Blow back designs are OK for pistol cartridges but the camber pressures of rifle cartridges you need a locking system.
Just look at the pistol designs like the High Point 45. Giant slide to hold the slide closed (by mass) for the few micro-seconds it need to get the projectile out of the barrel. That chamber pressure is less than 28,000 psi (that's 45 super). Trying to hold the bolt closed with bolt mass for a cartridge that has a chamber pressure of over 62,000 psi would have to be massive.