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.

The front of this bolt shows its dual- locking lugs protruding from either side. They turn into recesses within the front receiver ring to seal the bore and force all pressure — and the bullet — out the muzzle.
The front of this bolt shows its dual- locking lugs protruding from either side. They turn into recesses within the front receiver ring to seal the bore and force all pressure — and the bullet — out the muzzle.

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.

Modern Bolt-Actions Have Three Front-Locking Lugs

Modern bolt-actions, such as the Browning A-Bolt and X-Bolt among many others, have three equadistant front-locking lugs. While not necessarily stronger then dual lug set ups, they do provide a shorter bolt lift of 60-degrees instead of 90-degrees. The Weatherby Mark V action uses nine small, staggered lugs in three rows of three each. A smaller Weatherby action uses six staggered lugs in rows of two each.

Blaser R8 use a unique, 360-degree rotary lock up featuring 14 tiny lugs that engage a 360-degree recessed rim just behind the barrel chamber. A camming action inside the bolt body flares these lugs outward to secure the lock up. Most pump-action and autoloading rifles lock with a rotating bolt head that turns into locking recesses when the bolt is fully forward.

Regardless of how many lugs, how much they turn or flare, they are all considered front-locking lugs.

Rear-locking actions have one or more locking lugs closer to the back of the bolt where they turn into locking recesses in the rear receiver ring. A historically famous military rifle with rear-locking lugs is the British Lee Enfield. Most lever-actions rifles, such as the Winchester M94 and Mossberg 464 .30-30, lock at the rear of their long bolts. When the lever is completely closed, the rear of the bolt cams up to lock in the action wall or a breech block at the back of the bolt lifts up to lock it.

The Ruger M77/17 bolt appears to have its dual locking lugs in its middle, but they're really at the front of the bolt. The flat extension is the real breech block that seals the chamber.
The Ruger M77/17 bolt appears to have its dual locking lugs in its middle, but they’re really at the front of the bolt. The flat extension is the real breech block that seals the chamber.

Rimfire rifles frequently used some sort of rear-locking lug, often the bolt handle itself. Ruger’s popular M77/17 rifles in .22 LR, .17 HMR, .22 Win. Mag., and now .17 WSM, lock with dual lugs that are technically at the front of the bolt body ala the Win. M70, but an extended breech block attached to the front of the bolt face actually closes the breech, and this block can be more than 2 inches long. The locking lugs actually seal in the rear receiver ring.

True Accuracy Nuts Like Front-Locking Lugs

Obviously all these locking systems work, so what’s the difference? Probably not much, but true accuracy nuts insist on front-locking lugs for the absolute best chance for optimum accuracy. With front lugs, there is a solid and immediate tension with almost no chance for the bolt body to stretch or bend slightly when the shot is fired.

Rear-locking lugs theoretically allow a long bolt to flex, and that might compromise accuracy. Some claim that rear lugs are also not as strong as front lugs, but, again, manufacturers must pass rigid and stringent safety tests. I doubt they’d be selling guns if the locking mechanisms weren’t perfectly safe.

Should you worry whether your guns have front or rear-locking systems? Now that you know the difference, you get to decide.

Make sure you visit Sportsman’s Guide today for a great selection of gun parts and bolt-action rifles!

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