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Re: Groove diameter

Posted by Wayne McLerran on Sunday, 17 August 2014, at 7:51 p.m., in response to Re: Groove diameter, posted by Dick Hosmer on Thursday, 14 August 2014, at 2:44 a.m.


After checking the calibration of my v-anvil micrometer, which was right on, I gently measured a soft lead bore slug I just received from a TD owner. The groove diameter was measured 3 times (once using each of the three grooves). Then, using a standard micrometer I made 3 measurements across each land and opposite groove. Finally I measured across opposite land edges with a vernier caliper and standard micrometer to see if it agreed with the groove diameters. Here's the data :

Groove diameters: 0.4638", 0.4638", 0.4637" Across one land and opposite groove: 0.4575" Across opposite land edges: 0.4615"

The bottom line is the groove diameter is 0.4638" and bore diameter (land diameter) is 0.4512". The land heights are 0.0063".

BTW, I also measured another soft slug and a Cerrosafe cast made from two other TDs to determine/check if measuring across opposite land edges will provide the same measurement as using a v-anvil micrometer. As noted above, using the technique the measurement was off by 0.0023”. The slug I had from another TD was also off a little over 0.002". But measuring across opposite land edges on the much harder Cerrosafe cast was very close (within 0.0005”) to the same measurement when using a v-anvil micrometer. It was obvious the soft land edges of the slug will not stand up to even gentle measurements using a vernier caliper of standard micrometer.

So Dick, your premise/assumption that we discussed and I agreed with 4 years ago ( is technically correct but the technique of measuring across opposite land edges does not work very well if the medium used is pure lead or a real soft alloy. BTW, I just checked my collection of casts and slugs. When we initially exchanged emails on this subject I mentioned “slugging the bore”. I’m now sure that was a misnomer and I actually used a Cerrosafe cast, not a slug, made from the original bore of the 1877 carbine I later had relined. Hence the reason that at the time I agreed your premise was valid.

Regards, Wayne


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