When it comes to getting your barrel threaded, understanding concentricity is a must. These days there are all kinds of DIY kits to reduce the outside diameter of your barrel to prepare it for threading. This process is typically followed by some sort of thread guide and threading die. To read more about DIY threading see this post.
If you plan to attach anything to your muzzle, having straight AND concentric threads will make all the difference. Simply said, concentricity means the outside of the barrel is matched to the bore. Let’s get into what this means.
When I get a barrel setup in the lathe to thread, over half of the time the bore is centered, but the barrel outside diameter has a “wobble” of sorts…meaning the distance from the center of the bore to the outside surface of the barrel is not the same all the way around. Another way to think about this is to say the center hole (bore) was drilled “off center”. Without getting too technical about barrel manufacturing processes, let’s just say it isn’t that big of a deal…until you start thinking about threads.
When I get a barrel setup in the lathe to thread, over half of the time the bore is centered, but the barrel outside diameter has a “wobble” of sorts…meaning the distance from the center of the bore to the outside surface of the barrel is not the same all the way around. Another way to think about this is to say the center hole (bore) was drilled “off center”. Without getting too technical about barrel manufacturing processes, let’s just say it isn’t that big of a deal…until you start thinking about threads.
The solution? When using a lathe, the FIRST thing you do in the setup is get the barrel indicated off the bore (i.e. center the bore so it spins in the middle) and THEN cut the outside diameter of the barrel to match the bore evenly all the way around. Once the outside diameter is cut to match the bore it is concentric! Nice eh?
The next step after that is to determine the correct outside diameter to match the threads – which come next. See the article on thread pitch here.
Ultimately concentricity means the bore of the barrel and the outside surface of the barrel have been matched. If threading is added, it will also now be matched to the bore.
Does it matter? Well, by now the clear answer should be “yes” it matters…and that’s the scoop on concentricity.