Or, an Exercise in Splitting Hairs
For some bizarre reason, this topic has been ricocheting around in my head all day, so here goes...
Several firearms, mostly designed in Der Vaterland, use rollers in their operation. While they serve the same function (locking of the bolt), there are two distinct designs that use locking rollers: roller-locking and roller-delayed blowback. The former is employed most notably in the MG42 and CZ52, while the latter is a trademark of Heckler & Koch.
Roller-locking is a form of short recoil operation. In these guns, the barrel moves a short distance (a quarter inch or so in the case of the CZ52), at which point the rollers are allowed to unlock the bolt. In this way, these guns operate much like modern autopistols using the Browning tilting barrel locking method. Since the start of the unlocking process is delayed by the barrel movement, chamber pressures have dropped considerably from peak by the time the bolt unlocks. The spent case can be extracted with little effort. (As an aside, the MG42 uses a muzzle booster to cycle the barrel more forcefully - a feature seen in many other recoil-operated machineguns, and about the only time any sane person would want a recoil enhancer.)
Roller-delayed blowback is another can of worms entirely. In delayed blowback arms, the blowback action begins immediately; however, the action retards the rearward movement of the bolt in some manner, so that the bullet has time to leave the barrel and chamber pressure can drop before extraction. In these guns, the cartridge case begins to move rearward immediately. Since the chamber is at peak pressure, this presents a problem. At rifle pressures, the case, being a thin sheet of brass, pushes out against the chamber, and refuses to budge. If extraction is attempted at this point, the case can be torn in half, which is a good way to muck up the operator's day.
To combat this, cases must be lubricated. In the Schwarzlose machinegun, the cartridge feedway included an oil pump for this purpose, as did the thoroughly screwy Breda model 30 light machinegun. (I'm starting to think that the Italian military refused to adopt an arm unless it met some minimum screwiness quotient. How any of Beretta's arms were approved remains something of a mystery.) John Pedersen developed a toggle-locked semiautomatic rifle for the competition eventually won by the Garand; for proper operation, the rounds were coated in beeswax (which, being a dry lubricant, had the distinct advantage of not pulling dirt into the action). This approach, according to Major General Julian S. Hatcher, worked better in practice than in theory.
The good folks at CETME (and later H&K) sidestepped the lubrication problem with a fluted chamber. When fired, these flutes 'float' the cartridge case on high-pressure gas, which allows it to begin the extraction process in one piece while still under pressure. This has proven quite reliable, but has the side effect of trashing the brass. If you're a military and have millions in OPM to buy fresh ammo with, this is a non-issue. If you're a civilian with a working man's ammo budget and a Dillon 550, this is a distinct problem.
H&K also made the P9 and P9S handguns with what is unhelpfully described both as 'roller locking' or 'delayed blowback,' often in the same sentence. As far as I can tell, the bolt face is a separate piece, and the rollers lock it to the fixed barrel; when the slide has retracted enough, the rollers can cam inwards to unlock the slide from the barrel. Almost reminds me of the 'shifted power stroke' used by Pedersen on the Remington 51 and M1917 pistols. Now I *really* want a P9 to fiddle with... (if anyone can enlighten me vis-a-vis the P9 operation, I'd much apprectiate it.)