Can someone explain bore diameter vs. bullet?


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raindog
November 11, 2009, 04:24 PM
So let's say I have a .308 rifle (caliber chosen at random).

First question: does that mean the measurement is from the top of one land to the other? In other words, if we measured from the bottom of one groove to the bottom of another, it would be wider?

Second question: For a bore of .308", what size bullet would be ideal? .308"? Or would .309" or .310" or .311" be better, so the lands cut into the bullet and grip it as it moves down the barrel? Should the bullet ride "on the lands" or be cut into by the lands?

I have to assume that .307" or smaller would be very undesirable since it would mean the bullet was "skipping down the bore" and bouncing up and down off the lands...but I am speculating.

For my .308, I am loading Sierra #2200 bullets, which are "30 cal .308 diameter". So let's assume the barrel is brand new...the rifling will lightly contact the bullet - so the rifling really is just there to impart spin, right?

Now let's say it's 20,000 rounds later through the same barrel. Presumably the lands are worn down a bit. What is the effect? Well, reduced accuracy, I know that ;-) But I assume that means that the barrel is (a) not making good contact with the lands, (b) not spinning properly, and (c) perhaps having some reduced velocity as the "seal" is not good and the bullet isn't pushed as hard (?)

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R.W.Dale
November 11, 2009, 04:34 PM
You have that backwards


The bore diameter is the distance across the top of the rifling. Typiclly .300" for American 30 cal

the groove diameter is the measurment inside the grooves which should be .308" for your application

bullets do not "lightly" contact the rifling but rather get engraved hard by the grooves .004" by each land

rcmodel
November 11, 2009, 04:39 PM
Now let's say it's 20,000 rounds laterThe throat erosion right in front of the chamber would have made a .308 barrel worthless way before 20,000 rounds.

Wearing out a barrels rifling is a non-issue with most any centerfire high-velocity caliber, as the throat will be gone way before the rifling even begins to wear out further down the barrel.

rc

saturno_v
November 11, 2009, 05:01 PM
+1 rcmodel

Throat would be gone way way before rifling....

Jim K
November 11, 2009, 08:16 PM
Just think of the bore diameter being the diameter of the drill used to drill out the bore. The groove diameter is the diameter of the rifling cuts. A bullet in a modern breech loading rifle is groove diameter (.308" in this case) so it is actually forced into the barrel by gas pressure. In other words, it does not expand to fill the grooves like a muzzle loader bullet; it is already at the larger diameter.

Jim

Uncle Mike
November 11, 2009, 09:58 PM
http://telecom.hartford.edu/images/boredia.jpg

Try this...

The .308" would be across the 'groove' diameter-

It would be, give or take, around .300" across the 'bore' diameter- The rifling 'lands' being, let's say .004" tall on each side of the bore....

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