An in-spec M14 should NEVER double. Never. The sear and hammer arrangement is designed to bypass the trigger hooks with the hammer and grab the sear before the bolt travels far enough rearward to strip a second cartridge. In other words, if you get a double, you trigger group isn't working properly. Chances are that this sort of timing issue would relate to a stock that is too tall from the trigger group inletting to the receiver shelf - you should be able to correct it.
Slam-fire is another thing. ANY free-floating firing pin can slam-fire under the wrong conditions. Usually this relates to debris in the pin channel (for example primer cup fragments) causing the pin to not properly retract. In an M14, this debris jam would have to occur the moment the bolt rolls into lockup and the pin has cleared the safety bridge. Not a common occurrence, and in fact it would be very very rare IMHO. Even GI guns would be as susceptible, but as I mentioned, it's not something likely to happen.
SF can also happen if you use too-soft primers and you get a one in 1000 event where the pin gets enough forward momentum that the small dimple that M14 floating pins leave on primer cups is enough to touch one off, but this type of SF usually happens in battery, since the pin cannot physically go forward enough to hit the primer until it passes the safety bridge and locks up in battery. This, of course, assumes an in-spec safety bridge. It would manifest as a round going off and cycling the action when you let go of the charging handle. I've only ever heard of this happening when single-loading without a magazine in the gun. Generally a bad idea with the M14.
