I've noted this for quite some time as to why .308 projectiles are marked .308 on the boxes however my 4 micrometers say they are .307 in diameter. Either I have some bad mic's or?

The Mic. may need calibrated. pressure has a lot to do with precise measurement. Its a touchy touchy feely kind of thing.
check along the length of the bullet. you may be too close to the tip?
What brand bullet do you have? If they are seconds or bulk they could be undersize.

Best way to check is what do your micrometers measure other calibre bullets as?

I checked .284 which when mic'd came out as .284, .243 mic'd 243, .338 same, 375 same. Only .308 bullets came out as .307.

Sierra .30, .307, same for Speer, same for Hornady, same for Nosler. I mentioned it to other reloaders at my club and several guys said they are finding .30 military bullet some are as small as .306 as well as commercial bullets being .307. Hell even Russian, Chinese, etc. ammo mic's .310/.311.

PS, I'm gentle when using the mic.

Several reasons. Unlikely all 4 of your mikes are bad....
Send me three of them and I will tell you what they actually are.

How do they shoot? If they shoot OK - is it a problem?

some 30 cal barrels measure 307 too.

it isn't a big deal for the bullets to measure 307, I bet they come out the muzzle the same size as the barrel they are shot in.

I think they will shoot just fine.

Had a European made .375 H&H rifle that had a very tight barrel.

I made him 0.372 bullets and it shot very well.

No problem with accuracy. Heck I can always use another micrometer, however I still don't know why every other caliber is spot on.

quote:

Either I have some bad mic's or?

.308" bullets can be measured with a 0" to 1.000 micrometer. When closed the 0 to 1" micrometer measures zero.

F. Guffey

Yeah I agree you can zero your mic. Most 308 bullets today don't measure .308. They've not only changed bullets they've changed barrel dimension such a groove depth. Groove depth isn't as deep as it originally use to be. You had a good honest .004 groove depth in the old days and now they feel comfortable with .003

308, 307, I suggest both will be acceptable for all hunting..perhaps not at a bench rest shoot. It just not enough to make any difference in most rifles. If one doesn't shoot well don't use them, use whatever shoots best..your over thinking a non existent problem I suspect..