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Discussion Starter #1
In another threads at https://www.shootersforum.com/wildcat-cartridges/236415-25-08-25-souper.html

It was found the subject gun, bought sight unseen in a wildcat caliber, had a 'jugged' chamber which made extraction very hard even with a mild load.

To repair would take pulling the barrel and that is problematic in Jap actions so it was decided to lap the chamber straight and hope it was enough.

I had one fire-formed case to work with and you can't switch grits on a lap, so I decided to lap with 10 micron and polish with wet-dry.

Making the lap--

The fireformed case was de-primed and stuck in a Wilson case gauge but any die will work. By holding the die-gauge in the lathe the case is held in by friction on the taper as a center cutting end mill 'bore's out the primer pocket. I went 1/4" x .125 deep.

A 12" length of .500 TGP drill rod was turned down to .250 and .125 long to fit the 'primer pocket'.
JB Weld held them together over-night.

Only the body needed lapping so a squirt of 10micron diamond water-oil lapping paste (ebay-cheap) was applied to the case body and smeared with a finger.

After 20 seconds @ about 300 rpm, the lap was withdrawn and wiped down. It was clear to see both ends of the case was cutting but a band in the middle was not. Another 20 seconds made that band much more narrow and showed the case was nearly straight....it was straight enough! That little lap-over the shoulder means the already excess headspace would be further expanded if continued.

The chamber was washed, wiped, flushed and wiped again and inspected by flashlight. It's still rough, but now the ripples are closer to being flattened out and the big parts aren't all that much bigger. a slotted 3/8" dowel was strung with 400 w-d and spun about 30 seconds. More paper was added to bring it to bear near the rear of the chamber and given other 30 seconds. The chamber was cleaned again and a sulfur cast made.

The chamber cast came out clean with just a bump of the rod and all in one piece.

It needs a FP spring to safely shoot it. That'll be here Tuesday. If fire-forming shows it needs more, I'll have cases to make them. The rod can be reused over and over.
 

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A good tutorial on lapping with the photo record... cool!

I know the expense of the action wrench renders this unfeasable, with the sorry machining of the chamber it makes wonder how well the barrel shank and threads match the receiver?
 

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Discussion Starter #3 (Edited)
Exactly, Monte-- When you see a cross-threaded spark plug, it's hard to trust the engine re-build.

Someone asked how 'lapping' works--

There are two ways of 'lapping' and we're doing a combination. The lapping compound is abrasive. In this case, industrial diamonds. They look like crushed black gravel under the microscope and are suspended in a water soluble grease. On first contact with steel, the gravel tumbles and then sticks into the brass.....like sharp rocks in concrete. The slurry of grease, gravel and brass and steel chips eats away at the steel more than the brass because it 'sticks' to brass and 'skids' on steel (or tungsten carbide, for that matter). Lapping straightens and flattens.

This particular chamber still has a scratch (middle arrow-red). The left red arrow points to a case separation sometime in the past.
The black arrow points to a 'low place' in the chamber probably caused by 'bearing down' with the polishing stick trying to get the scratches.

It's easy to see what is doing the work and what isn't.

The good news is that the chamber is very 'tight', still.

To lap the body bigger without changing headspace, cut and file the case off at the shoulder, plus about .005. (Blue arrow) Use just the body portion as a lap and quit when in stops cutting. That will take out all but the deepest scratches but the case body will be bigger.
This chamber mouth is .469 with no visible swell of the case. It might need another lapping to be right.
 

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I did pretty much the exact thing with my 6 X 63 and could not get the results I wanted. I wound up having Jess re-bore it to 9.3 X 62.

Great info here.
 

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Discussion Starter #5
Being able to put the barrel in the lathe cuts 90% of this work. Polishing chambers is done with a hardwood dowel with abrasive wrapped around it. The trick is to keep the stick parallel to the chamber wall and use an in and out stroke just like repolishing brake cylinders or engine cylinders---a criss-cross pattern of polish marks that fade away in the final grits. A chamber should NOT be highly polished. Leave a little 'grip' with a final 600 wet or dry session. If you have to polish a new chamber more than two minutes (most take 20 seconds total), your reamer was bad and needs re-sharpened. ;) There's really very little to do with a good reamer. The problem comes from deep scratches from galled chips in factory chambers that cause hard bolt lift.
 
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