There still seems to be some confusion on how this and other pistol actions operate; I'll try to give a very coarse overview of the various layouts currently offered and the ones the R51 seems to be compared to
Tilting Barrel:
-1911; a pivoting link pushes the barrel up into the slide, causing grooved locking surfaces in the barrel and slide to mesh and lock together. Chamber pressure (resulting in bullet movement/recoil) drives the two meshed parts backward against the moving bullet until pressure drops, at which point the pivoting link drags the barrel down from the slide. The slide continues back on its own momentum (the barrel door-stops against the lower frame and comes to a rest) and the extractor mounted on the integral breechface drags the case from the chamber. The initial slide of the barrel backward rather than the pivot is why the return spring can't go around the barrel; it wouldn't be able to push the barrel forward relative to the frame (only the slide)
-Hi Power; same general concept, only a milled ramp integral on the underside of the barrel contacting a cross pin drags the barrel down from the slide
Rotating Barrel:
-As with the 1911, the locked slide/barrel are pushed backward against the bullet as a locked unit until it exits and pressures drop, at which point the a cam on the frame spins the barrel relative to the slide, and allows the two to disengage. Again, an integral breech block on the slide with an extractor drags the case out on its own retained momentum. This is essentially a rotational analog of the linear Browning tilt-barrel systems.
C96 Mauser:
-I personally think the C96 is closest in layout to the Pedersen action, though they do differ in function. Like the Pedersen, it has a breechblock separate from the slide. However, this antiquated and complex design also has a moving barrel and separate locking piece (more moving parts to obtain a similar result). In the Mauser, the barrel is the slide, and the separate breechblock is pressed up inside its rear portion and locked to it (much like a 1911 barrel to the slide) by a pivoting locking piece driven upward by a cam on the lower frame. To unlock, the locking piece must travel backward off the frame cam (along with the barrel and breechblock), and to do that, the slide/barrel can shift back a short ways. Once the breechblock and barrel decouple, the barrel stops and the breechblock continues on its own momentum to eject/feed.
Blowback:
-True Blowback; All pistols are technically blowbacks if they lack a gas piston. As with locked breeches, gas pressure on the bolt face ultimately is what operates the gun (we call them "recoil operated" because the pressure also drives the bullet forward, generating equal and opposite momentum on each --the "recoil" is the name given to the gun-side of that equation). Simple blowbacks rely on nothing but bolt mass (and to a lesser extent spring pre-load) to slow down the opening of the breech under pressure. Obviously, when higher pressures or slower moving bullets try to open the breech under pressure faster or a greater distance over a longer time, respectively, the bolt/slide mass must go up. The higher the momentum of the bullet, the higher too for the bolt, and battering of the slide stops quickly takes over unless heavy springs are used to cushion the frame (the real reason for heavy springs on blowbacks). For something as small as a pistol, the bolt rapidly gets too large to be practical much beyond cartridges of 9mm power levels. Locked breeches bring the number back down by essentially hiding the true momentum of the bullet from the gun by locking the breech and slide together while under pressure.
-Delayed Blowback; most systems use mechanical advantage (leverage) to effectively multiply the mass of the reciprocating system by forcing a secondary mass to travel faster than the portion actually resisting the breach. There aren't very many examples of this type in pistols because it is still too difficult to get enough delay for powerful cartridges and still retain reliable function, but it is seen commonly in rifles like the H&K G3 platform. The other method of delay that's been successfully implemented in several pistols is gas delay, in which some chamber pressure is tapped and used to oppose the rearward travel of the slide/bolt face. The H&K P7 and Steyr GB function very well, but these systems require very fine tuning of the design and a narrower band of ammo power levels for reliable/safe function than locked breeches enjoy. They are also susceptible to gas port fouling like any truly gas operated system.
FNH Five-seveN:
A weird hybrid of recoil operation and delayed blowback. The barrel and slide are linked, but are not locked. Initially, a lever connects the two, but at different distances from the pivot so that movement of the slide also results in a slightly lesser movement of the barrel in the same direction. This has the effect of slowing the rate at which a breech of given mass opens (delayed blowback) and also increases the overall reciprocating mass opposing the bullet's momentum (a benefit of a locked barrel/breech recoil system). The design is carefully calibrated so that the barrel and slide decouple while the case is still pressurized at a lower level, and the gun operates as a simple blowback thereafter.
Model 51/R51 (Pedersen):
Barrel is fixed. The slide mostly serves to add reciprocating mass to slow unlocking of the separate breechblock and guide its travel during feeding/ejection. When in battery, the slide holds the fully-forward breechblock downward, where it locks into the lower frame to oppose pressure in the fixed barrel. However, there is a small bit of slop between the contact surfaces of the breechblock and frame, which allows the breechblock/slide to shift backward briefly (like a simple blowback), just enough to impart momentum to the slide. At the instant the breechblock contacts the frame, the system is a true locked breech for the remainder of the pressure cycle. It is
not a delayed blowback; the breech is fixed during this portion of the cycle. The slide's momentum carries it back long enough for pressures to drop, at which time the breechblock is cammed upward from the frame and slides back with the slide to eject/feed. If you think of the slide as a bolt carrier and the breechblock as a tilting bolt, the system is more like an SKS or Ithaca M37 shotgun, only actuated by that initial stutter of blowback energy rather than a separate gas piston or manual pump action. Because the barrel is fixed, it can be used as the guide rod to keep the slide tensioned relative to the frame.
Hopefully this clears the water for some folks on exactly what is going on in these guns, and why describing the R51 as Blowback or Delayed Blowback is quite misleading. A fixed barrel does not equate to blowback, cheapness, harsh recoil, or any other inherent design flaw. It is, however, one less moving part to go wrong (though the separate breechblock is another --you can't win since the two have to separate somehow
).
TCB
