Open Bolt SMG Problem

T

TheConstructionist

Inactive
Hey everyone,

I just joined this forum, looks like a great place to learn from great people.

Anyway my question is concerning a custom open bolt system- If the firing pin is fixed in these system how does the pin not interfere with the shell loading up tightly against the face of the bolt?

The system im experimenting with is using .22 cal ammo and the pin is located at the top portion of the bolt. As the bolt slides forward the shell is picked up from the clip but stops from being properly centered in the bolt as the rim of the shell hits the protruding firing pin. The pin only protrudes about 1mm.

Thanks for the help and suggestions!
 
True, the more i read into it the more i realize its not really a good design. I was looking at my savage .22 and was just wondering how would a shell not hit the pin if it was fixed? And wouldn't the action of the bolt sliding forward hitting the back of the shell possibly set it off before it was chambered?

Open bolt unsafe= illegal, i get it now, but still curious though...
 
As above, if you're in the USA, an open bolt firearm is 100% illegal.

If you're elsewhere: Usually in open bolt .22 systems the firing pin isn't fixed for this reason. Most systems have one sort or another method of using a non-fixed firing pin.
These may be a delayed striker system, or a hammer fired system.
In either case, the firing pin is retracted until the bolt closes, then the striker is released or the hammer drops to strike the firing pin and fire the round.

The fixed firing pin only functions well in center-fire cartridges.
A famous example is the M1-A1 Thompson gun in which the firing pin is nothing more than a projection machined in the bolt face.
 
For purely technical interest, one open-bolt 22 that I have seen work very well is on the Gevarm A and E series of rifles; these use a wedge-shaped firing pin that runs the length of the face of the bolt, from top to bottom, so there isn't anything that the casehead needs to skip over during the feeding cycle. I've seen a number of these rifles converted to FA, and they cycle at around 1400 rpm very smoothly.
 
On that note, I've seen and fired an open bolt .22lr that had an extractor on either side of the bolt, fed from the bottom from 10/22 magazines, and had an ejector on top of the bolt (case left through an angled port down/right) and the FP was fixed on the bottom of the bolt.

The cartridge would be pushed mostly by the rim via the firing pin, to the feed ramp, and would be ignited just before the bolt pushed the cartridge the rest of the way into the chamber. Pretty good idea, overall. I never saw it fire out of battery, nor did it have any issues with extraction/ejection.

I wish that I had been buying guns back before 1986, the design he made on a F1 sure was neat. :)
 
I have seen one design on a .22rf that used a shorter firing pin and used a lever with center piviot that only drove the firing pin forward when the bolt fully closed.
 
That is the way the Thompsons worked up to the M1A1. (In fact, the "A1" was the change to the fixed firing pin.) The others had a triangular shaped rocker (called the hammer), which pivoted in the middle. When the lower end hit the frame, the upper end forced the firing pin forward.

Jim
 
The answer to the OP's question is that there is generally a lip on the lower part of the bolt on fixed firing-pin bolts. So, when the bolt is fully retracted and proceeds to move forward, the lip on the lower part of the bolt is the part that makes contact with the cartridge, pushing it out of the magazine and into the chamber. Just as the bolt is almost closed, and the cartridge is 98% in the chamber, the lip of the bolt slides under the cartridge and the fixed firing pin and bolt-face slam into the rear of the cartridge.

This is how it works on Mac 10's and UZI's. I'm assuming that most other open bolt systems use a similar system. So, unlike closed bolts, where the bolt face is simply flat with a hole for the firing pin to go through, the closed bolt face with protruding fixed firing pin is somewhat recessed behind the "lip" of the bolt.

This is also the reason why unknowledgable people mistakenly think that semi-auto closed bolt systems can easily be converted to full-auto. Just jerry-rigging the sear won't make if fire full-auto reliably for several design related reasons. But, ATF can make this happen with a jerry-rigged sear which is enough to get someone doing an illegal fudged-up conversion lots of jail time and fines.
 
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