Help with a strage question

[whomeno]

ok first off I slugged my hi point c9 barrel at .357. when I shoot and retrive a bullet from a factory round of 9 mm it has lan groves in it.so if the barrel is at .357 how does a bullet at .356 have lan groves?

[btroj]

Because the lands stick up above the bore.

The bore measures .357, the tops of the lands are probably more like .350.

Make sense now?

Key is that the lead bullet needs to fit snug to the bore, hence using a bullet .001 over bore diameter.

[whomeno]

ok, so I went to powder coating my bullets now.should i keep them at .358 or resize them to .356?

[btroj]

Keep at .358. Going to .356 is likely to give leading.

Powder coat isn't near as strong as a copper jacket. It only keeps the lead off the bore, it doesn't eliminate the danger of gas cutting due to undersized bullets.

[whomeno]

ok thanks for clearing things up for me. it all makes sense to me now.

[JSnover]

Glad you got it straight but you need a clarification. The bore is the smaller diameter, measured from or created by the tops of the lands. The groove diameter is the larger, measured at the bottom of the grooves. When you measured your slugs, you were reading the groove diameter; the larger of the two.
Sizing is based on the groove diameter.

[Hickok]

Think of the bore diameter, as the barrel when it was smooth and before it was rifled, it will be smaller. When the barrel gets rifled, the grooves are the larger spiral grooves impressed/or cut into the bore, hence they are the larger diameter. The original small bore diameter is now called the "lands". Bore diameter and lands are the same, being the smaller demension.

Therefore you now have lands and grooves!

A .30 cal barrel will usually be a .300" bore or land diameter, with a rifling depth of .004", thus giving a total groove diameter of .308", ( 2 x .004" rifling depth plus .300" bore diameter = .308")

This is the perfect spec for a .30 cal barrel, but in real life it can vary.

[blackthorn]

A more long winded explanation:
The three terms; Bore, Land, and Groove all have to do with features inside the barrel of a firearm, therefore, to generalize and state that ‘I have a 30-06 and its bore slugs .308’ is not correct on a very basic level. A firearm’s lands and grooves are distinct and totally separate parts.

BORE is correctly used in only two instances. One is to describe the measurement from the top of one land, to the top of the opposite land, or stated another way; it is the MINOR diameter within the barrel created when the grooves are cut. The second way ‘bore’ may be used correctly is when describing the entire interior surface of a barrel, as in, ‘the bore of my rifle shines.’ This usage includes both the lands and the grooves.

LANDS: The lands within the barrel of a firearm are the raised portions that form the minor diameter of the barrel’s interior, created when the grooves are cut into the interior configuration of the barrel.

GROOVE fits the dictionary definition exactly. It is a narrow area where material has been removed from a surface and the bottom lies at a lower level. Saying, ‘My groove diameter is .308’ describes the major internal diameter of the barrel, from groove to opposing groove. Or it can be described as an imaginary circle drawn perpendicular to the axis of the barrel and touching the bottom of each groove.

When the discussion gets around to the proper way to use the words: ‘bore’ and ‘groove’, it is easy to keep these two very different dimensions straight in your head if you consider the traditional steps in making the rifled barrel. First, a hole is bored through the barrel blank from end to end. This bored hole is the same diameter as the name of the caliber, so, for instance a .30-30 barrel will have a hole bored that is 0.300" in diameter. At this point you may have guessed that the initially drilled hole is the ‘bore’.

The next step is to cut grooves, a few thousandths of an inch deep into the inner wall of the bore to form the rifling. Those grooves will generate an overall inside diameter larger than the original ‘bore’ dimension. If the grooves are each 0.004" deep, that adds 0.008" to the .30 caliber ‘bore’ diameter (mentioned above) and results in a .308" bullet diameter requirement.

Grooves vary in depth with the caliber (and caliber refers to the bore diameter, .30-caliber being .300", for example). Small calibers have shallower grooves, which is why a .22-caliber bore will have about a .224" groove diameter, with each opposing groove being 0.002" deep. A .30-caliber, in contrast, will have a .308" groove diameter because the opposing grooves are each 0.004" deep. In between, the .25 and .27 calibers use grooves about 0.0035" inch deep, which makes their bores .257" and .277" respectively.