30 Cal bullets really .307"

[High Plains Drifter]

I have measured my 30 cal. bullets and get .3075 on Noslers, .307 on Hornady and the "MIL" spec FMJs from Wideners (I think they are made by General Dynamics), using two different calipers.

Any ideas?

 

[LoonWulf]

Little too much pressure? Mine usually read .308+ when checking with calipers. I usually read just ahead of the base of the bullet

 

[morcey2]

Do you have access to a micrometer? My calipers are often off by 0.001 or 0.002" which isn't a big deal (in most cases) when you're measuring 1.5"-plus, but a micrometer is much more accurate.

 

[Swampman]

For really precise and accurate measurements, a good micrometer is going to be a lot better than calipers.
Calipers are great for measuring case length, but for measuring bullet diameters a micrometer is generally the best tool to use.

The Nosler and Sierra .308" bullets that I've measured have almost without exception measured right at .308" with my Starrett micrometer.

I've measured pulled WWII .30 Carbine bullets that measured as small as .3065" and Polish 7.62x25 bullets that measured .307" (meant for weapons with .310" bores).

 

[Bart B.]

I've measured 30 caliber bullets with micrometers semiannually calibrated to a gauge block (.000010" tolerance) in a metrology lab. Got diameters from .3068" to .3078" (Lake City Arsenal 7.62 M80 bullets) to .3092" (Lapua D46 match bullet. Sierra match bullets have been .3082" to .3084" Arsenal 172-gr FMJBT match bullets at .3085" to .3086". Western Cartridge Company and Winchester 30 caliber match bullets from ..3086" to .3088". (Not surprized; Win 70 match barrels were .3084" to .3085" groove diameter.) Hunting bullets made in the USA from .3074" to .3081"

Best accuracy with all of them happened when the barrel's groove diameter was smaller than bullet diameter. But that's been well known for decades.

 

[243winxb]

Thanks to RC for the photo. RIP.

I had some Hornady 30-30 bullets , that were also .307" when measured.

Edit/add
Title of thread - 30 Cal bullets really .307"

Some match grade bullets will measure larger on the pressure ring. By as much as .0004" to .0008" larger. (243)

 

[ny32182]

Maybe I misread the implication in the question (if so, disregard), but I think the OP was expecting that ".30 caliber" bullets would all measure bang-on .300", and is about to discover that naming conventions for cartridges and generally referenced bore diameters OFTEN do not actually indicate that the bore or bullet is actually exactly that size.

Example, 7.62mm actually does measure .3" exactly, however:
-US orgin 30 calibers pretty much all (that I'm aware of) actually use .308" bullets, or about 7.82mm in reality.
-Russian origin 30 calibers use .311" bullets, or about 7.899mm.
-9mm handguns actually use bullets that are between .355 and .357", in reality, larger than 9mm...

Etc. I'm sure there are MANY more examples.

 

[DeadEye9]

Different manufacturers and their sub models within often all vary slightly in overall diameter when measured with a tight tolerance micrometer. Subtle differences are not as easily detected with calipers and such not dialed in quite as tight tolerance wise. In any event, this is why people will try various brands/styles of bullet in a given caliber to see what gives optimum accuracy from their rifle for the intended purpose of that cartridge. Unless you slug your Bore, and then do the testing to see exactly what diameter offers the best results from your rifle, these subtle differences in diameter mean little to the average shooter/reloader. Anything within the specifics caliber range for a given platform will "work" it's just some perform better than others and the diameter differences are one of the reasons behind this. Personally I wouldn't put too much effort into measuring and worrying about overall siZe as I would in trying various bullets and seeing what your particular firearms prefer.

Also for reference I grabbed a few projectiles off my shelf real quick in various calibers and measured, none were exactly on.
My Hornady .224" 62 FMJ measured .225"
Winchester .355" 124 FMJ measured .3545"
Hornady .308" 150gr SST measured .307"

 

[DeadEye9]

 

[Reloadron]

Maybe I misread the implication in the question (if so, disregard), but I think the OP was expecting that ".30 caliber" bullets would all measure bang-on .300", and is about to discover that naming conventions for cartridges and generally referenced bore diameters OFTEN do not actually indicate that the bore or bullet is actually exactly that size.

Example, 7.62mm actually does measure .3" exactly, however:
-US orgin 30 calibers pretty much all (that I'm aware of) actually use .308" bullets, or about 7.82mm in reality.
-Russian origin 30 calibers use .311" bullets, or about 7.899mm.
-9mm handguns actually use bullets that are between .355 and .357", in reality, larger than 9mm...

Etc. I'm sure there are MANY more examples.

Isn't a 30 caliber bore diameter 0.300" while the groove diameter is 0.308"? The difference being a land to land diameter (Bore) and a groove to groove diameter of 0.308". That is using the .308 Winchester SAAMI cartridge drawing as an example. The acceptable SAAMI bullet diameter is 0.3090" -0.0030". The 243 Winchester has a 0.243 groove diameter with a 0.237 bore diameter so go figure.

Anyway, the .308 Sierra 168 grain BTHP bullets I have sitting here measure 0.30820" while the Speer Gold Match 308 168 grain BT bullets come in right at 0.30805". That with my trusty Mitutoyo "dead on balls accurate" (Industry Technical Term) Digimatic Micrometer.

Ron

 

[Bart B.]

Note the term "caliber" originally meant the bore diameter in hundredths of an inch.

Many custom match barrels chambered for the 308 Win round have bores as small as .2970" and grooves as small as .3060". British Empire countries made such match barrels for their fullbore long range target rifles. They were not allowed commercial or hand loaded ammo; only arsenal M80 ball ammo. It's bullet diameters were sometimes almost 3060" diameter. They shot most accurate in those barrels at 3000 fps, slower in larger bore/groove sizes and would go subsonic before 900 yards.

 

[High Plains Drifter]

Thanks for the input, like many other things in our culture there really is no standardization, like in traffic laws, and aviation, depends on where you are at.