Gentlemen, I just went thru this about a year ago on a 336. This may not be the cause of anyone elses 336, but is was on mine. I converted a pistol grip rifle to a straight grip. I did use new factory parts. To make a long story short, it was popping open when fired because the bolt locking lug was not fully seating in the bolt and receiver lug recesses. To fix it, I had to grind down the contact pad on the top (as it sits in the rifle being held) of the lever that stops travel of the lever when the lever is closed. This fitting pushes the locking lug higher into the locking recess. It is easy to do. File .010" off of the pad and check the fit until you only have .005" to .010" clearance between the lever pad and the low tang. The lever popped open every time I fired it before doing this. After this fitting, it has never popped open when fired. RJ
