Creative Speculation

Soft-point bullets are more likely to break apart in the body and cause more damage, and hence a greater likelihood of death. The murder weapon could only use 6.5mm FMJ bullets. The Hague Conventions specify an army only use these types of bullets because there's a greater chance that the bullet will pass through the body without breaking up. That means it will only incapacitate rather than maim and kill.

A conspiracy from the usual suspects would have known this and have been able to produce a rifle with soft-point bullets to further guarantee the kill. Indeed, as Bugliosi points out, a good reason to suspect James Earl Ray was involved in a conspiracy to kill Martin Luther King, Jr is that he used a brand-new $700 rifle ($2000 in today's money) that did use soft-point bullets, one of which killed MLK. By contrast, Oswald had a $19 rifle, which would have been $57 today, that only used full metal jacket bullets.