Ruger Mk 1 slamfire?

[Ds J]

What causes a Ruger Mk 1 pistol to slamfire?

It has happened to me twice, despite meticulous cleaning and rigorous oiling.

The only thing I did not clean, simply to stay clear from completely stripping the pistol, is the trigger spring plunger.

Could this be the bugger?

[oafpatroll]

Firing pin stuck (or sticky) in fire position perhaps? Weak or missing rebound spring?

[Ds J]

The firing pin is most definitely not stuck.

The rebound spring might be the culprit, or dirt somewhere in the connector.

[223]

My Ruger Standard has never had any such issues in many years and many thousands of rounds, even the old dirty Swartklip ammo.

What I have experienced is some rounds firing with the slide slightly back, about 1mm.  This causes the cases to bulge just ahead of the rim and could result in a ruptured case.  This was due to some gunk in the chamber from the dirty ammo, causing the round to "jam" before the slide goes fully into battery.

In a scenario as above, the slide is stopped by the case, with all the pressure being absorbed by the case rim, which holds the priming compound.  Could this cause a slam fire?  Maybe, maybe not.  I don't know.

[Treeman]

As mentioned, a stuck firing pin releasing late to come back position, then having momentum to fire  or same pins, spring being to soft and continuing forward after slide stops. ( I do not know model - so just using systems I do know as proposals)
As AR mentioned, when the case does not seat all the way it can rupture around rim. This happens when people fire a lot of .22 short in their .22 and then go back to .22 LR, or some of the older ammo I know of used to also cause the problem. The problem was a diamond hard ring of carbon that builds and keeps building around the bullet and case first separation point - the shorts would form a ring some way back and then the longs met resistance when seating causing the case not to seat flush.
Even standard correct ammo builds this ring just after the case ends and it can really tighten the bullets seating resistance.
I bought a pistol as "fkd" once that did just as AR mentioned, blew rims when fired. I just knew I could fix it and did so with a brand new Parker Hale bronze brush or two (they hard - very hard brush's). This pistols barrel ring build up would stop a l- Bent (90 degree bent end) electric fence wire jag  when pulled through barrel.
Ya ! electric fence wire a very good amateur gunsmithing must have in box, almost the duct tape of metal objects. Clears ports and corners delivers a fine drop of oil hard to reach places etc