Though it’s nearing its 65th anniversary, the Remington 870 is still one of the most
versatile pump shotguns on the market today. One of the attributes that makes this shotgun
so flexible is the way its barrel is mounted. While many competing shotguns like the Ithaca
37 and Mossberg 500 have barrels that attach to the end of the magazine tube, the 870’s
barrels attach around the magazine tube, leaving its front end unobstructed.
And because all 870 magazine tubes are open-ended, their capacity can easily be expanded by simply
replacing the magazine cap with a magazine extension.
Unsurprisingly, magazine extension options abound for the bestselling 870.
Between factory Remington extensions and offerings from a slew of aftermarket companies, there
are well over a hundred different kits to choose from, with capacities ranging from
+1 to +18, and everything in-between. Though the construction and features of these
extensions vary, their basic designs tend to fall into two major varieties,
single-piece and two-piece.
It’s very helpful to understand the distinction
between these two types, as both have their respective advantages and disadvantages, and
choosing the best one for your purposes requires these attributes to be considered in the context
of the rest of your build.
Now, you might be confused by the terms “single-piece” and “two-piece”,
since an extension kit will usually include more than just one or two parts.
When talking about single- or two-piece extensions,
we’re not looking at the entire kit, just the two main functional elements of the extension
itself. These elements are the female threaded portion
that screws onto the magazine tube and secures the barrel, and the extended tube section
that provides the additional capacity. With single-piece extensions, these two components
are either machined as one seamless part, or separately-made parts that are permanently
attached. Either way, the result is one rigid item that
screws onto the end of the gun’s magazine tube and is tightened against the barrel guide
ring just like a magazine cap. Two-piece extensions break up these elements
into two separate components - a coupling that screws on like a magazine cap,
and then an extension tube that threads into the coupling.
The biggest advantages of single-piece extensions
are their simplicity and ease of installation. Because they’re one rigid piece, they’re
pretty much just elongated magazine caps, and the entire thing can lock down against
the barrel guide ring by engaging the barrel’s locking detent, or by using a lockwasher.
No need for a separate clamp if you don’t want to use one.
This makes them very easy to install and remove; tools are not usually needed.
But this simplicity comes with a trade-off. When installed, single-piece extensions tend
to have an internal gap at the joint between the base magazine tube and the extension’s tube.
This is due to the need for any cap or extension
to be able to be tightened against the barrel’s guide ring.
If the barrel is not properly secured, the gun will be unreliable and potentially unsafe.
Due to variations in the manufacture & attachment of the magazine tube, barrel, and barrel guide
ring, along with differences between factory & aftermarket barrels and receivers, the length
of magazine tube thread in front of the barrel guide ring can vary slightly; and magazine
caps and extensions must be designed with some amount of extra thread travel to ensure
full compatibility. For magazine caps, the gap that results from
this excess space is a non-issue because it’s at the very end of the magazine, and only
a few spring coils ever encounter it. But with magazine extensions, shell rims and
the magazine follower have to pass back and forth across this transition, and they can
hang up in this gap. Installing accessories underneath the extension
will make this gap even wider, and can increase the likelihood of hang-ups.
Good extension design and proper follower selection can mitigate the effects of this
gap, and ensure reliable function while avoiding compatibility issues.
But not all extensions are designed as well as they could be, and some types of followers
just don’t handle even small gaps reliably. If you’re going to use a single-piece extension,
especially a +2 or longer kit, it’s very important that you test it thoroughly with
whatever other magazine parts you’re planning on using.
Two-piece extensions, with their separate extension tubes and couplings, can allow for
a truly seamless internal transition if the extension is designed so that the two tube
sections butt directly against each other inside the coupling.
As long as the extension has enough thread, this joint will remain tight despite variations
in the length of exposed magazine tube, or the presence of under-cap accessories.
The two-piece design also makes these extensions much more versatile than single-piece kits.
If you use your shotgun for multiple applications that call for different magazine capacities,
a two-piece extension system can let you change extension tubes while keeping the same coupling,
bracket, end caps, and sometimes even the spring.
Many two-piece extension systems offer a variety of interchangeable extension tubes, and you
can make custom-length tubes relatively easily by shortening and capping a longer or unfinished tube.
Some systems with detachable extension tube caps
even let you chain multiple extension tubes together with additional couplings.
This modularity can also make repairs easier and less expensive, since any damaged components
can be individually replaced. If you want more configurations or need to
remedy irreparable damage with single-piece extensions, you’ll need to purchase additional
complete extension kits. And because a single-piece extension is in
essence a long magazine cap, the gun cannot function properly if it is removed, or works
loose under recoil. With two-piece extensions, the coupling does
all the work of holding the barrel on the gun, so the barrel does not have to be loosened
or removed when changing extension tubes. And if the extension tube works loose, it
won’t affect the gun’s core functionality.
In an emergency, the gun can even be used
as a single-shot without an extension tube installed.
So why aren’t all extensions two-piece if this system is so much more reliable and versatile?
Well, as always, there are trade-offs. Unlike single-piece extensions that can be
simply screwed on and off by hand like magazine caps, two-piece extensions are a fair bit
more involved to install and remove. The coupling and extension tube need to be
installed separately, and because only the coupling is secured against the barrel guide
ring, the extension tube will tend to back out under recoil.
To keep it in place, the tube often must be clamped to the barrel.
These clamps add weight and complexity, and usually require tools to remove or replace,
so you’ll need to have the right screwdriver or allen wrench on hand whenever you want
to take your gun apart. And you may have to pull off other parts and
accessories when installing or removing some clamps.
It’s also important for the clamp to be properly matched to the size and spacing of
your build’s extension and barrel, which can be difficult with heavy-profile or other
nonstandard barrels. Finally, not all two-piece extension kits
actually come with clamps, so this may be something you'll have to purchase separately.
Now, these clamps do help to reinforce the extension, and they’re a popular method
for mounting accessories; so for a lot of builds, this requirement may not be that big
of a deal. But there are other cases where a clamp isn’t
really necessary, and being forced to use one just makes things more complicated.
And while the butted internal mate of two-piece extensions is seamless and hang-up-free in
theory, it can be rather finicky in practice. Single-piece extensions are intended to function
with a gap here. Assuming they’re designed well and paired
with the right follower, a small amount of additional misalignment or gap width is not
a big deal. But because the tubes in two-piece extensions
are meant to fit right up against each other, even small gaps or discontinuities at the
joint can lead to hang-ups. In addition to good design and manufacturing,
proper installation is essential for achieving a good mate, and there are a number of potential
mistakes that can leave a gap inside the tube. Not threading the extension tube all the way
into the coupling, placing an overly-thick sling plate under the coupling when the extension
tube doesn’t have enough thread to make up the difference, snagging a spring coil
between the tube sections during installation, using a clamp that doesn’t match the size
and spacing of the mag tube and barrel; all of these are common causes of loading and
feeding problems that I’ve seen inexperienced owners run into.
Now, some two-piece magazine extensions may use a different joint concept where instead
of tightening against the magazine tube, the extension tube tightens down against an internal
ridge or lip that’s part of the coupling. Regarding advantages and disadvantages, this
type of two-piece extension would retain all the characteristics of other two-piece extensions,
except that its internal mate with the magazine tube would be much more like the mate of a
single-piece. I’ve yet to come across this type of two-piece
extension for the 870, but since I haven’t personally inspected every extension kit ever
made, I figured I’d mention the concept.
So as you can see, each type of extension has its respective advantages and disadvantages.
Single-piece extensions are fast and easy
to install and remove, and they will work with any 870 barrel out there; but because
they’re unable to adjust to normal manufacturing tolerances or the presence of certain accessories,
there will almost always be an internal gap between the tube sections that can cause hang-ups
if the extension is poorly designed, or the wrong follower is used.
And because the extensions are all one piece, the entire thing has to be replaced if it’s
damaged, or if you want a different capacity.
Two-piece extensions give you a more seamless
internal transition, along with a modular system that allows you to add additional extensions
or remedy damage for less expense. On the other hand, their reliability is highly
dependent on proper installation, and it’s easier - especially for inexperienced owners -
to make mistakes there. Their need for a reinforcing clamp or bracket
also adds complexity, and can make it difficult to pair them with nonstandard barrels.
Which one is the better choice for you? Well, that depends on a lot of things; the
specifics of your build, its intended application, and even your personal preferences.
Hopefully this video will get you started on figuring things out for yourself, or at
least help you ask the right questions while you’re planning your build.
I’ll be uploading more videos on various aspects of 870 magazine configurations in
the coming months, so if you’re from the future, you can check those out.
And as always, feel free to ask in the comments below or post over on the Rem870.com forums
if you have questions.
Until next time please have fun, and shoot safely and responsibly.