Much baseless criticism is levelled at the Lee Enfield action in US gun forums - and invariably by individuals who have never owned and shot one.  Particularly, some regularly play their duplicitous tricks as compulsive critics of the Lee Enfield and the .303  cartridge.  They will typically try to dismiss all Lees as "weak".  Then, when confronted about these accusations of weak, and when the fact that thousands of Lee Enfield No. 4 actions were re-barrelled for the 7.62x51 / .308W (62,000 psi) are levelled at them they will then retreat to: "Oh well, the number ones - or at least those that weren't made specifically to take a constant diet of 7.62 must be weak then".

 

Fact is, no Lee Enfield action was ever specifically manufactured to withstand 7mm Rem Mag, .300 Win Mag and .308W pressures. Good engineering in the No.4 Mk.1&2 simply happen to be able to do that day in and day out due to the reliability of the action.  The design has proven itself for continual use in the 60,000+ psi regime and has been doing that for the thousands of rounds that long range target shooting demands. I shot myself into the S.A. Air Force Bisley team with a No.4 Mk.1 that at the time had consumed in excess of 18,000 7.62x51 rounds through its Musgrave barrel.  History proves that the No.4 action is at least as strong as any other commercial action on the US market. More about this later.

 

The mind either lacking in learning - or by wilful filtering - refusing to understand the logic of mechanical physics for whatever personal reason typically puts up a defense merely for self-protection it seems.  Once this mindset is encountered there is no logical debate possible as it resorts to arguing and using baseless belittling of either the Lee action or the presenter of the facts. Belittling is the first safe space the compulsive, emotion-driven anti- .303 confrontationalist retreats into.

 

The explanation of  why the Lee action "has to be weak" is invariably based on the rear position of the locking lugs.  "It [bolt thrust] must compress the bolt on firing and therefor it will cause increased head space", is a recent but often repeated attack in a gun forum.  "It must...", they say - so we shall certainly examine that assertive assumption further on in this article.

 

There is no record that can act as base for this silly speculation. The unmatched accuracy at 1,000 yard events of the Lee Enfield and the 174gr Mk.VII bullet in any case belies this theory, and the consistent long life accuracy with the high pressure 7.62x51 cartridge puts a period to this argument.  EVERY action of every design stretches and compresses at EVERY firing.  As long as the pressure is below that which causes plastic deformation then that shock absorbing reaction in the length of supporting metal is exactly what gives the Lee Enfield its strength and durability.

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The Issue of Bolt Thrust

Say we had a sealed pressure vessel the exact shape and size of any cartridge case in hand while a compressor is pressurising it via a line and valve. At 55,000 psi we take the line off. There is equal pressure inside in all directions. The highest force inside is radially onto the side walls due to the large surface area. There  is NO resulstant pressure moment in the chamber either forward or rearwards.

 

If there is no weakness in the metallurgic integrity of the vessel the failure mode when the pressure is increased past the ultimate strength of the metal will be radially onto the large side wall surfaces and not rearwards or forward onto the small surface areas.  In a barrelled action the design weaknesses are the stress concentration radials of the acute angles of the threads in the action and the barrel. I have never heard of any rearwards failure by a bolt - always an up or down failure at the action grooves.

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Should one open the front end of the hand held pressure chamber and the gas escapes (like out of a balloon) that gas pressure acts as a force onto the surrounding air mass. The opposite reaction is thrust which would force the handheld pressure vessel in the opposite direction like it does to a balloon. Inside the barrel the gas expands in a controlled and relatively slow manner into the ever increasing volume behind the departing bullet. The pressure force in all directions decreases as the bullet moves out of the case mouth and past the peak pressure position about 1.5" further.

 

In the rifle the designed weak point in the temporary pressure vessel is the safety pressure plug (the bullet) in front of the cartridge.  The cartridge itself is tightly shaped inside the chamber and pressed radially to the sides due to the flexibility of the relatively thin brass walls. The highest force by far is radially onto the flexible case walls pressing them tightly against the chamber walls. This radial pressure acts like old style drum brakes on a vehicle, preventing or limiting rearward movement of the case.

 

The volume of the pressure chamber is slowly (in relative terms) being increased as the pressure relief plug (bullet) is pushed out and as it moves forward, while chamber pressure forces are slowly decreasing in all directions.  Pressure decreases progressively in ALL directions as the volume behind the moving bullet increases. There is NO resultant vector of pressure forces.  All the time the biggest pressure is radially to the sides, pressing the flexible, elastic brass tightly against the chamber walls.

As the total pressure which is the same in all directions and also on the base of the moving bullet decreases with increasing volume the case walls (and their braking grip) relax into their previous shape.  "Bolt Thrust" is negligible with normal operating pressures.

 

The actual value of that "bolt thrust" force:

At 55,000 psi the radial pressure force on the chamber wall surface by the walls of a .303 case is a whopping 152,000 lb. (55,000 psi x case outside surface area) - the very reason why over-pressure failures are always into the chamber walls in any rifle.  This radial force is countered by the thickness of the metal walls of the rifle's chamber.  This designed thickness is conditional on the quantum of the radial pressure plus the tangential stress vectors it causes, and the metallurgical integrity of the barrel steel.  Typically under European CIP specifications the average wall thickness around the cartridge web is about 2x calibre and tapers off forwards. The barrel profile can in fact be seen as a graph of the highest stressed sections.

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If there had been no braking effect to rearward movement of the case as is caused by this huge clinging radial force the total pressure on the bolt face via the inside rear end surface area of the case will be 6,930 lbs. force - or 30,130 psi.  Due to the braking effect of the radial pressure force on the case walls In practice the rearward force is a great deal less.  Let us consider the effect of these forces exerted onto the Lee Enfield bolt as if the braking effect did not exist, keeping in mind that the less than 1/2" chamber wall thickness is sufficient to contain the full 152,000 lb radial force of the 55,000 psi chamber pressure:

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Compression and Stretching Dynamics

In front locking lugs the rearward "thrust force" is countered by the compression resistance of the full length of the receiver rearwards of the lugs, aided by the compression resistance of the metal from the bolt face to the contact surface of the locking lugs with the receiver.  Furthermore, these are aided by the tensile resistance of the barrelled action forward of the lugs.

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On the Lee action with locking lugs to the rear of the bolt the pressure against the bolt face is countered by compression resistance offered by the full length of the bolt from face to the lugs, aided by tensile resistance of the full length of the receiver forward of the locking lugs (3 inches of locked steel) including the added rigidity from the mated barrel. There exists no popularly available empirical values for these compression and tensile forces and the relative ability for the above two designs to counter these without measurable elastic compression / extension and spring back. Rough calculations indicate that there is no difference in elastic response between the two lock-up designs, and in fact that the longer linear shock absorbing dynamics of the Lee action may render it more resilient to brittle failure than the front lock up design of the Mauser.​