Countouring a Rifle Barrel Blank

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.

unturned bbl blank

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.

The barrel is secured in a four-jaw chuck and a dial indicator is used to dial it in.

The barrel is secured in a four-jaw chuck and a dial indicator is used to dial it in.

Since this barrel was saw cut, I need to square up the ends.  I make a light cut across the face.

Since this barrel was saw cut, I need to square up the ends. I make a light cut across the face.

 

This is a range rod.  The rod holds a sized busing (which are sold in .0002" increments) to fit into the bore.  The rod is hardened and has a slight taper.

This is a range rod. The rod holds a sized busing (which are sold in .0002″ increments) to fit into the bore. The rod is hardened and has a slight taper.

The range rod is inserted into the bore.  Now the bore can be dialed-in.

The range rod is inserted into the bore. Now the bore can be dialed-in.

A 60-degree piloted center drill is used to cut a chamfer that will mate to the centers used on the lathe.  It is secured in a Manson floating reamer holder.

A 60-degree piloted center drill is used to cut a chamfer that will mate to the centers used on the lathe. It is secured in a Manson floating reamer holder.

The end of the barrel after the center drill cut a relief.

The end of the barrel after the center drill cut a relief.

A drive dog is now attached to the barrel and the barrel is mounted between centers.

A drive dog is now attached to the barrel and the barrel is mounted between centers.

This is what the barrel looks like between centers.

This is what the barrel looks like between centers.

The tail stock is now moved on the lathe the distance I calculated earlier.

The tail stock is now moved on the lathe the distance I calculated earlier.

With the tailstock offset it is time to start cutting some metal.  I like using carbide tooling for heavy work like this.  The finish may not be as nice as a high-speed steel tool, but the carbide is far more durable for this application.

With the tailstock offset it is time to start cutting some metal. I like using carbide tooling for heavy work like this. The finish may not be as nice as a high-speed steel tool, but the carbide is far more durable for this application.

This is what the barrel looks like after the first pass.  Notice how the finish changes on the barrel, this is because on the initial pass, the depth of cut is deep on the right side than it is on the left.

This is what the barrel looks like after the first pass. Notice how the finish changes on the barrel, this is because on the initial pass, the depth of cut is deep on the right side than it is on the left.

Since the barrel would chatter when I made cuts towards the center, I decided to use my steady rest to improve the quality of the cut.  I worked the barrel in three different sections.

Since the barrel would chatter when I made cuts towards the center, I decided to use my steady rest to improve the quality of the cut. I worked the barrel in three different sections.

Before I move the tail stock back into position I take time to remove any tooling marks.  I start with a lathe file.   Before filing, I make sure all of the oil is removed from the barrel.  Oil and files do not mix.

Before I move the tail stock back into position I take time to remove any tooling marks. I start with a lathe file. Before filing, I make sure all of the oil is removed from the barrel. Oil and files do not mix.

Abrasive cloth coated in Viper's Venom cutting oil follows the file.  This creates a nice finish.

Abrasive cloth coated in Viper’s Venom cutting oil follows the file. This creates a nice finish.

The tailstock is reset to its original location.  A straight tenon is cut on the muzzle end.  I will use this for two different operations; cutting the barrel shank, and chambering the barrel between centers.

The tailstock is reset to its original location. A straight tenon is cut on the muzzle end. I will use this for two different operations; cutting the barrel shank, and chambering the barrel between centers.

To turn the barrel shank I mount the tenon I cut on the muzzle end in the lathe's chuck.

To turn the barrel shank I mount the tenon I cut on the muzzle end in the lathe’s chuck.

This is what the shank looks like after tapering.  It is 1.250" in diameter, and needs to be turned down.

This is what the shank looks like after tapering. It is 1.250″ in diameter, and needs to be turned down.  I didn’t bother removing any tool marks where the taper ended.  These will all be removed when the shank is cut to diameter.

Turning the shank is a quick operation.  It is far easier than turning the taper.

Turning the shank is a quick operation. It is far easier than turning the taper.

The shank after turning.

The shank after turning.

I used 220 grit abrasive cloth and oil to polish the shank.

I used 220 grit abrasive cloth and oil to polish the shank.

finished blank taperedI’m happy with this one.  I’ll put up a post when I thread and chamber it.

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