Guns & Ammo© April, 1998
The other
day I was visiting a firing range and happened to look down and saw a
funny-looking case. I picked it up and looked at the head stamp. It was 7mm
Remington Magnum. But the neck was far too short and the neck diameter, at
least of what there is of it, was a good fit on a .308 bullet. What
happened?
Actually, if you've seen this sort of thing a few times before, it isn't
all that much of a mystery. Look at the photo of the case beside the "real"
7mm RM and the .300 Winchester. Notice that the shoulder of the "fire formed"
case is exactly where the shoulder is on the .300 Winchester. Notice also
that the overall length of the "fire formed" case is very close to the same
as the real 7mm RM. It's pretty obvious what has happened. Someone fired
a 7mm RM case in a .300 Winchester rifle.
Is he lucky he didn't blow his fool head off? In a way, yes. Using the wrong
ammo in any gun is a way of inviting disaster. But in this particular situation,
there wouldn't have been much of a problem except the accuracy would have
left something to be desired. A .284 bullet doesn't fit in a .308 bore very
well. It isn't going to get any spin or stability. For what it's worth, velocity
and pressure are both going to be lower than factory numbers for either
cartridge.
There are a whole host of combinations of right gun and wrong cartridge.
Some are just sort of funny, like the example above. Some can be dangerous,
very dangerous indeed. Take for instance a .350 Remington Magnum (the very
short "fire plug" magnum) in a .300 Winchester rifle. Will it chamber? It
sure will, and the results would be pretty bad when the pressure tries to
size a .358 bullet down to .30 caliber. I wouldn't even shoot that combination
in a remote fired pressure barrel unless I didn't want that particular barrel
any more.
Lets
take another, milder example. What about shooting a .308 in a .30-06? About
all that happens is that you get a pretty funny looking case. You might even
get pretty fair accuracy because the bullet at least fits the barrel. Hold
it a moment, you say, what about head space? Both the .308 and the .30-06
headspace on the shoulder, so will the .308 just fall into the .30-06. Two
things keep that from happening. The head diameter of the two rounds are
.473 inch, so the .308 will start to wedge into the '06 chamber before the
head gets very far past the breech end of the barrel. In guns of the Mauser
type, the powerful extractor is usually enough to hold the case head back
against the bolt so it can be fired. The photo of the .308 case "fire formed"
in a .30-06 chamber shows what this combination produces.
The .300 Savage will fit into a .308 gun, but the .308 won't chamber in the
.300 Savage. Actually the .300 Savage isn't a terrible fit in the .308. It
isn't right and shouldn't be tried, but this would be one of the most benign
"wrong" combinations you could have.
The Air Force and the Navy each have 20mm guns. It surprises a lot of people
to learn that the two services, at least during the Vietnam War, had different
kinds of 20mm ammunition. The rounds compared about like the enlarged version
of a .308 and a .30-06. Eventually someone wondered what would happen if
the Air Force (shorter) ammo was fired by accident in a Navy gun. In the
demonstration test that was run, the Air Force ammo came out of the Navy
gun looking just like the .308 from an '06. It wasn't especially dangerous
except that the gun wouldn't have worked properly and wouldn't have done
it's job. That could have been dangerous to the pilots who were depending
on the gun for fire suppression.
Let's make one thing very clear here. We're certainly not recommending that
anyone ever fire ammunition in the "wrong" chamber. The results are never
going to be great, and they could be catastrophic. Our purpose here is to
show what some of these combinations produce, so that if you ever see cases
coming out of someone's gun that look like these you might be able to get
things shut down before something bad happens.
Most of the really bad combinations involve a case that's shorter than the
chamber and a bullet that's bigger than the bore diameter. With reloaded
ammo, loading with a short overall length (very light bullet) makes some
combinations possible where the factory product wouldn't fit. Reloading
introduces another way to get in trouble with the wrong combination. If you
don't crimp the case mouth and the bullet isn't a real tight fit in the neck,
it can be pushed back into the neck as the bolt closes. A round that simply
won't chamber with a tightly held bullet sometimes can be chambered if the
bullet will telescope back into the case.
The belted magnum cases that are based on the .375 H&H head size (.532")
offer somewhat greater potential for mixing cartridge and gun, because they,
at least in theory, headspace on the belt. A shorter case (like the 7mm RM)
will drop into almost any longer magnum chamber and look like it fits perfectly.
That's exactly what happened with our first example. I found three of these
cases when I started looking and suspect the shooter didn't know he had a
problem until after three shots he realized that he couldn't hit the paper.
We
haven't mentioned shotguns. What happens when a 20 gauge case is dropped
into a 12 gauge bore and then a 12 gauge is put into the chamber and fired?
The results shown in the photo are what usually happens on the first shot.
(you don't get two or three tries at this) It's very easy to leave a "wrong"
size shell in your pocket and then get it into the gun by accident. If you
own more than one gauge of shotgun, you always have to be on the alert for
this all to common serious problem.
In a way, it is surprising that with all the ammo available today and all
the potentially bad combinations that exist there aren't more accidents.
Of course, if you owned one gun and have only the "right" ammo on hand, there's
no danger. But the shooter who owns a number of guns must always be on the
alert to prevent this kind of situation.
Fireforming of "improved" cartridges from the basic case is really a different
situation. I suppose it could be argued that fireforming any wildcat cartridge
out of some factory cartridge is an example of shooting the wrong ammo. But
this is hardly the same thing at all. In both the improved cartridge
and the true wildcat situation, we depend on the hope (but perhaps not the
fact) that the designer of the wildcat cartridge took a careful look at the
dimensions of the combination before he recommended the case forming process.
Usually, he will also recommend some procedure to maintain at least a temporary
control of headspace during the fireforming shot. True fireforming isn't
the same thing at all as just dumping a .308 into a .270 caliber gun (very
bad news) and wishing for the best.
There is at least one example of case-gun mismatching that's done frequently
and usually done on purpose. Lots of people use .38 special ammo for practice
in a .357 magnum revolver. Especially with target loads this doesn't seem
to do anything bad, and many guns shoot just about as well with the .38 specials
as they do with the .357 magnums.
Factories do try very hard to design new ammo with dimensions that at least
minimize the ways that people can do stupid things with their product. But
there are just too many cartridge shapes, and anyone in a moment of distraction
can make up a bad combination. It's up to us as shooters to get things right.
There's a very simple test to decide who has the best incentive to use the
correct ammo. Ask yourself who has his eye right behind the gun.
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