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1.  Today breach screws are invariably used with BL ammo (ie no metal cart case) and larger calibres (including naval and coast artillery in the first half of the 20th century)

2.  A century ago they were also used with QF ammo (ie with metal cart case) and smaller calibres

3.  Today some BL ammo is used with sliding block breaches (Germany and UK)

4.  Screw breaches have 'interrupted threads', this means that closing them only requires a fraction of a full circle turn to screw them in the breach block which means they work fast.  Various mechanisms are used to minmise the breach distance the lever for opening and closing  the breach has to be moved.  The Welin interupted thread and Asbury breach opening/closing mechanism has been used with US and UK guns since WW1.

5.  Screw breaches are fitted with a 'lock' (as in 'flintlock' or 'matchlock' of history), this holds a 'primer' (also called a 'tube' by some countries).   Primers are usually percussion but electrical have been used by naval and coast guns.

7.  When fired the primer sends a flame through the vent into the chamber where it ignites the cartridge in the bag of propellant.

Thanx a lot.. but infact I had already read that page.... This is rather basic info regarding Breech Mechanisms. I am looking fora bit more in-depth information about "Welin" and "Cylindrical"  Breech Screws.... Somehow I couldnt find that on net. I hope U coluld help me with that as well.

Still.... thanks anyway... and looking forwarrd to some more from from U

 

Interrupted thread and stepped segments with varying radii.  The design achieves a large bearing surface but needs only a small turning movement to close.