why did they cut the threads

[Woodsy]

I refer to my comments in my earlier post about it being a place for crud to collect. The groove through the thread is a by product of the broaching process for the square bolt face recess. The Lithgow L1A1 and the FN FAL both have this groove. The following photos show that bolt face recess is much larger than needed, and the side of the bolt is cut away. The purpose of all this clearance is to provide a 'crud receptacle' for operators who don't clean their rifles. The last photo also shows the crack from the locking shoulder hole, through the upper ejector block pin hole and into the body pivot recess. This is what happens after about 100,000 rounds (and 5 barrels) fired at deer from helicopters by 'rough as guts' meat shooters. The rifles were seldom cleaned properly and the gas relief sleeve was closed down to keep them running! In the pair photo the L1A1 action is on the right.

Mr E, if you wished to make the bolt face recess slightly narrower to aviod the groove in the threads, it would make no difference to the operation of the rifle in civilian terms. The groove is from a manufacturing process. It is a not a designed 'weak point'. I have seen a lot of horribly abused and damged FAL's and none have given way in the barrel ring, despite the best efforts of some rough b**stards!