The differences are subtle, and have confused many shooters. In short, the plating on TMJ bullets is thinner than on FMJ, and completely covers the core with no exposed lead. This reduces lead content in the air when firing indoors, which is why we recommend these for indoor practice.
The TMJ bullet is normally cast while the FMJ is sagged making the FMJ slightly more consistent in weight and diameter. The FMJ costs a little bit more to purchase which translates to a slight higher ammunition cost. We recommend these to everyone who desires a degree of precision in their shoorting