Measuring a Five Groove Barrel Slug

[rjathon]

I read where folks spin a barrel slug from a five groove Enfield barrel while measuring with a micrometer. I tried it numerous times to try to get some skill but don't trust my reading of .3164.

How accurate is this technique? Is there a better alternative?

Thank you.

[james nicholson]

Wrap the slug with a thin leaf from your feeler gauge then subtract the thickness of the leaf twice.

[JSnover]

What j n said. You can't spin a five-sided object and get a good measurement until you make it round.

[flintlocke]

rjathon, If you can get your mitts on a #0 Morse Taper female....coat the inside with a marking dye (I'm fond of aerosol liquid graphite)....as aligned and square as humanly possible, press your slug gently in the taper with the intent to make witness marks in your dye coating....measure the inside diameter of the witness marks left by your slug. But wait, it's not that easy, to be dead accurate the ends of your slug need to be trued, not much fun with soft lead. I have dreamed up a number of elaborate schemes but a big mill file and a vee block works as good as any. It will probably take some fiddling to find the right force to mark your dye but not deform your slug.
All that being said and done, if you form a rather long slug and measure it with the jaws of a caliper instead of a mic, jaws PARALLEL to the long axis of the bore, there is usually enough rifling twist to get an accurate reading. Good luck. Let me know what you find with Leverevolution in the .303. flintlocke

[W.R.Buchanan]

Measure across the groove and land of the slug or barrel, then use a pin to find the I'd of the bore.

Example: If the slug OD or barrel ID measures .308 and the bore is .303 then the rifling is .005 deep. .308+.005=.313

You need to do the measurement several times and get the same readings. If you don't, you don't have a valid reading, and need to do it over.

It is very easy to get bogus readings with calipers. That's why you take the same measurement several times,,,

Keeping pressure on the caliper while wiggling it slightly to insure that you are either getting the smallest OD or largest ID reading possible.

Just lightly touching the part with the caliper jaws is not accurate!!! You must have all the slack out and have the jaws square to the hole or OD of the part. It requires some pressure and movement to insure that this is taking place !!!

Also it doesn't happen every time,,, no matter how good you are, or how good you "think" you are.

Repeated measurements are the only way to insure accuracy!

If you get the same measurement 3 times in a row then you probably have gotten as close as you're going to get.

Note I said,,, "Three Times in a Row." not three out of ten.

Good Luck.

Randy

[Blammer]

follow these directions and do the math
It reads:
find A, subtract constant "C" from it which gives B.
Multiply B by constant .8944 to find B' or Bullet diameter.
Constant "C" is always unique to each V block.
http://i54.photobucket.com/albums/g81/blammer8mm/Vblock_zps3e6a5bbc.jpg
(http://s54.photobucket.com/user/blammer8mm/media/Vblock_zps3e6a5bbc.jpg.html)

[JWT]

I put together a quick AutoCAD drawing to test Blammer's method. The method checks out but the multiple is 0.828426.

158637 158638
Good timing on this post. I was thinking how to do this exact thing with a five groove Trapdoor.

[Blammer]

I"m stealing your picture! :)

thanks

[Nicholas]

Doesn't that depend on the angle of the V? I believe that is why Blammer said the constant C was unique to each V block.

[JWT]

The angle is important. In order for the formula and drawing I posted in #7 to work correctly the angle has to be 90degrees. The C constant is the distance from the bottom of the V to the surface plate the v-block rests on.

[DCM]

Gonna steal that pic too! Thanks JWT

[kywoodwrkr]

http://castboolits.gunloads.com/showthread.php?10665-Measuring-5-groove-Bbls&highlight=V-block

[Outpost75]

This may be helpful

159297

[Blammer]

ok, so who's up for mass producing some V blocks for sale. :)

[milrifle]

It seems like someone posted a few months ago that they made v blocks for 3 and 5 groove slug measuring and they came with the constant "C" stamped on the end of the block. Wish I remembered who it was. I tried searching, but couldn't find it. I toyed around with it when I got my 3 groove Snider, which I've yet to get shooting very well, and now I have a Trapdoor with 3 groove rifling, so it's time to get a v-block.