In firearms, action threadsrefer to the screw threads used to attach a barrel to a receiver, most commonly used in rifles and revolvers, but also on some pistols and shotguns. The threads on the receiver are often called receiver action threads and are usually internal threads, with the matching threads on the barrel being called barrel action threads and are usually external threads. This method of fixing a barrel to a receiver has been used extensively by firearms manufacturers since before the 20th century, and can be viewed as a traditional barrel mounting method. Action threads are not the only method of fixing a barrel to a receiver. Furthermore, some firearm designs have moving instead of fixed barrels. Action threads must not be confused with muzzle threads, which are used on the other end of the barrel for mounting accessories such as a flash hider, suppressor or muzzle break.
Designation
There are many systems for designating thread types. Threading can be specified by diameter, pitch, angle, length and fit tolerances. However, the use of action threads is not well standardized within the firearms industry, and threading can vary between manufacturers and models. For example, factory and aftermarket receivers using the Remington 700 footprint are produced with various types of action threads, all with a diameter, but with a pitch of either a 1.588 mm, 1.411 mm or 1.270 mm. Typically, many rifles use thread diameters in the range between. Many older rifles from the first half of the 20th century use a thread pitch around 2 mm, while many modern rifle use thread pitches around 1.5 mm. Fine threaded systems intended for hand tightening typically use thread pitches around 1 mm.
Mounting
Using action threads to mount a barrel to a receiver typically requires fitting by a competent gunsmith, and typically some machining has to be done. In this process it is important so set the correctheadspace. Correct mounting is important both for safety and accuracy. If the barrel, receiver and bolt are not fitted properly, severe and potential fatal problems can arise due to faulty headspace, e.g. cartridge overpressure and case rupture. Threaded barrels are often mounted to the receiver with a lot of torque, and will therefore generally require tools for assembly and disassembly, such as a suitable action wrench and a vise. Depending on the firearm, a recoil lug is sometimes fitted between the barrel and stock as part of the process, Cleaning up the receiver and barrel action threads is often done during "blueprinting" in order to increase accuracy.
Alternative methods
Several alternative mounting methods to using action threads exist, for example pressing the barrel into the receiver or using a barrel nut. Quick barrel change systems is an increasingly popular alternative, as seen in for example SIG Sauer 200 STR, Roessler Titan or Blaser R8. These methods typically only require simple hand tools or no tools at all. This can be an great advantage to competition shooters who regularly wear out barrels, or for hunters who want a modular rifle that can shoot several calibers. In these designs, the bolt locks directly into the barrel, and the manufacturer often guarantees that the barrel is headspaced correctly from the factory.
