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

Winchester Model 62A
Winchester Model 62A

Favorite Firearms: Granddad’s Winchester Model 62A

Once his grandfather's gun, a pump-action Winchester Model 62A became a personal favorite for NRA member Mike Shoemake.

Preview: Breek Arms Warhammer Micro Mod2 Charging Handle

Breek Arms recently introduced a Micro version of its Warhammer Mod2 AR-15 charging handle, with shortened bilateral latches that measure 2.4" across both handles.

Post-Black Friday 2022 Industry Deals

There is no shortage of post-Black Friday deals. Judging by the ongoing deals, gun and gear makers have no intention of losing that momentum for the 2022 holiday season.

The Armed Citizen® Dec. 5, 2022

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

Original & New: 100 Years Of FN CCW

In 1922, Fabrique Nationale unveiled the result of efforts to modernize its Model 1910, providing a cutting-edge handgun that, at the time, was expected to attract military and police contracts across Europe.

LC Carbine: Ruger Doubles Down On The 5.7x28

Ruger refers to the LC Carbine as a pistol-caliber carbine (PCC), and from the company’s very specific perspective—the LC being a derivative of and companion to its Ruger-5.7 pistol—it is.

Interests



Get the best of American Rifleman delivered to your inbox.