The roller-delayed blowback system can be confusing at times to understand, but I hope this video is educational in understanding how G3/MP5 type actions work. The term “roller-locked” is an incorrect description due to how this system is designed. Unlike the FAL and other popular .308 rifles of yesteryear, the G3/PTR91/C308 do not use a gas system to cycle the heavy .308 cartridge. It uses the power of its own recoil to cycle the action. It actually does this very well since the .308/7.62×51 NATO is very stout. The roller-delayed blowback system is not rare by any means, nor is it new. The MG42 used a similar system and the design was adapted to a later experimental rifle, which ended up being the influence for the design of the CETME rifle out of Spain. With a storied history and a good track record for reliability, this rifle also comes with some flaws, just like any other system. The rollers on the roller-delayed blowback system are the center of the systems function and how well it works, hence it being the first word in the description of the operation. Their size and the angle of the locking piece dictate what kind of loads will be able to cycle out of this rifle. I personally had issues cycling PDX-1 ammo out of my PTR. But this issue is simply caused by the fact that the springs, rollers, and locking piece were built to military spec, meaning it was built to function best with military ammo or hotter .308 rounds.
As with any system, if you take care of it, it will take care of you. Watch for the weak points, check the headspace occasionally, and keep it clean. If you do all this and keep it oiled in the right places, you will be golden.