disco stu wrote:straightshooter wrote:disco stu wrote:What's the comparison between these cap threads and those holding a barrel in the action?
The threaded joint of a barrel into an action is usually quite a close fit and has considerable preload, that is, it is done up so tight as to be considered immovable.
The end result is that the joint is not weakened by being subjected to repeated shocks from firing.
This is also assisted by there being an approximate balance in the substantial mass of the barrel with the substantial mass of the receiver and stock assembly.
Cheers. Yes, good points on the inertia of those parts vs the cap and the pre load, of which the cap really has none. What are typical thread dimensions of barrel threads?
The cap is preloaded against the shoulder of the barrel, the face of the cap is hard against the face of the barrel, and the inside of the rear of the cap is hard against the face of the chamber - that is how it is head-spaced. It's only hand-tight, but it is still in tight contact with the threads. If it were not fully tightened, the action can't be closed due to the pair of safety lugs behind the cap.
Barrel threads vary greatly. Frank DeHaas' book, Bolt Action Rifles has a good selection of barrel thread diagrams for older rifles. For most centrefire bolt-actions you're generally looking at roughly 22mm to 25mm major diameter, about 16mm to19mm of thread (barrels that use a lock nut have a lot longer thread), at ten to sixteen threads per inch. Some barrels are not threaded at all, they fit into the receiver and are held in place by a nut around the receiver, or some other attachment. Some barrels are simply press-fitted into the receiver with no other locking system - the Ruger Americans for example.
A few examples:
Sako L-46 is .855" major diameter, .845" long, 16TPI.
Pattern 14 Enfield is 1.125" major diameter, 1.084" minor diameter, .720" long, 10TPI square thread.
Springfield M1903/A3/A4 is 1.040" major diameter, .990" minor diameter, .734" long, 10TPI square thread.
Arisaka Type 99 7.7mm is 1.050" major diameter, .700" long, 17TPI.
No.1/No.4/No.5 Rifle is .997" major diameter, .687" long, 14TPI.
M98 Mauser is 1.100" major diameter, .625" long, 12TPI.
I don't have numbers for modern firearms, or for any .50BMG rifles, but they might be available online.