When I posted How to taper a rifle barrel, I had worked on a barrel that had been previously chambered. Normally, if someone was tapering a barrel, it would be an unturned blank that has not been chambered or threaded. In this post, I’ll be contouring (tapering) a straight blank.
The barrel will be getting a straight taper. The barrel is an unturned (straight 1.250″ diameter) gunsmith blank. A straight taper is the easiest taper to cut on a barrel. Barrels with curved profiles, such as sporter contours, are more challenging. Check out this link to see a few different examples of barrel contour specifications.
This barrel is a Shilen Match grade, 1:10 twist, stainless steel blank, from Brownells.
If your lathe is equipped with a tapering fixture, tapering is a breeze. I have a tapering fixture, but installation is time consuming, so I will not be using it. Instead, I’ll be using the offset tail stock method.
I am an XM3 fan, so, once again, I decided to copy the barrel taper found on the DARPA XM-3 rifle. According to “The Darpa XM-3″ by Steve Reichert, the 18.5 inch long barrel has a modified Hart #7 contour. It has a 2″ straight shank and a muzzle diameter of .850″. From his article, I wasn’t able to determine if the shank is 1.250″ or 1.200″, so I elected to use 1.200″.
Since my blank will have a finished length of 18.5″, to help the machining process, I decided to add 2″ to the chamber end and 1″ to the muzzle, for a total length of 21.5″. I will be removing this excess material once I chamber the barrel for the rifle.
Since the source states a 2″ shank and 18.5″ overall length, I know the taper is 16.5″ long (18.5″-2″=16.5″). At the shank, it is 1.200″ and the muzzle is .850″. This means the barrel tapers .350″ (1.200″-.850″=.350″) over 16.5″. Since the lathe will remove material on both sides of the barrel, I will divide this in half (.350″/2=.175″). To determine the taper per inch, I simply divide this number by the length of the taper (.175″/16.5″= .0106) and get a value of .0106″ per inch of barrel. Since my blank is 21.5″ long, I’ll need to offset the end of the barrel .228″ (21.5x.0106″=.228″).
Finally, I’ll need to calculate the muzzle diameter. Since the XM-3 has a .850″ muzzle diameter at 18.5″, I need to determine its diameter another inch away from the shank. I simply subtract .021″ (.0106×2=.021″), from .850″ for a muzzle diameter of .829″ on this blank.
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All lathe work is conducted on a Grizzly gunsmith’s lathe.
I’m happy with this one. I’ll put up a post when I thread and chamber it.
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