Re: barrel threading
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Wannashootit</div><div class="ubbcode-body">Is there anything specialized about threading a barrel that would require a gunsmith, vs. a (quality!) machine shop?
Just askin, it doesn't appear to be anything that a skilled machinist couldn't do as well as a gunsmith? Is there something specialized about a gunsmith's lathe as far as it's ability to hold a barrel that a machine shop wouldn't have? </div></div>
<span style="font-weight: bold">YES! </span>
Gun barrel wall thickness varies. Sometimes it is so bad you can see the thicker wall at the muzzle, other times it might be within just .010 inch. Factory barrels are worse than custom barrels. Here is why you want a gunsmith to do the threading.
A machinist will just chuck up the barrel in a lathe. Most people assume that the outer circumference is concentric with the bore. So the machinist puts the barrel in a 3-jaw, self centering chuck. The machinist may not have the spider at the back of the head to make the barrel muzzle is concentric with the throat. Then the machinist will cut the threads based on the outer circumference of the barrel, not realizing the bore is turning non-concentric. Sure the muzzle brake will screw on fine, and even look right, but if the outer part of barrel, which was used as a base to get the barrel concentric was off .010" + then the bullet may strike the passageway inside of the muzzle brake.
A gunsmith, while using a common lathe, knows that he must set up the barrel with the bore (muzzle exit) concentric to center of the chuck. (The hole in the barrel, not the outer body of the barrel). The gunsmith will set up the barrel to turn 'between centers' or in other words a conical shaped center will be inserted into the bore at the muzzle end, with the chambered end lined up at the same center line with another 'conical center' with a lathe dog to drive the turning of the barrel. The threads are cut concentric to the bore, with the bore concentric all of the way to the chamber using a spider set up.
Then the barrel is inserted into a self centering chuck using the outerdimension (O.D.) of the barrel to properly contour the muzzle brake. Then the <span style="font-style: italic">over</span>sized circumferenced brake is installed, and outer dimensions trimmed down to meet the contour of the barrel. This is also why most pre-packaged brakes are larger in diameter than your barrels muzzle.
One additional thing. If the brake is a type that must be 'timed' (Ports must exit in certain directions) then the 'Smith will know that and trim the brake, or barrel to get the brake timed just right. The 'smith will also know how to keep the internal brake shoulder -to- the external barrel shoulder gap to a minumum.
And last, if the barrel has to have a new crown cut, a 'smith will have the knowledge on how to do that, where a machinist will not.
I know Brownells sells dies to thread barrels but it is too easy to mess up the end of the barrel, or tap it crooked. I urge all barrel threading to be cut on a lathe.
I know a garage gunsmith who spent many hours designing and then machining 'the perfect' muzzle brake. He made all of the mistakes a machinist could make as listed above. He wanted to show me his work and fired the first shot into a pond. The bullet went first, then dug into the brakes ports and tore it from then end of the barrel. The brake flew into the pond never to be found. When I asked him how he did the threading, he said he used the outer dimension of the barrel to cut the threads, and not the bore.
Needless to say he destroyed the threads at the end of the barrel, but nothing else was damaged (aside from his pride). I had to cut a little over an inch off of the barrel (luckily he had that to spare), re-thread the barrel and install the comp properly.
I hope this explained the process well enough.
A pic would be more effective in the explanation.