Modifications for precision rifles

[Tomcat088]

I’ve recently spoken with some people who have been interested in tactical rifles and the reasoning behind some of the modifications made to actions, barrels, etc. I wanted to try to answer this question with some depth, but not too much for the people who aren’t necessarily interested in becoming home gunsmiths. I am NOT a gunsmith, but I sit in the shop and work with friends that are, and I have assisted in building my own rifle from the ground up. I’m familiar with all of the processes and at least one way to do them, and the reasoning behind it. I want to point out that some of these techniques don’t have a “set” way to do them, although the principles behind them are usually similar. So… just because you don’t see something done in a specific manner doesn’t mean it’s not correct or “true”. Keep in mind by tight tolerances we’re talking about thousandths of an inch and sometimes even a couple of ten thousandths, depending on the smith, applications, equipment, etc; very small amounts.

I’ll talk in vague terms regarding effects on the action and accuracy because some of these issues are even debated amongst top smiths of the WORLD. I’m going to talk mostly in terms of the Remington 700 action because it’s one of the most customized actions for tactical rifles in the United States; it’s also the one that I’m most familiar with. Some other actions may not need the techniques commonly done to the Remington 700, and others may need different techniques done to them in order to reach favorable results. I hope this brief overview helps you understand some of the terms and particulars.

Always remember that in order to have an accurate rifle, you MUST have consistency. There shouldn’t be any stress or binding in any part of the setup, as well as no movement or wobble of the action in the stock. The process of “blueprinting” that I will describe will help ensure that. Blueprinting is a term that is often used to mean “cleaning up the tolerances of the action”. Because barrels, actions and other parts are often made on an assembly line with the expectation that parts need to mate closely with other parts, there must be looser tolerances than optimum and some “slop” so that these parts always fit together without costly custom fitting of every firearm. This little bit of slop does hurt accuracy on the most minute levels and results in a slightly “loose” fit in terms of precision machining. It’s not something you’d typically make out with your eye or by feel, but you can see it once you have these parts in a lathe and spinning. The goal is to have everything on the receiver square, but not only square, but concentric and “true” to everything else on the receiver. These terms may seem a little vague now, but I’ll be elaborating shortly.

When you remove the barrel from an action/receiver on a Remington 700, there are threads on the interior part of the action and the barrel threads in. The outside edge of the receiver (the face), makes contact with a recoil lug (more on that later) that contacts the barrel. This face needs to be square (perfectly flat), so that the barrel can make even contact with the entire receiver. This is the joint where the separate parts become one, so it needs to be perfect. Even contact ensures that the barrel doesn’t point, wobble and rock to any one side, and it needs to have a large, even area to distribute the forces and harmonics acting on the barrel and action. Barrels and actions “ring” the way that a bell does, but it tends to be on a level that we can’t hear. Consistent and even harmonics make for consistency and accuracy. An even face also means that there is no pressure or stress added to the barrel or action, because pressure is evenly distributed over the whole face of the action.

After the face is squared off, the next usual step is to work the integral lugs of the receiver and the threads. The integral lugs are on the inside of the receiver, and they are the flat area that your bolt lugs (ears that stick out on the bolt toward the chamber) make contact. It should be noted that the bolt lugs also have to be cleaned up so that they are also flat, but we’ll get into that later. Again, the goal of squaring up the integral lugs is that the bolt makes flat even contact with the action. This is basically the “seam” where the bolt and action become one. The lugs have to be parallel and true with each other, so that the bolt makes even contact, this is important for several reasons. If the bolt does not make even contact with the receiver, it will sit “cock eyed” in the action. This means that the bolt is actually not quite in a straight line with the chamber that is reamed in the barrel, and when the brass is pushed back on the bolt face during firing, only 1 lug will bear the brunt of the tremendous pressure. This can effect harmonics, can induce stress, and cause the bolt to “settle and shift” while the bullet is still traveling down the barrel. This will effect where the bullet impacts, and how consistent the rifle is. In factory rifles, there is actually a fair bit of slop on the lugs; it doesn’t usually cause extraction issues, but in some cases it can, and it also can be hard on brass.

The other technique that is commonly done at the same time as squaring up the receiver face and integral lugs is the process of cleaning up the receiver threads. The threads can be thought of as the “glue” that holds the action and the barrel together. They must be clean so that the barrel doesn’t wobble, and so that ALL of the threads make good even contact. This kind of seems like a small deal to most people, but it’s very important. If the barrel wobbles or shifts, it will obviously make the rifle inconsistent and it will change the alignment of everything else in the rifle. You want all the threads to make even contact so that there is not any stress added to the barrel (especially around the chamber area), and so that the harmonics are again even and consistent between the barrel and the action; they must be mated as close to perfect as possible. One thing to keep in mind is that the threads not only have to be clean, but they also have to be concentric and true to the receiver face and the integral lugs; ALL THREE THINGS. If the threads are not square to the receiver face, it could be impossible to tighten the barrel up to the action and square off the face; or if you happened to do it, the threads would be ruined and wobble after that. IF you did manage to do it because the threads weren’t that far off, you could induce a GREAT amount of stress into the barrel around the chamber. This stress can shift and change as the barrel starts to heat up (which it does easily in this area where pressures are very high), and/or if you ever take your barrel off. This is because you will tighten it differently and the stresses will also change in the chamber area. All of this leads to inconsistency and stress; which are what you are trying to eliminate.

The receiver is faced off, integral lugs faced off, and threads cleaned up or redone all without the receiver ever coming out of the lathe. It can take some work to get a receiver to spin perfectly true in the lathe, and once you have it in right, you want everything to be “concentric” to the other parts. You want all of these things to be true and in EXACTLY the same line (which is straight). This keeps everything lined up, so that the bore of the barrel, barrel, action, bolt, firing pin, etc all line up perfectly. This not only makes them become one piece, not just several pieces slapped together; it also helps them have no extra stress added to them and have good consistent harmonics. Here is a link to a prominent tactical rifle smith who talks about his process that he goes through to true up the receiver and the bolt. http://www.louisianaprecisionrifles....eprinting.aspx He is not the only smith or necessarily the “best smith” (there are lots of GREAT smiths, including William); he merely had this up handy in an easy to read manner so I posted it here. Here are also some embedded links so that you can see the process occur and maybe understand it a little better.