ring is 0.2" wider overall than the B-8, but I can't get overall dimensions of the two eight rings with my caliper.