The Miracle of Detachable Box Magazine Shotguns

Feeding shotshells reliably from a box magazine has long been the goal of gunmakers. Find out why it has been such a difficult engineering hurdle.

A Remington 870 DM with hardwood furniture is also expected next year.
This Remington 870 DM with hardwood furniture is expected to hit stores next year.mfg photo

The big news in the gun industry right now is the introduction of the Remington Model 870 pump action shotgun with a detachable box magazine. The addition of a detachable magazine adds a new dimension to how a shotgun can be used.

Reloads and Unloading

It allows rapid reloading of course, but it also allows quick change of ammo. In a tactical situation that means you can change from buckshot to slugs. In hunting you may be out chasing upland birds when you scatter a flock of fall turkeys and need to replace your #8 shotshells with magnum #4 shot. A detachable box magazine allows the easy switch of ammo without working the action multiple times to remove the existing ammo in the gun.

It’s also much safer to unload the shotgun by removing the magazine and opening the action to eject the chambered shotshell, than it is to cycle the gun multiple times to empty the tube magazine.

So the question that begs to be asked is, why now? How come we did not see a detachable box magazine in a pump action shotgun from a mainstream manufacturer years ago?

Feeding Shells

The answer is in the ammo. Shotshells are a gun engineer’s nightmare. While rifle cartridges are made from metal, shotshells are not (save for the expensive, full brass hull shells out there). They are made of plastic, which catches on anything and everything when trying to feed them from a metal or polymer magazine into a steel shotgun chamber.

Metallic cartridges with spitzer shaped bullets are pointed and often tapered; shotshells are cylindrical with a blunt leading edge. Sometimes they can even be mushroom shaped.

There is a large variation in the shape with the various shotshells as well. The shape of the star crimp on a target load can create a much different nose profile than the roll crimp used on a slug. All these are factors in the feeding of a shotshell through from the magazine to the chamber.

An example of a Saiga 12 autoloading shotgun, which is fed from a detachable box mag.
An example of a Saiga 12 autoloading shotgun, which is fed from a detachable box mag.photo from rockislandauction.com

Shotshells have a rim, which is necessary for them to headspace in the chamber of the shotgun. Rimmed shells do not work well in a box magazine as they can hook over one another and jam up the works. To work correctly, the rimmed shells must be carefully fed into the magazine so the rims are stacked back to front to allow feeding. Then they must stay in that configuration. It all sounds much simpler than it is in the real world.

Another consideration is length. Common shotshells today range from 2-3/4 inches to 3-1/2 inches in length. Even at that, there is no consistency.

One brand of 2-3/4 shotshell may be a different actual length than a competing brand.

Finally, there is the weight of the shotshell. A 2-ounce 3-inch turkey load weighs much more than a 7/8 ounce 2-3/4 inch target load. This is a big factor in how a box magazine is able to feed the shotshells up through the magazine to the action.

Sidewinder Venom kit from Adaptive Tactical on a Mossberg 500 shotgun
The Sidewinder Venom kit from Adaptive Tactical on a Mossberg 500 shotgun with a 10-round drum magazine. The aftermarket product replaces the shotguns magazine tube and adds a magwell in front of the receiver. It functions well, but adds considerable weight to the shotgun and the positioning of the magwell forces the support hand into a someone overextended position.file photo

Recoil

Another huge factor is recoil. Recoil forces can wreak havoc with the positioning of the shotshell in a box magazine, which in turns messes up the feeding process.

Shotgun designers solved the problem of feeding shotshells in repeating shotguns years ago by using simple geometry and mechanics to manage the shotshell through the feeding process. The shotshells are fed out of a tube magazine where they are stacked nose to tail. They tend to behave well in these magazines, as they are already in line with recoil forces so that the energy imparted to any shotshell in the magazine by recoil has little to no effect in their positioning relative to the feeding process.

When the shotguns cycles, the shotshells move in a straight linear motion from the magazine on to a carrier which then rises up to align the shotshell with the chamber of the shotgun. The shotshell is then pushed forward with a reverse linear motion into the chamber and when the bolt locks into place the gun is ready to fire.

The six- (left) and three-round magazines from Remington engineered for the new line of 870 DM shotguns.
The six- (left) and three-round magazines from Remington engineered for the new line of 870 DM shotguns.mfg photo

Feeding from a box magazine is much different. The ammo is stacked one on top of the other in a vertical column. The top shotshell, the one that must feed into the chamber, is positioned well below the chamber of the gun.

The shotshell is angled so the front or leading edge is higher than the rear. The shotshell is captured in the box magazine by the feed lips and must be pushed forward to become free from the magazine. As it’s pushed forward the front of the shotshell must move up to the chamber, followed by rear of the shotshell until it is parallel enough with the chamber to feed into it freely. There is no lifter to position the shotshell, so it must be guided mechanically to not only rise higher inside the action, but to move to a parallel position to the chamber.

Remington Debuts 870 Shotgun with Box Magazine: Full Review and Range Test
The M870 pump gun has gotten a substantial update with a new version that feeds from a quick-change detachable magazine. A full line is coming in 2018, including a TAC-14 variant.
Remington Debuts 870 Shotgun with Box Magazine: Full Review and Range Test

Because of their shape and blunt nose shotshells cannot enter the chamber with any tilt, as many metallic cartridges can. They also do not follow directions well as the plastic will catch, drag and generally act like a child having a tantrum as they are guided along through the feeding process.

For these reasons, some drum magazines have proven more reliable in some shotgun platforms, but they are large, heavy, often difficult to load, and usually quite expensive.

Feeding from a detachable box magazine is a complicated process that requires a lot of geometry, mechanics and a little voodoo to accomplish. When you add in all the variables with shotshells, it becomes a nightmare.

The Fix

Still, it would seem that Remington has figured it all out. I recently spoke with a couple of Remington engineers; Andrew Haskin, Director R&D Long Guns and Vincent Norton, Principal Design Engineer on how they managed to accomplish what was before thought to be impossible.

They explained that they were able to use new technology, including high speed photography in conjunction with computer modeling, to identify and predict the issues that would and did occur. From that, they were able to engineer solutions to the problems. They modified the Model 870 shotgun to use detachable box magazines and engineered the magazines themselves as well.

A hunter version of the 870 DM is coming next year, with a camp finish and thumbhole stock.
This hunting version of the 870 DM is coming next year, with a camo finish and thumbhole stock. It will ship with both a six-round and a three-round magazine.mfg photo

The magazines are able to control the shotshells, even with length variations and recoil forces, so that the top shotshell in the magazine presents correctly each time to feed. The shotgun design was modified to work with the magazine system to strip the top shotshell from the magazine and feed it into the chamber of the shotgun.

Reports are that they have it working very well and that this detachable box magazine system is every bit as reliable as the conventional 870 design. If that’s true, this is pretty revolutionary stuff. I would expect that copy-cat guns will follow soon and that the market will offer several detachable box magazine shotguns to pick from.