Spomer on Shooting: Rear-Locking Lugs Explained
In the world of firearms, the phrases rear-locking action and rear-locking lugs has nothing to do with brakes on a motorcycle or the backdoor of your house. Both refer to the location where a gun's breech locks to seal cartridges in a firearm's chamber. In general, rear-locking lugs make rifles inherently less accurate than front-locking lugs but not always.
Most of us have seen the bolt from a Mauser, Winchester M70, Remington M700, or similar bolt-action rifles. They show two steel lumps, bars or lugs near the front of the bolt body. When you push the bolt handle forward on a rifle like this, it picks a round from the magazine and pushes it into the chamber. When you're fully forward and shove the bolt handle down, those two lugs are turning into recesses in the front of the receiver ring (or back of the barrel on some rifles), locking the chamber like a vault. This, of course, holds in the expanding powder gases upon firing, seals the breech against powder blow back, and forces all energy out the muzzle, which is what gives a bullet its velocity and kinetic energy.
Because these locking lugs are at the front of the bolt body, they are known as front-locking lugs. Some old military rifles, such as the Krag-Jorgensens used by the U.S. military in the Spanish-American War, had just one forward locking lug and usually some kind of backup or safety lug elsewhere on the bolt body.