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Wednesday, 19 July 2017

Slugging A Rifled Bore & Measuring the Result:

I was inspired to research this skill (Me? ..inspired?) when I last had to use my brass knocker to remove a popped bullet from stuck in my Ruger .327 Federal Magnum revolvers 4 inch bore .. I was shooting-off the last of the way-under-powered .32" Long wimp rounds that were loaded to the Manual's supposed MAXIMUM LOAD recommendation! - and had fired the primer only squib load that had arrived in the guns cylinder.

- I dropped the spent pill into my pocket thinking that it might be interesting to mike-it-up when I got home .. then later I found an informative story 'SLUG, MIKE, MATCH' in the 1979 Gun Digest. that aroused my interest further.

- Well my particular slug came-out at .312" as measured by my old Mitutoyo - as it should I guess.

Slugging a bore is fairly simple task of hammering into the bore a soft lead ball or cylinder (slug) and pushing it through and out of the barrel to then measure the groove diameter .. which will strongly indicate the best size bullet to use for maximum accuracy in that particular barrel.

Note: Best that you use a metal rod rather than a wooden dowel .. wooden rods have been known to splinter and jam the slug into the barrel almost permanently/ solidly!.

A Slugged Bullet With An Unused One.

Piece of cake - easy-peasy .. until you find that your bore has an odd number of grooves eh.  Then you can't just mike or simply measure a slug from a 3 Groove or 5 Groove barrel.
- because there is a 'land' opposite each 'groove' -  remember the 'slug' is a reversed image of the bore.

I did not have that problem with this SP 101 Ruger in 327 Fed. Magnum but you never know when it might arise in future.
There are somewhat complicated ways of doing this measurement using a V-Block of known angles and dimension. Link:

- but I reckon to use a vernier or dial/digital type caliper for this job because the narrow jaws will fit into the slugs 'groove' and over it's opposite 'land' too. This will give a measurement for the known bore diameter plus one groove depth ... all you need to do is add that figure (groove depth) again and you've got the groove diameter or desired bullet diameter. Maybe.

- or perhaps you could use known size precision drill blanks (+- expected size) to sit in the V and measure-compare with your slug in any unknown V-Block?

- Will that work? - I'll try it someday - when I take-up long range rifle shooting eh.

- Or you could find a real expert engineer with decent measuring facilities or ring gauges to voluntarily take-on the task.

Marty K.