trouble slugging 5R barrel - undersized results

Measuring an odd rifling count will always be tough.

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See if you an find someone with a set of gauge pins and check the minor bore size. That should be .300 no matter how many rifling so the barrel has. If it is undersized, call the manufacturer.

Have you shot the firearm? How is the accuracy? Tight bores using cast lead may lead up the bore. May not be a problem at all for jacketed bullets.

I know its 5 groove\land but I am rotating slug in calipers to get a max measurement.

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well no matter how you rotate it you're going to measure one land and one groove, so it sounds like it's probably .308 like it should be.

Have you shot the firearm? How is the accuracy?

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Really like to know this answer.

I would go .309" sizing die and use a Lyman "M" die, if available.

Several ways to check diameter. V-mics, Gage pins, v-block and indicator and finally, comparator. Some versions will work better than others dependent on barrel twist.

.308 unfired bullets are Supposed to drop freely down unsized fired brass.

The bullet measures .308 before firing.
The I.D. of a fired & expanded case is supposed to be bigger then that.

And it should drop freely into an unsized case.
It should not fit in a sized case without using a reloading die to put it there.

What exactly makes you think your barrel is undersize?

rc

My precision measurement contact says a Starrett T483 type of micrometer or equivalent will do the trick. Alas, if you were to buy one "not-new" expect to pay around $400 from an Ebay vendor :-(

Failing that, he suggested a good local centre-less grinding shop, they'll have the requisite measurement tools. For a "one-of" cheaper to pay for a measurement.

Anyone think this would work?

gidaeon said:

I am fresh out of machinist friends owning and knowing how to use the above tools ;-)

I noticed my slugs drop easily all the way down into fired unsized 30-06 and .308
cases - not that it gives me new info other than my slugs are smaller than other 30 throats

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You may have a way to measure easily without specialized tools.

I don't know if this will work, but costs you little more than a a bit to time to try it.

Take one of your slugs from your bore-slugging and verify that it drops into the neck of a fired, unsized case, then run the case into a sizing die, just a little, so the case is sized down only .001". (Measure the case mouth opening). See if the slug still drops through.

Repeat until the slug no longer drops through.

It would be tricky to do the .001" sizing and you might wind up re-expanding or chamfering a few cases to get the measuring right, but you may get a god measurement out of it.

Alternatively, you could drill an undersized hole in a sheet of something or other (sheet metal, stiff plastic, even cardboard might work) and ream the hole until the slug drops through and measure the hole. Ensuring the hole is not eccentric would be essential.

Caveat: I have never done this, nor heard from anyone who has, so this is entirely theoretical. But you could try a known object (like a slug that you passed through a barrel that has an even number of lands and grooves) to see if you can make the methodology work.

Good luck.

Lost Sheep

^ won't work


Because the neck sizing part of a rifle die isn't tapered. It's an all or nothing affair.


OP can you seat your slug into a rifle case and use a simple before and after neck diameter measurement to get an approximate measure?

R.W.Dale said:

^ won't work

Because the neck sizing part of a rifle die isn't tapered. It's an all or nothing affair.

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Thanks for reading and for replying.

That's why it would take an extremely delicate hand on the press to

run the case into a sizing die, just a little, so the case is sized down only .001"

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and repeated tries.

But you have detected the primary weakness in my idea. To get brass to form down only .001" at at time with a sizing die might well be impossible.

Lost Sheep