Monthly Archives: October 2013

Tricks the Pros Use When Rechambering a Barrel.

Chamber with ejector removed.

Many a chamber reamer has been damaged or broken when a novice gunsmith tried to rechamber a barrel with an extractor or ejector cut.  In this post I will explain how to get a high quality job without danger of damaging the reamer.

At right is the breech end of an NEF barrel that is about to be rechambered.  The mistake that is commonly made is to try to insert the chamber reamer into this existing chamber without preparing the chamber area.  Reamers are not made to work on an interrupted cut such as an extractor cut.  Trying to use a conventional drill bit will not work in this situation either, it will simply do a lot of damage and make a mess of the job.

There are two easy ways to handle this problem and end up with a nice clean chamber.

  1. Use a piloted counterbore to cut a recess that will accept the chamber reamer, eliminating the extractor cut.  The problem with this method is that you would need a specialty tool for every shoulder diameter that you might decide to rechamber for.
  2. Place the barrel in the lathe and use a boring bar or a simple boring tool ground for the purpose.  This method has the advantage of working on any cartridge combination that you might encounter.

Measure the bore to work with the reamer.

Bore out the area of the extractor cut to a dimension very close that of the shoulder diameter of the reamer you will be using.  The idea again is to prevent the reamer from cutting an interrupted cut.  As the shoulder of the reamer engages the chamber it will then cut uniformly and without chatter.  If you attempt to cut the chamber without performing this preparation each flute of the reamer will bang against the extractor cut as it comes around.  In most cases this will at a minimum damage the reamer, worst case it will brake the reamer.

hand ground boring tool.

At right is a simple hand ground lathe bit that will work for this job.  The under side of the tool must be relieved so that it can clearance the inside of the chamber area.  This is a finesse job, only remove as much as you need to get the reamer in full contact with the barrel.

What it looks like when bored correctly.

Here is the chamber area after the boring work is done and before the reamer has been used.  Note that we did not cut away any unnecessary material, only that which will make the reamer cut properly.

Chatter is a common complaint when rechambering a barrel.  The pilot is often either not engaged in the bore of the barrel or it does not fit the barrel properly.  Proper pilot diameter is .0005″ to .001″ smaller than the bore diameter (across the lands).  This allows for a slip fit to the bore.  An undersized pilot will promote chatter.

Finished chamber

A simple way to stop chatter that will not damage the tool is to wrap the reamer with a strip of wax paper.  The wax paper acts as a dampener against the chatter which is caused by vibration.  Do use cutting oil as normal when using the wax paper.

The chamber below completely cleaned up the old rim cut from the rimmed cartridge.  Of course the extractor would have to be modified for the rimless case.

Side note:  Reamers for straight wall cases like black powder and pistol type cartridges are prone to damage from the problems addressed in this post.  In addition they are prone to damage near the rim cutter when chips become trapped in the extractor cut.  So if your working with such reamers take extra care to keep chips cleared, especially when nearing the last few cuts.

Leave a comment

Filed under accuracy, Firearms, Gunsmithing, How To, Rifles, tools

A Couple of Lessons on Chamber Reamers

Forced pilot is damaged

Chamber reamers are pretty complex tools that incorporate all the features of the chamber into one form cutting tool.

At right is pictured the tip of a chamber reamer.  You can see from left to right the shoulder, neck, throat, and pilot.  Note that the pilot appears to be short for the reamer.  It was not short when the pilot bushing was new.

A novice used this tool, the bushing was too tight a fit for the bore.  Proper fit is .0005″ to .001″ under the bore diameter.  That makes for a nice slip fit of the bushing to the bore of the barrel.  The pilot bushing rides on the lands of the barrel.

How do I know the bushing was too tight for the bore?

Evidence of poor pilot fit.

Simple, the bushing was forced back onto the cutting edge hard enough that the throat actually cut the back of the pilot.  Note in the picture at left, the same busing off the reamer.  You can see where the tool cut the bushing.  This portion of the reamer is not very sharp as it was never intended to cut anything.  So that is how I know this bushing was forced into a tight bore.

The primary reason for using removable pilot bushings is so that you can match the pilot to the bore of your barrel.  No need to force things.

You know this guy has no idea how a reamer works…

GEDSC DIGITAL CAMERA

The reamer at right was “sharpened” by a “gunsmith”.  The large flat running down the center of the picture has two grinds.  The fat grind closer to the top of the picture us a relief grind, meaning it will never touch the barrel, it is clearance ground to make sure that chips will not get caught behind the cutting edge.

The narrow grind just below the relief grind is where the actual cutting edge is located.  This grind is also relief ground just slightly.  Only the very edge where the grind meets the flute is actually touching the barrel during chambering.  If you stone on the outside grinds of the reamer the dimensions change very quickly because the geometry of the cutting edge and the clearance grind.  Never stone on the outside edge unless you have been trained to do so.

If you look inside the flute on top of the cutting edge where the chips gather during cutting that is the area that you can stone without changing the dimensions of the reamer (in the picture here that would be visible above the grinds we just discussed, of course each flute is one cutting edge).  Again, because of the geometry of the reamer stoning on this inside edge changes dimensions such a tiny amount that it should not create any problems with chamber size if you don’t get carried away.  Normally all that is needed is the cleaning of metal built up on the cutting edge, no real stoning of the reamer itself.

When I look at the picture of the reamer above, I laugh because you can see where the “gunsmith” stoned on the relief grind.  Since this part of the reamer never touches the barrel at anytime it is clear that this guy had no idea how the tool works.  If you don’t know how a tool works, it’s a safe bet you have no business trying to sharpen it.  Send it to the reamer maker if your not sure, its cheaper that an angry customer.

2 Comments

Filed under Gunsmithing, How To, tools, Uncategorized