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 60A
Winchester Model 60A

Favorite Firearms: A Winchester Model 60A Named “Li’l Ole Bess”

My favorite firearm has been in my family for four generations. It’s a Winchester Model 60A single-shot, bolt-action, .22-cal. rimfire chambered for the Short, Long and Long Rifle cartridges.

Review: Winchester Model 70 Long Range MB

It is not uncommon for useful technical improvements in one arena to be adopted by and integrated into another. Materials and manufacturing processes originally devised for the aerospace industry are now commonly used to manufacture firearms.

Editor’s Choice: Ruger Wrangler Birdshead

Sturm, Ruger & Co. is expanding its handy and affordable Wrangler single-action .22 Long Rifle revolver line with a more compact version featuring a gracefully curved bird’s head grip and shortened barrel.

New for 2022: Marlin Model 1895 Trapper

Marlin added to its classic lever-action rifle line with the launch of the Model 1895 Trapper, chambered in .45-70 Gov't.

Beretta Offers Discount For Military, Law Enforcement And First Responders

Beretta USA is offering a 15-percent discount on its website to those who serve in the military, law enforcement and as first responders.

The Armed Citizen® May 16, 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.

Interests



Get the best of American Rifleman delivered to your inbox.