The Physics of Blowback Pistol Design

I was recently Googling for information to help me better understand design principles of blowback pistols and found this article on the physics of blowback design.  For the most part, it’s a problem of balancing the linear momentum of the bullet and expanding gas with the linear momentum of the sliding part of the firearm.  The friction of the bullet going down the barrel is another significant factor.  For most pistols (Glock, 1911, SIG P226, Beretta M9, etc.), that’s the slide, but it can also be the bolt in some fixed barrel designs (Ruger Mark II).

Here are some things I learned:

The phrase “every action has an equal and opposite reaction” is basic physics and instrumental in semi-auto firearm design. Linear momentum can be used to calculate the required mass of the moving part of a semi-automatic. Linear Momentum is defined as: Linear Momentum = mass * velocity. Because a bullet has much less mass than the moving part of a semi-auto, it has a very different velocity.  Basically, you balance the linear momentum of the slide with the linear momentum of the bullet.  Example:

9mm 115-gr. bullet with a muzzle velocity of 1,225 fps
Conversion: 115-gr.=0.263 oz.
Typical slide velocity is 12 fps according the article above.
(0.263 oz.) * (1,225 fps) = (weight of slide, oz.) * (12 fps)
weight of slide = 26.9 oz. = 1.7 lb.

That’s somewhat heavier than actual, but close. Remember that the friction of the bullet moving down the barrel reduces the required slide weight in the total scheme of things.

Pure blowback, unlocked breech designs are normally limited to smaller calibers, like .22LR. Barrels are normally fixed in simple blowback designs. The Ruger Mark II and Mark III pistols are prime examples. This limitation is driven by the need for the breech to stay closed until the bullet leaves the barrel.  If the breech opens before the bullet leaves the barrel, the high pressure in the barrel can rupture the cartridge case causing a “Kaboom!” sending hot gases and/or metal parts towards the shooter’s face.  The cartridge case is not designed for high pressure. It relies on the barrel and bolt face to provide support while high pressure exists in the barrel.

Larger pistol cartridges, 9mm and above, typically used a “locked breech” design in addition to the fundamental blowback design because of higher working pressures. A locked breech forces the barrel to move a small distance, usually a few millimeters or less, in conjunction with the slide as the slide travels rearward.

Blowback design works best with straight-walled cartridges like .22LR, 9mm, and .45 ACP. Bottle-neck cartridges like .223, .30-06, and .50BMG have a significant bolt-thrust problem due to the difference in diameters of the bullet base and the metal cartridge base. Read the linked article above for more info on bolt thrust.

Screen shot 2014-06-02 at 1.07.46 PMCase rupture due to lack of support. Very high pressure exists in the barrel prior to the bullet leaving. Once the bullet leaves, pressure in the barrel drops very quickly to atmospheric. The case must be properly supported while high pressure exists or the case will rupture.

Screen shot 2014-06-02 at 1.08.16 PMResult of a Kaboom! in a Beretta 92. This pistol design is proven. Some Kabooms! are caused by high pressure rounds. I avoid +P+ rounds in semi-autos for that reason.

Screen shot 2014-06-02 at 1.11.35 PM
Glock changed its barrel design when reports of Kabooms! occured in .40S&W and
.45ACP designs. Kabooms! have since been virtually eliminated. Note: LWD stands
for Lone Wolf Distributors which makes drop-in barrels for Glock pistols.

Screen shot 2014-06-02 at 1.26.33 PM
Cutaway view of a M1911. The two notches on top of the barrel fit into the associated notches in the slide causing the barrel to move with the slide for a few millimeters prior to tilting down. Barrel tilt is caused be the rotating link, “A”. Since the barrel moves with the slide for a split-second, the bullet is mostly guaranteed to leave the barrel prior to the breech opening.

Screen shot 2014-06-02 at 1.31.25 PMb
X-ray view of a Sig 226 showing its locked breech design where the barrel lug at the ejection port causes the barrel to move with the slide for a few millimeters. Here is a great illustration of the Glock locked breech in action.

When a cartridge is fired, the bullet is driven forward and the breechblock is driven backward. In a fixed-breech firearm, like a revolver or a bolt-action rifle, the whole gun is driven backward.  Fixed-breech firearms have more recoil because most of the energy affecting the breechblock is absorbed by your shoulder or hand.  In a semi-auto, some of the energy is used to cycle the firearm which reduces the energy transfered to your shoulder or hand resulting in less recoil

For Unlocked Breech, Recoil Operated firearms, like the Ruger Mark II, the straight blowback slide moves a very, very short distance under pressure.  Slide movement is delayed enough because of the slide’s mass being significantly greater than the mass of the bullet (remember, think .22LR).  The breech movement is delayed enough to give the bullet time to exit and pressure to drop to or nearly atmospheric.  Once that happens, the breech can safely open. In much the same way as the Locked Breech, Recoil Operated design, the slide makes full travel on its momentum that was conserved during the brief instant that it was being accelerated backward by the force from the bullet.

The only real difference between locked and unlocked breech design is the method used to delay the slide and the opening of the breech. The blowback uses high slide mass and/or heavy action springs. The locked breech uses slide mass and the bullet’s drag on the barrel in conjunction with a design that causes the barrel to move slightly backward with the slide to delay opening the breech. The action spring has very little effect on delaying the breech from opening. This is why a locked breech pistol can usually be safely fired without a recoil/action spring but the unlocked breech design usually can’t.

Don’t you love physics!