[Preacherman]

The discussion about the Marine Magnum barrel band in this thread (http://www.thehighroad.org/showthread.php?t=179174) got me thinking.

I agree with the importance of clamping a magazine extension to the barrel to avoid potentially catastrophic separation of extension and magazine when knocked around. I've also seen slings attached to these clamps, which seems like a logical place to put the sling mounting point. However, there is a persistent "rumor" (for want of a better word) out there that these magazine-extension-to-barrel clamps can affect the weapon's zero, which will change every time the clamp is removed (for cleaning and maintenance) and re-attached. Just as an example (I've seen it in many other places too), GG&G state the following on their Web site for their sling clamp devices (http://www.gggaz.com/products/shotgun_slingatt.php):

... experience has demonstrated that the use of sling clamps that go around both the barrel and magazine extension tube of a fighting shotgun will spread the barrel and tube by varying amounts, depending upon how much tension is applied to the clamp’s retaining screw. As this is difficult to gauge, the gun’s zero will change every time the clamp is removed for maintenance and then reinstalled.

Can anyone speak to this? I don't see how a clamp such as this would affect weapon zero, particularly if one puts it back in the same place every time, and under approximately the same tension. Oh, I can understand that if the clamp were wrongly sized, so that, if too large, it pushed the magazine extension tube and barrel apart, or if too small, that it pulled them together, this could certainly produce a change in zero: but I've not encountered this problem before.

I'd be interested to hear from those who've done this, as to whether or not there's any truth to this often-repeated "fact".

[Scottmkiv]

I could see how someting like this would be a problem with long range slug shots (100+ yards) but I can't imagine it changing things enough to matter at double or even triple your average defensive shots. Who cares if you are an inch off at 20 yards?

[Preacherman]

Interesting... I posted this same question to the API List, and got some good replies there.

One poster referred me to Fr. Frog's Shotgun Page (http://www.frfrogspad.com/shotgun.htm) (Fr. Frog's pages (http://www.frfrogspad.com/) are very worthwhile firearms reading, if you haven't encountered them before), where he states the following:

Besides the barrel itself, another item that can affect patterning and group sizes is the hanging of extension magazines on the shotgun. A poorly fitted extension magazine or one with a misaligned clamping strap can drastically alter your weapon's performance. During the class I found that a misaligned clamp was moving my point of impact over eighteen inches to the left with both slugs and buckshot. It also opened the shot pattern by almost three inches and the fifty yard slug groups by three to four inches!

Even with a properly fitted extension magazine and clamp, the tension of the clamp screw can alter your point of impact. Always replace the clamp in exactly the same position and tighten all screws to the same tension each time. If your shotgun is so fitted, check its performance with and without the clamp. You will no doubt be surprised at what happens.

If the extension tube is properly manufactured and fitted very tightly there is really no reason to have to use the clamp. However, it does serve to keep branches or other debris out of the space between the barrel and magazine tube when creeping through the woods and as a sling mounting point. If your extension tube has a tendency to work loose a set screw can be added to hold the tube in place.

However, most fighting shotguns are fitted with a sling and thus need the clamp. If your clamp is one of the one-piece units like the Remington factory one or the Uncle Mike's one piece you might want to try this trick. Adjust the upper part of the clamping strap to give a 32nd of an inch or so of clearance (the thickness of a matchbook cover works fine) on either side of the barrel when the bottom half is very tight on the magazine tube. You may also want to carefully fit a piece of steel tubing over the clamp screw and between the sides of the clamp to prevent over tightening. This modification has worked very well on several shotguns I am familiar with.

Except possibly for prolonged military engagements, extension magazines do not provide any real tactical advantage especially if one learns the "shoot one - load one" drill. They are probably best avoided, but unfortunately they are the "in" thing these days just like high capacity magazines in semiautomatic pistols.

Another respondent stated:

I attended Randy Cain's shotgun class in July 2000 and I distinctly recall him advising against the clamps, even those without the sling attachment. This is why the wilson/scattergun is built substantially thicker than most others such as the choate. The wilson is stable enough as is and doesn't need a support clamp.

When Randy speaks about shotguns, I listen.

So it looks like there's definitely something to this theory, even though on my clamped-extended-mag 870 in South Africa, it never posed a problem.

[Fred Fuller]

I haven't had any problems with a bunch of them on various shotguns over more years than I like to remember. It CAN happen of course, but given proper care and experimenting with a given setup before getting serious it shouldn't be an issue. I wouldn't put a two-shot or three-shot extension on a gun WITHOUT a clamp, though.

lpl/nc

[Chris Rhines]

I think that if a clamp/magazine tube support were properly made, any impact shift could be minimized.

Most of the variation in POI that comes with spring-type mag clamps comes from different amounts of torque and different locations on the barrel. This causes the barrel to deflect in a way that's tough to predict.

If I were going to make one, I would take a block of 7075T6 aluminum and machine it to profile (leaving it open at the top), then bore two holes, one for the barrel and one for the mag tube. Each one would be reamed to 0.001" over the tube diameter and would fasten to the tube with a couple of set screws, say three for the barrel and three for the mag tube. I'd also put a couple tiny detents in the mag tube so the support could be installed at the same place each time.

- Chris

[fellow14]

I would say that it would not be significant if it happened at all. Benelli put clamps on their shotguns that came with +4 extensions I can't imagine any reputable company doing something like that if it would be bad for accuracy. Especially a company that prides itself in making better than average shotguns. I would say if it was going to make a significant shift in POI it would probably happen on guns with long (28") barrels and longer (>+4) extensions thus when fully loaded hanging more weight off the barrel.