Fixed firing pin??

[Aunt Bee]

I have an SKS that had a free floating firing pin that I added a spring to because I heard "horror stories" about in case it stuck out(due to rust, dirt, etc) it would possibly fire"out of battery". My gun question is--- A British Sten Sub MG has a fixed firing pin and fires from an open bolt. It picks up the 9 mm from the mag just before it slams home. Why isn't this dangerous to the shooter also??

 

[Gik-tal]

Most full auto sub-guns do fire that way. The first of the line the Thompson was open bolt and fixed in, this is more reliable, in that less moving parts are needed, a big plus for cranking them out fast. As for saftey, I've shot free float firing pin guns for a long time, the SKS the AK, AR-16/M-16; the M-1 Garand and have yet to ever see one fire out of battery because of the firing pin. This can cause a rare but occasional burp of two rounds firing with one pull of the trigger, usally due to weak primers and or dirty weapon, but usally bad ammo.

 

[Aunt Bee]

Thanks- but those free floating were inside the bolt & functioning properly. What about one that is FIXED outside (past) the bolt head as in the sub MG- it must strike the primer BEFORE seating the ctrdg in the chamber. PS- thanks for the re-ply!

 

[Owen]

1. Low Pressure cartridges.

2. The lag between the impact and the ignition of the cartridge provides enough time for the bolt to get where it needs to be.

3. The thick case walls of the 9mm and .45 at the head end of the cartridge don't require support to hold the pressure.

 

[BBBBill]

...What about one that is FIXED outside (past) the bolt head as in the sub MG- it must strike the primer BEFORE seating the ctrdg in the chamber...

The process is called "advanced primer ignition." Strike might be a bit misleading. The round is moved out of the magazine by the lower shoulder of the bolt face which protrudes from the bolt face as much or more than the firing pin. As the round feeds forward/upward, the rim slides up riding on that lower shoulder and eventually reaches the firing pin, continuing to slide up across the firing pin as it enters the chamber. Just as the round is almost fully chambered, the base of the round is aligned with the bolt face and the primer comes in line with the firing pin. As the bolt continues forward, the extractor snaps over the rim and the firing pin begins to crush the primer. The bolt is only a few thousandths from being fully closed at this point, but it is still runnig home quickly under spring tension and inertia. The primer ignites the powder, the powder begind to burn (possibly expanding the case a bit, sealing the chamber and slowing the bolt). The powder has not had time to fully combust before the bolt fully closes. Some suggest that this sequence actually cushions the blow of the bolt slamming home. As the bolt slams home, the powder completes its burn launching the bullet and beginning the extraction/ejection cycle. All of this is happening in milli-seconds.

 

[Guncollector1982]

Kind of off subject but sorta related i guess. I bought a trapdoor that hadnt been fired in a long time i checked it over and found the firing pin froze forward in it. Could been kinda bad had the wrong person slapped a cartridge in and flopped the door shut. Hosed it with penetrating oil and worked it back and forth and it freed good as new.

 

[Aunt Bee]

Thank you all- espc. BBBBill---- NOW I understand!!

 

[Jim K]

Correct on the API, but to clarify a bit. The early primer ignition actually fires the primer and begins powder ignition BEFORE the bolt fully closes, so the backward movement of the case must fight not just the weight of the bolt and spring, but the forward momentum of the bolt as well. The result is to make the bolt act as if it is a lot heavier than it is. That in turn allows the bolt to be relatively light and the operating spring to be light enough for easy manual cocking.

When API is not present, that is when firing from a closed bolt (as required by BATFE rules), the gun must either have a heavier bolt or have springs much heavier than used in the open bolt models. For example, the standard Thompson M1928 SMG can be easily cocked with one finger, where the Kahr Arms semi-auto requires a hefty effort to do so.

Of course, blowback operation of any kind is only feasible with low pressure cartridges; with higher pressure cartridges or less breechblock weight, a breech lock of some kind is necessary. The same .45 cartridge that can function in a blowback SMG with a heavy bolt normally requires a locked breech in a pistol unless the slide is made very heavy (Hi-Point). Blowback operation is generally not feasible at all with rifle power cartridges. I once read that a .30-'06 blowback would require a 30 pound bolt, a bit heavy for your favorite deer rifle.

Jim