Terminology: Bore Diameter/Groove Diameter

by
posted on March 15, 2011
wiley-clapp.jpg

Here's the definition, right out of the NRA Firearms Sourcebook. It is “...the minor interior diameter of a barrel that is the diameter of a circle formed by the tops of the lands.” That is the bore diameter, but groove diameter is “...the diameter of a circle circumscribed by the bottom of the grooves...”

Look at it this way—a gunmaker starts with a thick-walled steel tube when he or she goes about making a barrel for a firearm. The tube is gun-drilled, straight and true to a dimension that is a few thousandths less than the diameter of the bullet. This dimension will be just about the same as the bore diameter in the finished barrel. He then forces or draws a button or cutter through the barrel with a twisting motion in order to impart spiral grooves evenly down the bore. This action removes or displaces enough metal as to produce consistently deep, evenly-spaced grooves from end to end. The dimension from the bottom of the groove to the bottom of the opposite groove is the groove diameter.

When the gun is fired, a bullet is forced into the barrel under great pressure. The bullet has a diameter very close to the groove diameter of the barrel, so it is a tight fit. Expanding powder gasses force it forward, thereby engraving it into the rifling and causing it to turn. The fit is tight enough to prevent powder gasses from getting around the bullet as it traverses the barrel, so it comes out of the barrel spinning at a rate imparted by the twist rate of those spiral grooves.

There are two diameters in a barrel. The greater (major) diameter is the groove; the lesser (minor) is the bore.

Latest

10mm round
10mm round

Handloads: A 10 mm Auto Loaded For Bear

The fear of a bear attack has likely sold more 10 mm Auto handguns than all firearm advertising combined. The 10 mm does deliver some impressive ballistics for a cartridge chambered in semi-automatic handguns.

The Rifleman Report: Creative Minds At Work

As all of us who experience this “mortal coil” eventually learn, the days seem more fleeting with each passing year. For those of us who make a living observing and reporting about the firearm industry, they eventually result in a somewhat disorganized pile of memories about companies, products and the people who create them.

Smith & Wesson Issues Safety Alert For Response Carbines

Smith & Wesson has identified a condition in which an out-of-battery discharge can occur when certain Response bolts fail to fully close before the trigger is pulled.

Review: GForce LVR410

With a long and storied history in the United States, lever-action carbines continue to be favorites among modern American shooting sports enthusiasts. This evaluation takes a closer look at the 24"-barreled LVR410, which is being imported by GForce Arms, Inc. of Reno, Nev.

Editor’s Choice: “In Country | My Memories Of Vietnam And After”

I met author Forrest R. Lindsey at a gun show where he graciously provided a signed copy of this remarkable account of his experiences as a young man enlisting in the Marine Corps a year after high school in 1965.

The Armed Citizen® Feb. 26, 2024

Read today's "The Armed Citizen" entry for real stories of law-abiding citizens, past and present, who used their firearms to save lives.

Interests



Get the best of American Rifleman delivered to your inbox.