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Installing a Flash Hider On a Pre-Ban Barrel or How to Thread the Muzzle of a Firearm

By Eric Kiesler

Threading a barrel for a compensator or in this case a flash hider is a relatively simple operation that can be accomplished by any competent machinist. For this example I will be threading an AR-15 carbine barrel and installing a standard A2 flash hider.


Now that the draconian “assault weapons ban” has expired I am once again free to do this with out fear of legal repercussions/incarceration. I will be modifying the barrel in a 4 jaw chuck in a lathe, so first the barrel must be disassembled in order to fit into the lathe, our Front Sight Bench Block (#080-000-252) can be helpful.

The thread spec for the AR-15 and flash hider is ½” x 28; a one half inch outside diameter with twenty-eight threads per inch. If the barrels outside diameter were already the correct ½” diameter it could potentially be threaded “by hand” with a Thread Alignment Tool (#246-000-031) and a Die (#080-598-529) and Die Stock (#325-100-185). It could be accomplished but it would be preferable to use a lathe. In this case the barrel diameter is .750” at the muzzle so it must be turned down in the lathe. The lathe is preferable as it allows the barrel to be threaded almost perfectly concentric with the bore; also the alignment tool pilots off of the bore and this could damage the bore if done heavy handedly.

I decided to modify the barrel by holding it in the 4 jaw chuck. I could have held the barrel between centers, which would have saved me the trouble of disassembling it. Rather than have the center bearing against the barrel extension and having to sweat a possible misalignment, tolerance stacking, and crown damage I went with the 4 jaw chuck set up. Using the 4 jaw will make checking for correct thread fit much easier as the barrel will not have to be removed from the lathe to check. I suppose I could have removed the barrel extension or fabricated a chamber mandrel, but that would add considerably to the cost of the job.

I placed the barrel in the 4 jaw chuck with some paper shims between the chuck jaws and the barrel to protect the finish. I centered the barrel with in about .015” by eye then I found a gage pin that was a perfect slip fit into the bore, in this case .2185”. With the gage pin protruding somewhat from the bore I used a dial indicator to get within .0005”.

If you do not have gage pins you could use the Barrel Centering Bar (#391-001-000) or you may be able to get away with simply indicating the outside diameter. In my case the bore was out .0005” from the out side diameter, insignificant as far as the flash hider installation is concerned.

Once I was satisfied that the barrel is spinning true to the bore I measured the flash hider to determine the length the threads will need to be, in this case I will thread .625” of the muzzle. I coated the OD of the barrel with Dykem Blue (#262-100-004) and scribed a line .600” behind the muzzle.

I will rough turn the muzzle to a diameter of .5” back to the scribed line. Staying .025” away from the finished dimension allows me to quickly turn down the diameter. My average cut removes about .040” off of the diameter and the barrel stays relatively cool. If your chips are purple or even straw colored you may be taking too much of a cut. Any time you machine steel you impart stress, by keeping the heat down we reduce the stress. Blue colored chips are fine if we are turning plumber’s pipe but not desirable on a gun barrel. When turning up to a square shoulder I like to set my compound at 90 degrees, this allows me to face the shoulder back in a controlled manor. Once I’ve turned the OD to .5” I measure the length of the turned section, then I simply remove the necessary amount from the shoulder by dialing it into the compound and facing it off. I typically face from the inside corner of the shoulder outwards, this pulls any burr out rather than push it into the corner. Speaking of corner you do not want the lathe bit you are using to have a dead sharp point, I give the tip a slight radius (1/32”) so as to reduce the stress riser. Some Gunsmiths like to turn a relief where the threads begin because it is impossible to turn threads all the way up to the shoulder, I do not like turning a relief. While a relief may make the job of threading up to a shoulder seem easier, it reduces the barrel diameter unnecessarily and potentially adds more stress risers. In this case the flash hider has a counterbore already machined into it so it should not be necessary to relive the barrel.

Once the shank length and diameter have been correctly turned I set the lathe up to thread 28 LPI. I grind my own 60 degree threading bits from a blank; we also carry pre ground Lathe Bits (#080-658-225) if you prefer not to grind your own. I coated the barrel with Dykem again. I touched off with my tool bit and zeroed the cross feed dial. At this point with my dials zeroed I took a threading pass across the turned shank, the tool bit just scratched the Dykem. I than check the LPI with my Thread Pitch Gauge (#493-480-028) before proceeding.

If everything looks good I continue threading as usual, this is the time to find out if you’re set up is incorrect. I like to use Do Drill (#083-007-016) on the shank when threading. It is a good thick cutting oil, I keep it in a Squeeze Bottle (#084-037-002). I turn the lathe on and apply a little to the area to be threaded; it is so thick that while the lathe is spinning the surface tension of the oil keeps it on the part.

 

Between each cut I brush the threads off with a tooth brush, check, than reapply oil. Near my lathe I have a chart that gives estimated tool bit depth settings for the lathe compound when cutting 60 degree threads; the chart pertains to relatively coarse threads. It stated that 24 threads per inch typically require .0308” with the compound set at 29 degrees. In my case I had to dial in .026” on the compound before the 28 TPI flash hider screwed on.

This is an AR-15 barrel so I will ultimately use peel washers to “time” the Flash Hider (#078-015-005). The flash hider is closed at the bottom and we want the ports to be at the top when we finally tighten it onto the muzzle. Stacking peel washers between the shoulder of the barrel and the flash hider will alter where the ports end up when the final torque is applied. If this were a sporting rifle barrel rather than use peel washers I would face material off of the shoulder of the barrel just behind the muzzle threads to get the flash hider to time up. With a thread of ½” x 28 I know that removing .035” from the shoulder would allow the flash hider to screw on 1 more full turn .017” would get me about ½ a turn. This figure is determined by dividing 1 (turn) by 28 (threads), 1 divided by 28 = .0357”.  These figures would allow one to estimate the amount of material that needs to be removed to rotate (screw) the flash hider or muzzle break into its correct orientation. The torque spec for installing a flash on an AR using the Armorers tool is 15-20 ft-lbs; I typically use the German specification “goudhntite”.

60 degree center gauge: (#345-778-060)
Threading bit: (#080-658-225)
Centering bar: (#080-000-258)
Crown savers: (#080-000-379)

If you had a pre threaded bbl and a flash hider that only screwed on part way you might run a tap into the flash hider. Along those lines we occasionally we get a question regarding the H designation on the taps we sell.  The “H Limit” pertains to the thread fit; it designates the applicable maximum and minimum size the threads may be as established by ANSI (American National Standards Institute). Typically a tap will have a minimum major diameter that is larger than the basic major diameter specified. If you are not well versed in thread nomenclature it can be rather confusing.  Basically the larger the number after the ‘H’ the bigger the tap is. When installing a flash hider the H designation of the tap used to fabricate the flash hider above was not known. Theoretically the blue print for the flash hider would specify the fit of the threads and the appropriate sized tap could be selected for production, the tapped hole would than be checked with a go and no go gage. The higher the number after the ‘H’ (i.e. H1 to H2) the larger the tap will be. Generally Gunsmiths do not concern themselves with the ‘H’ designations when fitting threaded parts. In the above case the threads were cut on the lathe and I did my best to have nice tight threads, in my estimation I have 75% or better thread fit. As I was threading the barrel the closer I got to the final dimension the smaller the cut I took, ultimately I took .0005” at a time until the flash hider screwed on. If you want to get real crazy you could lap the parts together for the best fit, this is usually unnecessary and for a Gunsmith does not make economic sense.