Dual Firing Pin Mainspring Concept

 Common Compression Springs

Compression springs, sometimes referred to as coil springs, are the typical energy source for functioning firearms and are the typical energy source to drive firing pins forward to achieve ignition.  Firearms are essentially completely static until this energy source is released initiating the “chain reaction” we know as discharging a firearm.  Interior ballistic theory suggests the repeatability and “taming” of this energy and subsequent movements and impacts of internal components will be conducive to the inherent accuracy potential of a firearm, particularly in the case of precision rifles.

As a compression spring is deflected linearly (compressed) it stores linear energy needed for its purposes.  However, there is also radial deflection occurring during linear deflection, creating the storage of radial energy.  This is especially noticeably with heavier force springs like the bolt action firing pin mainspring.  There is a lot of linear deflection upon the installation of the mainspring and the inside diameter actually increases in size clearly exhibiting radial deflection.  The stored radial energy released during decompression of the mainspring, (which could be thought of as the spring unwinding), creates torqueing, considered yet another undesirable phenomenon occurring just at the critical moment of cartridge ignition.

Bellville Washers

In this regard, Bellville washers, also known as disk springs, were thought to have an advantage over the more common compression spring.  Although Bellville washers are often used in static applications, (like lock-washers), they are also stacked in a series opposite each other and used in dynamic applications, compressing and decompressing just like a compression spring.  The advantage is there is no radial deflection, providing a completely pure source of linear energy.  The disadvantages of Bellville washers used in a bolt action application are twofold:  First, there is a great deal of force needed to achieve at least the minimum amount of compression required to effect proper firing pin travel.  Second, energy rapidly decreases during decompression, reducing to a dramatically low level at the point of impact to the cartridge’s primer.  By comparison, the energy of a compression spring is at a much more consistent level throughout firing pin movement.  To compensate, additional compression of the Bellville washers may be used as well as minimizing the amount of linear movement during decompression.  This creates a heavy bolt lift during cocking the action while the short free fall of the firing pin coupled with the rapid loss of energy during free fall still creates the potential of light hits, even to the point of misfires.

Solution:  Negating Radial Energy

CST pitched the Dual Firing Pin Mainspring concept to David Tubb at Camp Perry several years ago.  The idea was to address the radial energy issue while capitalizing on the efficient delivery of energy provided with every day compression springs.  The solution is the use of two springs of the exact same dimensions; overall length, wire diameter, and pitch; and used together to provide the same movement and energy as a single spring, (essentially a single mainspring cut in two equal halves).  The key difference of the two springs is they are wound in opposite directions.  These two springs are then stacked together with a floating bushing separating them. As the spring column is compressed, the radial energy is dispersed opposite directions by each spring, actually twisting the floating bushing during linear movement.  Thus, all torquing occurring during compression and decompression of the spring column is absorbed with the floating bushing and not transferred to the action during the critical moment of firing pin impact. 

Click on the left picture below and observe the radial twisting being absorbed by the floating bushing.  (Watch the black mark on the bushing).

CST is proud that the Dual Firing Pin Mainspring concept was picked up by David Tubb and is now available by Superior Shooting Systems as their "DUO Firing Pin Spring Sets" and are standard equipment in the new  CST Tubb Gun Receiver Assembly.  As an added bonus, the spring sets are of quality chrome silicon material, providing less than 2% loss of energy over 500,000 cycles, vastly outperforming the conventional music wire spring.

Special thanks is extended to Superior Shooting Systems, Inc. at www.davidtubb.com for providing quality Chrome Silicon DUO Firing Pin Spring Sets, not only providing a practical solution to radial energy considerations, but a truly superior quality spring as a welcome bonus.