Sherman tank cannon projectile, WWII, 75mm M61, $5.

Collectibles from the Second World War are hot! But they're not always easy to figure out. So, when you find something you can't quite identify, you should look for writing on it, and this is especially true for old war memorabilia and collectibles. If you can't find any writing, look for numbers. The only markings on this Sherman tank cannon projectile are on the copper driving band: 1944 75MM M61. What does all that stuff mean?

1944. Since the military has always loved to put dates on their stuff, always look for production dates, inspection dates, expiration dates or dates that could help give you any kind of time 'window.' The United States was involved in the Second World War from the time the Japanese bombed Pearl Harbor in December of 1941 until the U.S. dropped the atomic bombs on them in August of 1945, and since 1944 was right there in the middle of hostilities, it probably indicates the year our projectile was made.

75mm. Some of us who were out of school long before the metric system gained such popularity in the U.S. still suffer fits when we try to convert back and forth between the two systems. All that converting between feet, pounds, litres and centimeters makes us want to scream, but the one little trick that makes my life easier on a regular basis is knowing that there are 25 millimeters in an inch (actually 25.4, but 25 is close enough.) This tidbit of knowledge comes in handy once more here because it told me right off that the '75mm' here most likely indicates this projectile's diameter, because it looks to be exactly 3" across.

M61. Okay, The 'M' here simply has to stand for military, because the U.S. Army seems to name just about everything in its inventory something or other starting with 'M.' All I could assume is that the M61 here was the Army's designation for this particular tank round. I'd have to hit the Internet search engines for this one.

As it turned out, what I'd bought at that yard sale was a 75mm tank projectile from a World War II vintage M4 (There's that 'M' thing again) Sherman tank, which was the main battle tank used by the U.S. forces in the war. As used in the Sherman, this projectile would be mounted atop a large brass case that held the powder (see the small photo inset,) looking like a much larger version of a standard rifle cartridge. This particular projectile, the 'M61,' is known as an APC, or 'Armor Piercing Capped' round. The 'AP' part of this designation refers to armor piercing because unlike 'high explosive' tank rounds that rely on a focused explosive charge to literally burn a hole through an enemy tank's armor, armor piercing rounds simply rely on kinetic energy to punch a hole through the target, just like a huge rifle bullet.

At the very rear of this projectile, you can see a hollow cavity where an explosive charge and fuze were intended to be screwed into place. As you can see, the charge is missing, rendering this round an inert chunk of steel. Interestingly, the real purpose of this explosive charge was to ensure that the projectile would fragment right after penetrating the target, thus providing a sort of 'shrapnel effect,' much like a hand grenade tossed into a room. Note that the explosive charge doesn't help the projectile defeat another tank's armor at all!

The 'C' part of this shell's APC designation stands for capped, for the thin steel cap over its front end. This cap's sole purpose is to provide a pointed, aerodynamic form to help the projectile maintain its velocity and energy enroute to the target. On impact with anything, this pointed cap is instantly obliterated, allowing the projectile's blunt point to make contact with the target.

Using a blunt tip to penetrate armor instead of a sharp, pointed one may seem strange, but one way to increase the effectiveness of an armor plate against an incoming projectile is to face harden it, so that the outer surface of the armor is much harder than the rest of the armor underneath. On impact with this very hard surface, the tip of an aerodynamic (long and pointed) projectile is very likely to shatter, crippling its ability to get through the rest of the armor. So, the trick is to use a blunt tip that's less liable to break up upon impact, and then make it aerodynamic with a thin, pointed nose cone.

Now, let's talk a bit about that 'driving band' I referred to above, which is the copper band that wraps around the projectile near the base. This band is larger in diameter than the rest of the projectile, so that the barrel grooves will have something to get a grip on. In the lower series of photos above, notice the series of spiral grooves that run along the inside of the pistol barrel. As a projectile (whether rifle bullet, pistol bullet, artillery shell or a tank round) is fired through its barrel, grooves along the inside of the barrel dig into the sides of the projectile and impart a spin to it, much as a quarterback imparts a spin when he passes the football. Without this stabilizing spin, the projectile, just like a badly-thrown football, would wobble and flop around, ruining accuracy and reducing range.

In the below photo of the two pistol bullets, notice the lengthwise marks that were literally squeezed onto them as they passed down the gun's barrel. On the small, relatively soft lead or copper-jacketed bullets fired from rifles and pistols, the rifling gets its grip by simply digging right into the softer metal. But what if the projectile being fired is steel that's as hard as the barrel's metal? Unlike lead or copper, you can't just dig into the side of a hard steel projectile without inviting lots of problems, like immense heat, incredible pressures and prematurely worn out barrels. Since the barrel's lands need something softer to dig into, a copper driving band is secured around the base of the projectile, which the lands can dig into and impart the needed spin to stabilize the projectile. Another important function of the driving band is to seal the base of the projectile, preventing the immense pressures of the burning gases from escaping around its sides as it moves down the barrel.

Okay, why all the attention to this driving band thing? Well, imagine that you're one of the fanatical collectors out there on eBay who collect all different kinds of military stuff. You'd know that unlike all the uniforms, backpacks, ration books, helmets, rifles, pistols and other things that the zillions of returning servicemen always ended up bringing back with them, it wasn't too common for Uncle Joe to scurry home after the war with 15 pounds of unfired tank round in his back pocket.

So, even though a fired-and-recovered tank projectile would be a pretty unusual find, what we have here is a 'virgin' unfired example, which will be of much greater interest to the collectors of this sort of thing. This projectile ended up selling on eBay for $265.

Photo of WWII Sherman main battle tank M61 75mm projectile