things my caliper told me...

[chemist308]

I was playing with my new Lyman caliper and my bullet puller. According to the caliper, the largest interior diameter of my 9mm barrel (on bottom of rifling grooves of opposing sides of barrel) was 0.355 in. I used my new bullet puller to disassemble a factory 9mm shell. Guess what it measured at. Would you believe 0.354 in?

Now if this thing is accurate on both sides--and I'm not discounting the possibility of error here--I'm throwing a slug that is just a hair under the barrel diameter. I always thought that bullets were supposed to be at least 0.001 in over the interior barrel diameter--is that an accurate assumption? I mean 9 mm is 0.354 in, so isn't that why 9 mm molds are 0.356 in? Wonder if this could be contributing to my accuracy issues with this gun...

Edit to answer txh: The factory rounds are indeed jacketed.

[thx997303]

Your factory round is likely jacketed, and jacketed rounds are a different thing entirely.

Yes your lead boolits should be at least .001" over groove diameter of your bore.

And I would slug your barrel, it will be more accurate than merely measuring your bore with a caliper.

[Ricochet]

Jacketed bullets work fine under groove diameter. Their jackets are stiff enough that the lands of the bore support them adequately. Military rifles were often deliberately made with groove diameters several thousandths larger than their intended bullets. Best accuracy is found with a good fit, but a loose one is less fussy about things like fouling, dirt and rust and still shoots well enough.

[44man]

Good advise here, you can NOT measure the bore directly with a caliper. Chances are the bore is closer to .356", maybe more. So for cast you should look towards .357".
Ignore jacketed bullets.

[beagle]

I'd be willing to bet that if you pull a 9mm cast from a loaded round, it will have shrunk from say a .357" loaded bullet to at least .355".

I know that measuring pulled cast bullets from .30 Carbine rounds show a shrinkage from .310" to .308" in my case.

They tend to get sized down during seating.

And yes, calipers and mikes can tell you a lot about what's going on with your bullet casting and loading./beagle

[StarMetal]

I'd be willing to bet that if you pull a 9mm cast from a loaded round, it will have shrunk from say a .357" loaded bullet to at least .355".

I know that measuring pulled cast bullets from .30 Carbine rounds show a shrinkage from .310" to .308" in my case.

They tend to get sized down during seating.

And yes, calipers and mikes can tell you a lot about what's going on with your bullet casting and loading./beagle

beagle,

The 30 carbine case is very robust. It's a strong case and I believe that's why it sizes a bullet down more then others. If you can get away with it expand you case up larger and maybe you're bullets are too soft?

Joe

[thx997303]

I have noticed that with 9mm and mil brass it sizes my .360" as cast boolits down to .357" the diameter outside the case is .358" Of course my bore is .354" at the chamber and .355" at the muzzle.

I do get some leading at the muzzle, but nothing to worry too much about, and accuracy does not degrade after shooting a few.

Is mil brass just strong or something?