The bolt acceleration occurs mainly while the powder gas pressures are high and are exerting a force of many thousands of pounds on the bolt.

Although it is true that the driving spring absorbs the kinetic energy of the recoiling bolt and thus limits the total distance it moves, the resistance of the spring does not have any real effect in the early phase of the cycle of operation. In many descriptions of blowback actions, it is strongly implied that the driving spring contributes a substantial portion of the resistance which limits acceleration imparted to the bolt by the powder gases. вЂњNOTE: There is one point which requires special clarification at this time. From the previously cited Chinn Vol 4, page 15 (underline added by me): Note that even a tiny 22 long rifle cartridge pushes on the bolt head with a force of about a thousand pounds, so you can utterly forget about springs (at least, any spring you could possibly cock by hand!), or friction, or magnets, etc. This force can come from tricky-to-machine locking lugs, but weвЂ™d like to just use bolt inertia. Pushing back against the case head with enough force. Chinn says heavy grease is needed light oil tends to get squished off the high spots. Greasing the cartridges, so the cartridge tends to slide out of the chamber instead of sticking to the walls. You can stop the case head from separating by:ДЇluting the chamber, like the HK MP5 or G3, which equalizes the pressure inside and outside the chamber.

This вЂњcase head separationвЂќ can and has killed people, for example by flinging the bolt at high velocity back through the shooterвЂ™s eye. If it stretches too far, the case head may separate from the body of the case, and spray hot gas at tens of thousands of PSI in all directions. This is bad, because if the bolt moves back under pressure, then the cartridge tends to stretch out. Note that the chamber pressure pushes back on the bolt with the same pressure that it pushes the bullet down the barrel. The tens of thousands of pounds of pressure inside the chamber only last for a millisecond or two, and theyвЂ™re what push the bullet down the barrel.Д¬hamber pressure pushes bullet down the barrel Pressure versus time curve for firearm chamber peak at 1/2ms You can record your own pressure curve with a strain gauge like a Pressure Trace, but traces from different cartridges and guns are surprisingly similar, and different mostly in peak pressure (from about 10Kpsi to 60Kpsi). HereвЂ™s a typical cartridge pressure curve. Also, after a certain power level, they weigh more. So why arenвЂ™t all guns blowback? Well, blowback guns do have this little tendency to explode if designed incorrectly. The federal government forbids most use of open-bolt (fixed firing pin) designs, since theyвЂ™re extremely easy to make fully automatic. So you can build working blowback bolts from crappy materials like mild steel (of course, harder steel will wear better).Д«lowback designs are legal in most US states, as long as you use a semiauto hammer or striker.

The force on the bolt face is mostly just compression, instead of the tension at the back of the lugs (see Dan Lilja or Varmint Al for locking bolt analysis). There is no вЂњheadspaceвЂќ, or cartridge slop before the bolt hits the locking lugs. You donвЂ™t need to machine locking lugs into the bolt or chamber. There are no mating or rotating parts in fact, the only moving part can be the bolt! This is a public-domain government publication, so IвЂ™m reproducing the figures here directly.Д«lowback guns are actually a lot easier to build in a garage than locking bolt guns, because: HereвЂ™s a schematic view of a blowback gun, from George ChinnвЂ™s 1955 masterpiece, вЂњThe Machine GunвЂќ, volume 4, part X. вЂњBlowbackвЂќ guns, by contrast, just use the inertia of the bolt to hold the chamber closed. This includes bolt-action or break-action rifles, as well as rotating-bolt semiautos like the AK or AR. вЂњMost high-powered guns have a locking bolt, where locking lugs hold the chamber closed during firing. Reproduced from the Website вЂњOrionвЂ™s HammerвЂќ Massive bolt addition used to harness the 10mm in blowback

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