technical and safety Question.

I took a No.4 Mk2 converted to 7,62 x 51 to the range today fo rthe first time since I bought it. On one round the firing pin didn't strike the primer, the cocking piece went to what I assume is a 'half cocked' position and sat there. having never been faced with this before I was a bit non plussed. My solution was to re cock the rifle by pulling the cocking piece back with my fingers. I didn't try ot reopen the bolt, both because I didn't know whether it would, but mostly because I didn't want to take the risk of the pin impactinging the primer as I opened the bolt as I wasn't sure whether that could actually happen give the working of teh bolt. So questions. 1. Can that happen, and is there a safer course of action that the one is used. 2. if this turns out to be a regular occurrence what can cause it (I assume prior owener monkeying around trying to 'improve' the trigger, but what specifically can he have done to make this happen? and how would I resolve this?

You probably didn't quite have the bolt handle down far enough. There is an in-built safety feature- the cam grooves under the back of the bolt body (the same one that cocks the bolt on opening) has a lug between the long and short cam grooves. The cocking piece lug will be stopped by it- it will not allow firing pin protrusion unless the bolt is rotated to lock position. I've done some pics of them before.. If you can't work it out from my rough description I'll find the pics.

Your course of action to manually re-cock the cocking piece is about the only thing you can do. Always with the muzzle safely pointing downrange.

As long as the cocking piece has the lug, and the bolt is set up right to start with, the firing pin cannot strike the primer once you lift the bolt handle.

As Son said this is what happens when the bolt isn't completely closed. It could be that you didn't close it properly, or it could be that it couldn't close properly on the round you were using because it wasn't full length sized or because the headspace has been set a bit tight. If you've got gauges or know someone who has it might be worth checking the headspace. Most likely you just didn't close it properly, but an easy fix if it's not.

I'll back safety catch half engaged.

Whereabouts are you? It may be that the trigger and sear have been bubba'd, and that the rifle needs someone to look at it.

You have just encountered a feature called 'mechanical safety' That't why it's there and how it works. My betting is that you hadn't closed the bolt fully. The remainder, such as trigger pull etc is a bit of a red herring because it was after you'd squeezed the trigger. Nope. bolt not fully closed and mechanical safety comes into operation.