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

NRA Annual Meetings & Exhibits, Atlanta
NRA Annual Meetings & Exhibits, Atlanta

The Rifleman Report: New Developments

Our official coverage of new products for the current year is set for next month, but we included two substantial new developments in this issue that have the potential to make significant ripples in the firearm business even before then.

Arkansas To Begin Mandatory Gun Safety Lessons In Schools

Arkansas Gov. Sarah Huckabee Sanders signed into law requiring public and open enrollment public charter schools to provide age-appropriate firearm safety instruction beginning during the 2025-2026 school year.

Adams Arms Set To Relaunch

Alexandria Pro-Fab, a contract machine shop based in Minnesota, announced that it has purchased the assets of Adams Arms, along with all of the company’s intellectual property, and it plans to relaunch the company in the second quarter of 2025.

I Have This Old Gun: Bayard Auto Pistol

The Herstal, Belgium, firm of Anciens Etablissements Pieper was apparently expecting big things when, in 1908, it named its new semi-automatic pocket pistol “Bayard.”

The Armed Citizen® March 31, 2025

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

New For 2025: Benelli USA Nova 3 Tactical

This latest update to Benelli’s pump-action shotgun gives users a few upgrades over previous generations, making the new NOVA 3 the most exciting pump-gun yet to come from the Italian maker.

Interests



Get the best of American Rifleman delivered to your inbox.