Lengthening a shotgun’s forcing cone

Success, the forcing cone is now lengthened.
Lengthening (Extending) a shotgun’s forcing cone

The forcing cone is the tapered area just forward of the chamber that allows the shot an area in which to transition into the bore.  As the shot then travels through the barrel, it passes through the choke at the end of the barrel.

A lengthened forcing cone provides a smoother transition for the shot leaving the chamber and results in less perceived recoil for the shooter.  Since the transition is at a gentler angle, there is also less deformation of the shot pellets.  An outstanding cutaway picture of a lengthened forcing cone can be found here.

For this project we ordered the following tools from Brownells:

  1. Clymer long forcing cone reamer
  2. Forcing cone Flex Hone, medium and fine grit
  3. Flex Hone oil
  4. Do-Drill cutting oil

Before attempting to use a reamer on a barrel, make sure the barrel isn’t chrome lined.  An easy way to tell is to apply some cold blue to the bore and see if it takes.  Additionally, make sure the forcing cone reamer will not remove too much material, leaving the barrel wall dangerously thin.  Consult a gunsmith and have your work checked to make sure it is safe.

a

The extended forcing cone reamer and a tap handle.

You can't use too much cutting oil when turning reamers by hand

We like using Do-Drill cutting oil from Brownells

Using rubber vice jaws we secure the barrel vertically with a bucket beneath the muzzle to catch any excess cutting oil or shavings

Using rubber vise jaws, we secure the barrel vertically with a bucket beneath the muzzle to catch any excess cutting oil or shavings

We use an acid brush to apply a liberal coat of Do Drill

We use an acid brush to apply a liberal coat of Do-Drill to the reamer

 

Carefully insert the reamer into the breach end of the barrel and turn.  Every five turns we removed the reamer, cleaned off the shavings and re applied oil

We carefully insert the reamer into the breach end of the barrel and turn. Every five turns we removed the reamer, cleaned off the shavings and reapplied some oil

Making progress, here you can see the first metal we cut from the barrel

Making progress, here you can see the first metal we cut from the barrel

Almost there

Almost there

Success, the forcing cone is now lengthened.

Success, the forcing cone is now lengthened.

Once the forcing cone is cut, the surfaces need to be honed.  We have had excellent success with flex hones from Brownells.  The flex hones have a series of abrasive balls which create a slurry in a special oil, polishing the interior surfaces of the bore when driven with an electric drill.

Flex hones, (Left to right) Medium grit bore, fine bore, fine chamber, fine forcing cone.

Flex Hones, (Left to right) Medium grit bore, fine bore, fine chamber, fine forcing cone. For this project we will use medium (not shown) and fine forcing cone hones as well as a fine chamber hone.

Using an electric drill, we ran the medium hones first.  Cleaned the slurry from the barrel, then ran the fine hones.

Using an electric drill, we ran the medium hones first. Cleaned the slurry from the barrel, then ran the fine hones.

Once the honing process is complete, we clean the bore and inspect our work.