c-g-designs

The MYTH of CASE STRETCHING In Match Single Shot Rear Locking Actions

        Posted on May 14, 2013 by David  (The Shooting Shed Jpurnal).

        Robert Chombart emailed this to me today and  I have decided to publish the article in its entirety as it covers some interesting  points on the differences between modern rear locking rifle actions, specifically the M41 CG INCH action compared to the military rear locking action specifically the SMLE.

‘’The MYTH of CASE STRETCHING In Match Single Shot Rear Locking Actions

        Robert Chombart

Bolt compression resistance:

        I have calculated the compression resistance of the INCH Bolt.

        The cross sections of the Bolt decreases in steps from front to rear. At 11 mm ahead of the Lugs bearing abutment (against the counter-lugs in the Receiver), it is still over a total of 35 tonnes before any compression could occur (Yes, 35,000 kilograms or 77,260 pounds). This represents some 7 times the total pressure against the Bolt face of a Magnum-sized case head, 9 times a .308 and 19 times those of a .223, and still 6 times those of a .338 Lapua Magnum. This is for the total pressure exerted on rear thrust. Converted to the .308 bolt face, with its surface of .175 sq/inch, it represents 395,500 lbs/sq inch of capacity.

        Comparing to military or sporting Rear Locking Actions:  In the case of an excessive pressure exerted axially against one end of circular solid (the bolt body), is heavy enough to create a reduction in length, this solid increase in diameter (or bent if the force is not fully centred, or the solid, itself eventually not symmetric in shape). This phenomenon is called ‘Buckling” or ‘Flambage’ in my mother’s language.

        But if contained with close tolerances within another solid, thus preventing the radial expansion, any eventual reduction in length is impossible to occur because the cylinder cannot expand or flex. This is the case for the INCH  (and other rigid rear-lockers) in which the generously dimensioned bolt is maintained straight at close tolerance in the Receiver, with limited openings and generous cross sections.

        This is different in the military or some sporting repeater rear locking actions, the example being the SMLE, in which the small sectioned Bolt (1/3 those of the INCH)  is not maintained due to the large and long openings for magazine well and loading/ejection port. The Bolt is only slightly maintained (with generous tolerances too) at front and rear… with also a threaded junction in between (non-rotating bolt head screwed in the bolt). There is nothing here to contain the central buckling of the bolt. Also the receiver flexes under stresses on those actions as well because of their asymmetric construction and horizontal locking lug arrangement. The SMLE’s lugs bolting is the only of its kind being horizontal when closed.

        Choice of materials: The choice of material and treatments used in the construction also play an important role in general compression resistance. The material and treatment used for the INCH (as it was for the Millenium) has a Rm of 185 kg/sq.mm @ 52HRC (363,130 Lbs/sq.inch) as compared to the 120 kg/s.mm @ 38 HRC (170,680 Lbs/sq.inch) of a hardened 4140 receiver.

        Conclusion:  One important conclusion which can be drawn from the above is that, the smallest cross section of the Bolt being the only one to consider and being only 11mm (.433’) long, this distance being inferior to those Bolt Recess / Rear of Lugs of a Front Locking action, it is a nonsense to attribute any case stretching tendency to a well designed Match Single Shot Rear Locking Action, notwithstanding the superior metal characteristics.

        To repeat the main important aspect, the Bolt being fully enclosed in the action, the later acting as a super-strong outer sleeve/collar preventing any inflation or flex (buckling) of the Bolt.

 R.G.C