Understanding the Bolt Cycle

Explanation of how the Nucleus works and why 

To enhance the consumer’s understanding of a bolt action, I would like to explain exactly how the Nucleus works and why it works as it does.


The Nucleus is a three-lug bolt action and as such, a constraint is imposed on the amount of available bolt rotation. I designed the Nucleus to have 72 degrees of bolt rotation. In so doing, I stuck a balance between the rotation allocated to various functions that the action must perform. I think the balance struck is quite good because many Nucleus customers who are very experienced with bolt action rifles have demonstrated extremely high levels of proficiency with Nucleus based rifles. They have also demonstrated the accuracy potential of Nucleus based rifles by regularly shooting groups that closely approach the limit of what is possible with their ammunition.


The bolt cycle begins by lifting the bolt handle. In so doing, the bolt shroud, the cocking piece, and the bolt body rotate freely until the shroud contacts the receiver and the cocking piece is forced into contact with both the cocking cams of the bolt body and a slot cut into the bottom of the bolt shroud through which the cocking piece protrudes. This initial and small amount of rotation eliminates the clearance between the bolt shroud, cocking piece, and bolt body so that the reward camming of the coking piece-striker assembly can begin. It is at this point that the shooter will feel significant resistance to lifting the bolt handle because she must now do the work that will be stored as potential energy by the striker spring. While doing this work, there are at many pairs of surfaces within the action contacting one another under load. Hence, reducing friction between parts, even by small amounts, using surface treatments such as nitriding, or properly done DLC, or just good old-fashioned grease can significantly increase efficiency and decrease the effort required to lift the bolt handle. However, the largest contributor to effects resulting in heavy bolt lift is the pitch of the cocking cams machined into the bolt.


The pitch is defined as the distance the striker assembly will move axially (along the length of the bolt) for some bolt rotation. Mathematically, that amounts to the linear motion of the striker divided by the angular motion bolt and may have units of your choosing such as millimeters/radian or inches/degree. It is important to note, that as the pitch increases, the force required to lift the bolt handle will increase non-linearly. For example, the change in the force required to lift the bolt handle if the pitch is increased from, let’s say 2.0 to 2.5 inches/revolution, will be greater than it otherwise would have been if the pitch is increased from 1.0 to 1.5 degrees. A change of 0.5 in/rev has a greater detrimental effect at high pitch values that it does at low pitch values. Therefore, cocking cam pitch is something we pay close attention to.


The effects of increasing the cocking cam pitch are not limited to the interface between the cocking piece and the cams. Increasing the pitch increases the contact forces between numerous pairs of mating surfaces that slide against one another when the bolt handle is lifted. This increase in contact forces correspondingly increases the friction forces that work to impede the rotation of the bolt within the receiver.


As the bolt handle is lifted, the nose of the cocking piece will climb the cocking cam of the bolt until it comes to bear against the flat aft surface of the bolt. While this happens, the root of the bolt handle encounters the extraction cam which is machined into the receiver. The extraction cam serves to drive the entire bolt assembly rearward in order to extract the case from the chamber. The amount of bolt rotation allocated to the extraction cam and the pitch of the extraction cam is determined by the type of extractor used and its functional characteristics, the types of cartridges used, the body taper of said cartridges, and the estimated likelihood that reloaders will exceed maximum recommended pressures. The design of the extraction cam will drive the design of the forward closing cams machined into the forward end of the receiver. During the first 15-18 degrees of closing bolt rotation, the bolt locking lugs will engage the closing cams to drive a slightly oversized cartridge into the chamber. If the cartridge can freely enter the chamber, then the closing cams offer the clearance necessary to rotate the bolt closed as it spirals forward at a rate determined by the pitch of extraction cam at the aft end of the receiver.


Did you get all that? Good.


Now, with all that in mind, consider the following. The Nucleus was designed as a field action, and as such, it was designed under the assumption that the shooter will operate the bolt quickly. Afterall, I designed this action for precision rifle match competitors participating in timed events. Therefore, while designing the camming characteristics of the Nucleus, I chose to take advantage of the forward momentum of both the bolt and the shooters hand. That is why you feel a bump immediately after beginning to rotate the bolt closed if you choose to rotate the bolt slowly. I chose a design having the bump simply because doing so makes it possible to reduce the pitch of the cocking cam which decreases the effort required to lift the bolt handle. That bump results from the noses of the cocking piece falling partially back into the cocking cams machined into the back of the bolt before the locking lugs gain purchase within the receiver. Once there, the entire bolt assembly is essentially trapped between the cocking cams and the forward closing cams. If the bolt is operated slowly, this bump characteristic is more distinct and perceptible than it otherwise is if the bolt is cycled as I designed it to be, that is rapidly, or at least with alacrity.


That said, there are other ways to reduce the cocking cam pitch but those also come at a price. For example, I could have designed the Nucleus with a fat bolt, that is one having a main body diameter equal to the outside diameter of its locking lugs. Such a bolt would offer more space for a cocking cam but at the expense of compromised feeding. Optimal feeding characteristics result from a bolt body diameter equal to the head diameter of the cartridge being fed. Fat bolt designs also make controlled-round feeding much less effective if not completely ineffective, again because of the large disparity between bolt body diameter and case head diameter.


Remember, within 72 degrees, we must find the room for lost motion due to clearance between parts, effective primary extraction which also drives the closing cam geometries, and enough contact between the bolt lugs and the receiver to render the action safe when firing. The cocking cam doesn’t have to remain within the bounds of rotation reserved for bolt purchase which would make bump-free closure possible. Thus, I deemed it more advantageous to reduce bolt lift by elongating the cocking cam because of available bolt and hand momentum. Moreover, the tactile characteristics of the bolt cycle is discussed at length with experienced shooters during development. Once the cycle is understood, shooters tend to be receptive of ideas that may be new to them.


While cycling the bolt rapidly, I, and many others, do not notice the bump. If anyone has a good reason for cycling the bolt slowly, please share it, but do so with an explanation of why disturbing the rifle while closing the bolt is worse than disturbing it while opening the bolt. And be mindful of the fact that the magnitude of the forces encountered, and therefore the disturbance experienced while opening and retracting the bolt is much larger than that experienced while closing the bolt, assuming of course that the cartridge fits properly within the chamber.


And again, if you cycle the bolt quickly, as I assumed you would, the forward momentum of your hand and of the bolt does the work of driving through the bump for you, especially when using the heavy rifles commonplace among competitive shooters. While driving the bolt forward, the root of the bolt handle will engage the extraction cam, and the hand and bolt momentum will drive the bolt along the extraction cam initiating the rotation to drive the bolt over the bump to its closed position. Done properly, the bump is essentially imperceptible.


Lastly, just because you can purchase a bolt-timing job for bump free closure or watch a YouTube video sales pitch of it, that does not make bump-free closure correct or even desirable. As Bohem pointed out, literally millions of bolt action rifles have been produced with both cock-on-open and cock-on-close characteristics (Mauser, Springfield, Remington, etc.) and the fastest cycling rifle, the Lee Enfield, is 100% cock-on-close taking full advantage momentum. The guys designing bolt action rifles during the late 19th and early 20th centuries were really smart and virtually all of their work is still relevant today.


Yet another factor that determines the tactile characteristics of the bolt cycle is the interaction between the cocking piece and the sear (or sear bar as commonly referred to) of the fire control (trigger) assembly. I designed the Nucleus to operate with the Remington factory Model 700 fire control assembly. Understandably, aftermarket M700 triggers will seldom drop into an action without a noticeable change to the tactile characteristics of the bolt cycle. I really can’t do anything about this because I don’t make aftermarket M700 triggers. I know that some of you have observed the cocking piece dropping slightly when closing the bolt. Perhaps in the future we can offer cocking pieces that engage the sear a bit further forward.


That said, as a manufacturer of M700 compatible actions, I would encourage my industry colleagues to come together and establish a proper standard for the M700 sear – coking piece interface. Interested parties know how to contact me.


Regarding lubrication and light strikes (failure to fire), it is generally a good idea to lubricate all sliding surfaces within the action EXCEPT for the shaft of the striker (firing pin) that passes through the bolt shroud. Applying a viscous (thick) grease to this area will likely result in light strikes. Significant energy is required to shear a viscous fluid within a small gap at high rates. This energy can only come from the main spring and therefore light strikes can result. After receiving your action, fully disassemble your bolt and clean it with a solvent. Ensure that the striker and the bolt shroud bore through which it passes is dry and oil free. Carefully lubricate all other sliding surfaces with your favorite gun oil or grease. Also note that oils can dissolve grease causing it to flow into the gap between the striker and the bolt shroud.


Lastly, regarding public discourse and etiquette on forums, I would like to state the following. By his own admission, TexasTightwad, the person who initiated this thread lacks experience with bolt action rifles but nevertheless took it upon himself to publicly label functions that he doesn’t fully understand as defects. I find this offensive because, as I hope this reply has demonstrated, I and others put a great deal of thought into American Rifle Company’s products. Had he pointed out something of which we were unaware, my tone would be different. He, and all of us, would be well advised to refrain from such public discourse until a comprehensive understand of the matter in question becomes available. Demonstrating ignorance is fine. One does so by simply asking a question. But it is not OK to be presumptuous and publicly demonstrate a lack of awareness that others may possess knowledge that you do not. Doing so can misinform those trying to learn or may lead knowledgeable people to call your intellect and trustworthiness into question.


TexasTightwad did contact us multiple times, but I can only respond to such inquires as time and interest allows me to. I did speak to him yesterday and encouraged him to send his action to us so that we may inspect it.


As you can imaging, composing this reply took considerable time. That said, I do realize that American Rifle Company leaves much to be desired regarding customer service. Maybe someday, we’ll be able to offer the level of service approaching that of MidwayUSA. But for now, I think that our limited resources are better spent designing great products and the processes for making them.


Ted