Are My Slugging Measurements Correct? (30-30 Winchester)

[UtopiaTexasG19]

I just slugged my 336 in 30-30 Winchester with a very soft BHN 5 cast bullet that had been sized to .310 and had a devil of a time getting it through the well oiled barrel. No matter which area I measure with my micrometer I get a reading of .307 Is this possible? I know the micrometer works properly as I measured the slug after running it through the sizing die and got the correct measurement. Thanks...

[Bret4207]

Well, it's working right if we assume your sizing die is correctly marked, they aren't all exactly on. If you don't have a standard to test the mic against, try a bullet from a major producer like Sierra. Sierra's have always been pretty close to their advertised diameters IME.

As for you barrel, it's certainly possible.

[leadman]

If you have any catalogs from long ago you will notice that the 30-30 bullets were .307" diameter.
I had an older Winchester 94 that measured a hair over .307".
Don't know why they are now .308".

[725]

Sierra used to sell .307 jacketed for the .30 cal rifles.

[geargnasher]

My 336 12-groove (1966 vintage IIRC) has a groove of .3092 and a bore of .3025". Typically the Micro-groove barrels run from .308-3095", I'd do as Bret suggested and measure a jacketed bullet to use as a "standard" for micrometer checking.

Gear

[stubshaft]

My 1958 model 64 in 30/30 slugged .307", and can remember buying j words from Sierra and Speer that were .307".

[303Guy]

A simple 'standard' to compare measurements from on a micrometer that will measure .307 is zero. Make sure the faces are clean and check the zero. Then remember also that lead is very compressable. However, it seems like your question has been answered - there is nothing wrong with either your instrument or your measuring technique. The groove diameter is indeed .307.

[Lizard333]

I have a 53 Winchester 30-30 that I use my boolits sized to .309 and they shoot perfect, no leading. The older barrels were often .307, with changes happening on the new newer barrels. You can still shoot jacketed rounds that are .308 with no issues. I used to shoot Sierra 170 gn round nose through mine exclusively. That is us uniil I started shooting cast boolits. Now, all my Winchester 94 sees are 311041 sized to .309, gc, with 29 gn of 335, pushing them 2030 fps. They shoot like a dream!

[Bret4207]

Pretty sure I still have 2 boxes of .307 Sierra 30-30 bullets.

[9.3X62AL]

I use throat diameter whenever possible to determine my sizing diameter for a given firearm's boolits. To paraphrase Senor Buckshot's oft-repeated phrase......."When The Big Light hits the boolit, it will find a way to fit if a little oversized".