Understanding shotgun bores, gauges, chamber sizes, and cartridge lengths


Bores & Gauges

The term “bore” refers to the diameter of the barrel. “Gauge” refers to the volume of lead that will fit down the bore:

  • A 12 gauge will fit a solid sphere/ball of lead that weighs 1/12 of a pound;
  • A 20 gauge will fit a solid sphere of lead that weighs 1/20 of a pound.

The higher the gauge size, the smaller the spheres of lead

A shotgun “bore” is measured in inches, and only a .410 shotgun uses this actual measurement.

A 12-gauge shotgun’s bore is between 0.720″ and 0.740″ so the terms “12 bore” isn’t actually correct even though it was a common term until recently.


The chamber refers to the part of your shotgun where the cartridge will sit, and the chamber size refers to the length of the fired shotgun cartridge.

It is important that you know what size your chamber is as putting a cartridge in that is too big for the chamber can be very dangerous. The pressures will be too great for the gun and it could literally explode in your face.

Modern shotguns typically have a 3″ chamber (76.2mm) so will happily take 70mm cartridges. You can shoot a 65mm cartridge in a 70mm chamber, but you cannot shoot a 70mm cartridge in a 65mm chamber.

Shot size & volume

The higher the number the smaller the size of the shot, and the volume or payload of the shot is expressed in grams. For example, a typical clay shooting cartridge would be a 12-gauge cartridge loaded with 28 grams (1oz) of No.7 1/2 shot.

Generally speaking, the greater the weight of the shot the greater the recoil will be (remember your physics). Recoil is complex and will depend on various factors, however, including type and weight of the shotgun.

Whether you are looking for clay shooting, archery, quad biking, rage buggies or team building events, we are at hand to help and advise. Contact Andy Tarr on 07727 238 526 or email Safe and Sound Clay Pigeon Shooting at andy@safeandsoundclaypigeonshooting.com.