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!

[Peter Laidler]

While I don't profess to know exactly what the thread clearance channel is, I wonder if I might be so bold as to slightly disagree with woody about the slot being a clearance channel of sorts. That's because to be a clearance channel for crud, oil brass shavings etc etc you'd HAVE to have an exit point otherwise it'd just fill up with crud in short order and you'd be back to square 1. And STILL no means of clearing it out!. And there is no exit point here as the 'opening' is covered by the barrel when it's screwed on.

When you think of clearance points, think Sterling SMG barrel.breech area where crud can be pushed forwards and under the breech area

[Woodsy]

Peter, I did not mean that the groove itself was designed for crud accumulation, but that the width of the bolt recess, coupled with the scallop on the side of the bolt, was for this purpose. The groove in the threads is because of the width of the broach required to cut the recess.

[Peter Laidler]

Yep. got that woodsy after reading it again........

[ActionYobbo]

best answer I have had so far was from a tool maker (never worked on guns) who said the reason that is cut that ways is because thats what was on the drawings.

[TactAdv]

Might it possibly be a gas escape path for ruptured cartridge?? Is there any communication with the barrel chamber face there?
-TomH

[mrclark303]

best answer I have had so far was from a tool maker (never worked on guns) who said the reason that is cut that ways is because thats what was on the drawings.

Perhaps you're right MR E, after all, as Peter has mentioned before about all design changes from the original Fal drawings needing to be paid for by the Commonwealth.