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.
- 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.
- 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.
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.
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.
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.
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.