Apples and Oranges and Rifle Actions.

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A comparative of the relative strengths of front and rear locking rifle actions.

Someone recently asked me about the relative strengths and weaknesses of the conventional front locking rifle action design versus the less common rear locking designs which prompted me to write this article. I have for the most opted to look at modern actions as opposed to the older albeit very well known British, Danish and French designed rear locking actions and Robert Chombart has very kindly provided the data used for the basis of this discussion so as such this is his work and I have simply added or amended some parts.

With modern rifle actions in mind this comparison will be a direct comparison of the widely copied and cloned Remington 700 action versus the Model 41 C.G INCH. Also before people jump up and down and complain the Remington 700 is a mass produced action and it is not fair to compare it to a custom target action I will add the justification that is there are many, many custom actions that are really just accurised/blueprinted Remington 700 clones with very little changes at heart other than material changes and better control of the manufacturing process.

So firstly some direct comparisons between the C.G INCH, the Remington 700 and the forces at work

Receiver and Bolt Shear Resistance.

At the Action lugs:

C.G INCH shear area: 480sq.mm

REMINGTON 700: 423sq.mm

C.G INCH Calculated total shear resistance: 86,5 metric tonnes

Remington 700 (if 42 HRC): 56,5 metric tons

C.G INCH calculated total Lug bearing angle: 165°

Remington 700 measured total lug bearing angle: 132°

At the barrel shank:

The Remington 700 has a barrel tenon diameter of 1.0625”/26,98mm and the action diameter is 1,350”/34,29mm, this gives a cross section at this level: 351sq/mm.

Steel is 42CrMo4. Assuming it is hardened to 42HRC/134kgs/mm2 the total shear resistance at this level is 351x134kgs/mm2 = 47 metric tons.

The INCH has a barrel tenon diameter of 28,575mm/1.125” and the action diameter is 38mm/1.496” The steel is 42NiCrMo16 treated to 50HRC or 180kgs/mm2 so the cross section at this level is 492sq/mm, the shear resistance at this level is: 492×180 = 88,5 metric tons.

At the bolt lugs:

C.G INCH is 42NiCrMo16 at 180kgs/mm2.

REMINGTON 700 is presumably 4340 at 150kgs/mm2.

C.G INCH BOLT Shear area at the lugs is 301mm2

REMINGTON 700 Bolt Shear area at the lugs is 220mm2

C.G INCH Bolt Shear resistance: 301×180 = 54,2 metric tons

REMINGTON 700 Bolt Shear Resistance:220×150 = 33 metric tons

As this is a comparison loosely based on the 2 lug Remington 700 type action versus the 3 lug INCH action let’s look at the general mechanical differences between the front and rear locking in a bit more detail.

Angle of pressure (forces transmission)

In a Front Locking system, the angle formed by the two (or multiple) Lugs bearing surfaces from the centre of the bottom of the case head recess is a very wide angle. On a well known Full-bore action, it is 98° at the root of the Lugs and 112° at the outside diameter of the Lugs, there are probably very slight variations from one model to another however in principle the effect of these differences is negligible.

It can immediately be seen that the forces spread at those angles creates a ‘flexing’ effect as the force is exerted at the centre and the hold on the outside as per the above angles.

In a Rear Locking system, this angle is very shallow. On the INCH it is 12° at the root of the Lugs and 14° at the outside diameter of the Lugs. This closed angle creates a buttress effect allowing forces to be transmitted in an almost straight line with the bore axis of the massive cross sections of the Bolt and Receiver.

Bolt travel

The transfer of the lugs to the rear allows an important reduction in Bolt travel. On Front Locking systems a lengthwise allowance must be made for the Bolt Lugs and Receiver Counter lugs, which the Bolt head has to override to reach the closed position. The cartridge to be chambered has to accomplish the same increased movement.

On the Rear Locking system, the Lugs are situated behind the Loading Port so the cartridge can be fed directly.

On our previously discussed ‘typical’ front locking design Action the Bolt travel is going to be around 107,0mm/4.210” versus the significantly shorter 85,0mm/3.346” on the C.G INCH, this is with a provision for longer cartridges cases.

So what are the advantages of a reduced bolt stroke? The first and obvious one is the reduced disruption to the shooter’s position when a new round is chambered, the reduced stroke also has a significant impact on the Cheek pieces profiles and heights possibilities, as this reduced Bolt travel allows smaller lighter cheek pieces with more adjustment to be used.

Bolt Face and feeding

The absence of Lugs on a rear locker allows a perfectly cylindrical Bolt Head and allows the use of a wider extractor. A Cone breach is an obvious solution which is not usually readily achieved in front locking designs. Another advantage of a rear lug design action is the feeding of rounds due to the absence of Lug raceways, it makes for a smooth feeding of rounds.

Lugs arrangement

The Rear Locking of the C.G INCH allows a specific Lug disposition as when closed, each Bolt Lug is situated geometrically, with one lug at top (12 O-clock when closed) and the two lowers symmetrically in the horizontal plane (4.30 and 7.30 O-clock respectively). This means that each Lug is situated behind a consequent section of the Receiver and also backed by a circular strong section of the Body behind them.

Compare this to a three lug Front Locking designed action where one of the lugs is always immediately ahead of the Loading Port or in the case of a two Lugs design bearing vertically in the axis of two uneven side sections, the support of the R/H one (RH action) will be considerably reduced by the Loading Port cross-section.
Barrel Shank

The INCH reduced Bolt travel allows a significant increase in the barrel shank length at 35,0mm/1.378” however overall length of the action is shorter in its class at 187,0mm/7.368” OAL.

The C.G INCH has two closely toleranced cylindrical registers, one behind the threaded middle part, the other at the front centre the barrel, the middle threaded section only being used for what a thread is intended for, namely screwing and tightening. This arrangement makes barrel changes extremely easy and with absolutely no zero shift.

Also, the inside collar allow a double bearing of the barrel shoulders: against the Receiver front face, and against the inside collar. This means the shoulder bearing surface is at least 30% more than a conventionally fitted barrel.

So endeth the first lesson on the relative differences between a conventional front locker and a rear locking action and I do hope this has cleared up some questions and misconceptions.

Finally a big thank you to Robert Chombart for taking the time and having the patience to explain this and allowing me to share this information.

More to follow.