#eidson

https://boxartden.com/gallery/index.php/Model-Kit-Artists/A-Scott-Eidson

We measure the rate at which most things turn in revolutions-per-minute (RPM).

My earliest recollection of this term involved phonograph records. I'm old enough to have spent my formative years listening to 78 rpm discs. In fact, my grandmother had a wind-up Edison machine that I still have. It looks exactly like this.

My parents' 78 record player was much more advanced — it ran on electricity, included an AM-FM-SW radio, and allowed for playing several discs in a row automatically. We had a Magnavox, but the closest I could find was this Zenith. That's actually me listening to the record. (Nah, I was much cuter.)

High tech, indeed.

Then things slowed down. First we had Long Playing records (LPs) that turned 33⅓ RPM. Those were developed mostly by Columbia.

The LP led to the birth of Hi-Fi. All early Hi-Fi systems were owned by beautiful young women who only wore negligees or very tight Capri pants. They always wore mules. The best sound came when lying on the floor or upside down in a sculptured chair.

Not to be outdone, RCA countered with the 45 RPM "single" with a great big hole in the middle.

Something else we're familiar with is the good, old-fashioned ceiling fan. I have two. A report from the Florida Solar Energy Center showed ceiling fans typically run as slow as 45 rpm (like an Elvis record) to as fast as 200 rpm. A typical high speed is about 150 rpm, or double the speed of Granny Roach's Edison. The ones at Toomer's Drugs in Auburn are nice and slow.

What about car tires? How fast does the wheel of a car turn? Go ahead, guess.

Most car tires are about 28 inches across. That means the circumference (don't give up on me!) is close to 80 inches. In other words, the cars moves 80 inches every time the wheels turn one time.

Let's use 60 miles-per-hour because that's also one mile-per-minute. That means the car travels 5,280 ft (one mile) in one minute. That would also be 5,280 x 12 = 63,360 inches. So 63,360 ÷ 80 = 792.

So a typical car wheel at 60 mph turns about 800 rpm.

I'm guessing that so far there have been no major surprises.

OK, what about a rifle bullet? They spin. So how fast do they spin?

Go ahead and guess.

Nope, you're wrong (unless you're a gun nut like me).

C'mon, admit it. You guessed 1,000 rpm. Or 5,000 rpm. Or maybe even 20,000 rpm. Not even close.

Let's do some easy math (not always an oxymoron).

The most popular rifle cartridge in the U.S. is the .223 Remington. That's also the Army's main rifle cartridge (only they call it the 5.56 NATO). It shoots a bullet that typically travels about 3350 feet per second.

If you've ever looked through a rifle barrel (or watched a James Bond movie), you've seen the spiral grooves that make the bullet spin. Early guns didn't have those grooves, called (what else?) rifling.

When the British fought the Americans in the Revolutionary War, they wore red jackets and fired smooth bore guns. Those guns weren't very accurate very far away.

Why? Well, it's like a football thrown with a spiral in comparison to a knuckleball pitch in baseball. The better the spiral, the straighter the flight of the football.

Conversely, the (non-rotating) knuckleball jumps and flies all over the place. Neither the pitcher, catcher, batter, or umpire knows where it will go.

The Americans had developed the "Kentucky" rifle with grooves inside the barrel to make the bullet spin. It worked wonderfully well and they were able to shoot accurately from a great distance at the red coats with British soldiers inside.

The standard rate of twist for the .223 Remington is 1-in-12. There is one full twist every 12 inches. That is to say, the bullet turns one revolution every foot.

So if the bullet travels 3350 feet in one second, that is a rate of 3350 x 60 = 201,000 feet per minute. Remember the bullet turns one time per foot, so it is spinning at over 200,000 rpm.

You didn't even come close to guessing that, now did you?

And just like ceiling fans, some rifle bullets turn slower and some faster.

A black powder .45-70 at 1200 feet per second with a 1-in-20 twist is 43,200 rpm. At the other extreme, a Barnes 36 grain Varmint Grenade at 3695 fps in a military rifle with a 1-in-7 twist would be turning 380,000 rpm. (Before any of you other gun cranks jump me for that last statement, be aware that Barnes publishes that data, so they apparently tested it.)

Personally I think that last bullet would turn into smoke about ten yards past the end of the barrel, but I haven't tested it.