Bill S said:
Having the pin stuck in the forward position DID NOT prevent succeeding cartridges from being picked up - so I wouldn't count on it being any different in a 1919. At least with a 1919 you can grab the belt and twist it, preventing it from feeding. But those first few seconds while your mind figures out what just happened still give you enough time for things to go bad.
To give you an explanation; in this case it is due to the differences in bolt faces on each weapon.
The SKS uses a recesses face bolt that completely encircles the rim of the cartridge.
When the bolt begins to strip a cartridge from the magazine the rim of the bolt contacts theis lower lip on the bolt and as it is pushed from the magazine it slides upwards as it travels forward. At the time the cartridge aligns and begins to seat itself in the bolt it is already over 70% within the chamber and has the full momentum of the bolt being driven behind it.
Once the cartridge is chambered and the kneck of the case shoulders up within the chamber the remaining bolt travel drives the stuck pin into the primer and ignites the charge. The only problem here is that the bolt has not fully engaged the locking shoulder and the result is a uncontrolable slam fire. This will ruin an SKS because normally the locking surface on the bolt only has the opportunity to engage a fraction of the shoulder in the reciever and the result is about 2,800psi exerted on a ledge half or less than what it is supposed to engage. I've seen and repaired a few locking shoulders that the owners have ruined from repetive slam firing.
Firing in this manner is very similar to a submachine gun bolt in operation. Most open bolt submachinegun bolts have a fully recessed bolt face with either feed lips or a fully supported ledge that allows the cartridge to slide up and back into the cupped bolt face as the cartridges chambers up and then drives the fixed pin into the primer. It is a totally unlocked cycle who's operation requires a heavy bolt to control rate of fire and dampening.
Anytime you convert a sub gun to semi you need to remove the feed lips or ledge around the bolt face where the cartridges feed to allow the cartridges toslide up the face directly. This prevents slam fires because should the pin stick, the rim of the cartridge will slide up and either jam the gun or force the pin back into it's recess. This is a saftey issue too and not just to appease the legal side of it. Normally semi subgun bolts require a good bit to be turned or hacked out thus reducing the original overall weight. If you have a slam fire in such a weapon you are looking at a significantly increased ROF which further decreases your chances of regaining control and also imposes greater stress on the action of the weapon and that could result in parts flying back at you. Do not skimp on removing this if you build one and if possible try to replicate the original weight you displace from the bolt with steel spacers or lead to ensure a smooth feed. I've always drilled and poured lead plugs or added shims in the backs of the bolts in my past projects that really required the necessary weight and I've never had any of them hiccup or be finicky on ammo.
The 1919 cannot slam fire via a stuck pin because the bolt face is a straight flat surface in which the cartridges are fed and slide down. If a pin were to stick the rim of the cartridge would hit the protruding pin and would not allow the case to chamber and shoulder up. In a "slam fire" I had with one of my 1919's it was due to a collapsed and broken firing pin spring and a galled up sear. The internal spring broke and coiled into itself and the sear rounded off and wouldn't catch it. The 1919 firing pin is large and heavy so it just slid back and forth till I yanked the ammunition. Naturally the bolt will slam closed on an empty chamber and when it does, it will throw your firing pin forward abruptly which will cause it to lodge/stick forward. To some this might look like the reason for the slam fire but it's not, it's more likely due to a broken spring or sear.
What most likely happened in your case was that the spring failed and as the bolt impacted the buffer the pin got slung back, as it moved forward, picked up a round and chambered it, and at this halt the pin got thrown forward again into the primer. It will continue eating like this until the food is all gone..........greedy *******
