I've been reading about filing away the magazine lips to prevent jam ups. Here is another possible fix. If you have the same problem this is the correct fix and not the magazine lip one.
I had a jam up problem with a Mark ii. Sometimes the bolt stopped about 1/4 inch from the barrrel and other times the bullet slid up over the barrel hole. I compared it to another Mark ii I have which never jammed. After removing the mainspring housing I noticed the bolt in the good gun slid very freely while the jamming gun had a small amount of friction. Its a little bit hard to explain but heres how I fixed it.
Remove the barrel and the grip side covers. Insert the magazine. The top of the magazine needs a small amount of lateral clearance or left to right movement to prevent friction on the bolt as it slides forward. In other words the top portion of the magazine needs some slop as it is wiggled from the right to left sides of the gun. Its not much, maybe only 1/32 of an inch. The good gun had some slop but the jamming gun was very tight. On the each side of the grip assembly is and oval shaped stamped indentation. The magazine slides past these indentations and is held in place by them. On the jamming gun the left side indentation was made a little too deep and held the magazine firmly in place. I filed away the inside of the left indentation until there was a small amount of magazine slop and the bolt started to slide freely. Now the gun fires and reloads perfectly every time. Probably other people have had this same problem and I hope it helps. But dont go filing away on your gun unless your absolutely sure this is your problem.
