Blowback Bolt Weight
Introduction
All firearms need some mechanism for locking the rear part of the barrel, so that the pressure can only escape by pushing the projectile forward. This part that does the locking is usually called "breech" or sometimes "seal".
This locking is not only necessary for the function of the gun itself, which is to accelerate a projectile towards the front of the barrel, but it is also required for the safety of the user.
If this breech or seal, is too weak, then the cartridge, hot gasses and likely even parts of the breaking breech material are shot back towards the operator of the gun. As actio = reactio, the energy of this backward directed shot has the same energy as the projectile that is shot forward.
Direct Blowback designs use the inertia of the bolt mass to keep a sufficient part of the cartridge inside the barrel's chamber until the projectile has left the barrel. While the explosion inside the barrel pushes the projectile forward, it pushes the cartridge case backward with approximately the same energy. As mass has inertia, the bolt that's pushed reardward by the cartridge base takes a brief timespan (a few milliseconds) to accelerate backwards.
We look at a pracitcal example: consider a 9x19 barrel with a length of 25.4cm (10 inch) and a 9x19 cartridge in the chamber. Just behind the cartridge, there's a steel bolt with a mass of 600g. This steel bolt is the breech. It is not locked and only pushed towards the barrel chamber by a spring.
When the cartridge is fired, the projectile moves forward and, at the same time, the cartridge case moves and accelerates the bolt backwards. As the bolt has a mass of 600g and is initially at rest, it takes a few milliseconds to overcome the inertia of the bolt's mass. Also, due to its mass, the same energy that moves the comparably light project forward at high speed will move the heavy bolt backwards only at low speed.
When the 9x19 projectile has traveled a distance of 25.4cm and leaves the front of the barrel, the bolt and cartrdige in front of it will have been pushed backwards about 3.5mm. If we look at the cross section of a 9x19 cartridge case, we'll see that the unsupported part of the cartridge case is still within the chamber of the barrel. This is important because the thin cartridge case alone cannot support the high pressure that's still within the barrel and it would rupture, exposing the user to potentially lethal high velocity metal fragments.
In unlocked or direct blockback designs, the bolt and cartridge case will always move backwards while the cartridge case, the high pressure combustion gasses and projectile are still within the barrel. The goal is simply to slow things down at the operator side of the barrel, so that cartridge case is sufficiently supported at the sides (by the barrel's chamber) and at the back (by the bolt face) until the projectile leaves the barrel and the high pressure inside the barrel vents through the now-open forward side of the barrel.
At the same time, the bolt must move backwards fast enough so that it can cycle a new cartridge. The bolt must move behind the magazine to catch up a new cartridge and the cartridge must move backwards enough to get hit by the ejector at sufficient velocity to throw it out of the receiver.
Bolt Weights
This list contains the measured weights of actual production-gun bolts that are proven safe.
The weights refer to the complete bolt assembly without the charging handle and recoil spring.
| Caliber | Gun | Bolt Weight (gram) | Action Type |
|---|---|---|---|
| 9x19 | Sten MK2 | 630 | Open Bolt |
| .45 ACP | M3 "Grease Gun" | 880 | Open Bolt |
| 9x19 | Uzi | 660 | Open Bolt |
| 9x19 | F1 | 700 | Open Bolt |
| 9x19 | Owen | 670 | Open Bolt |
| 9x19 | MP40 | 650 | Open Bolt |
| .22LR | Voere 2005 | 210 | Open Bolt |
| .22LR | Gevarm A6 | 205 | Open Bolt |
| 9x19 | Sterling | 610 | Open Bolt |
Advise
The best advise for designing a safe and secure bolt is to look what professional companies have designed.
Many of the formulas that circulate on the web are not quite correct.
It is also a common flaw in DIY guns to have, sometimes vastly, underweight bolts. A bolt too light is not only dangerous because it might open the chamber when there's still high pressure inside, but it also increases wear because the bolt moves back too fast.
For 9x19, a common bolt blowback mass is around 650-700g. This includes the bolt handle and attachments to the bolt that are part of the moving mass, i.e. the firing pin and the extractor.
For .22LR, a common blowback mass is 200-220g.
Always err on the safe side.
The worst thing that can happen if you choose a heavier weight compared to a lighter one is that your gun won't cycle or eject reliably because the bolt doesn't move far enough backward or not quick enough to throw out the old case or it won't cycle reliably with low-powered ammo like subsonic ammo.
However, if you choose a bolt that's too light for the highest powered commercial ammo of a certain caliber, it is certain that you'll have excessive wear on your components because of a very fast moving bolt, as well as the possibility of a serious detonation out of the chamber.