Sunday, April 19, 2009

More Ballistic BS

I recently read an article at that stated that the U.S. Army is going to re-barrell their 7.62NATO M-24 sniper rifles to fire the .300 Winchester Magnum cartridge. According to the geniuses who came up with this idea, this will provide 50% greater effective range. Upon reading that statement, my bullshit detector not only went off, it almost exploded. This is a typical example of a "fact" pulled directly from someone's ass and then waved around and defended as a scientific certitude.

Just so everyone is on the same page the 7.62NATO/.308 Winchester and the .300 Winchester Magnum are both .30 caliber(.308) cartridges. This means that they fire the exact same projectiles, any bullet that can be loaded into one can be loaded in the other. The only ballistic difference is that the .300 WM propels it's bullet slightly faster. Assuming the same bullet weight and type the .300WM pushes it out 400fps faster than the 7.62NATO/.308. This velocity difference remains the same no matter what weight or type of bullet is used. By the way, all of the ballistic facts I'm using were extracted from my Sierra reloading manual, NOT a lower body orifice.

For comparison purposes I used the .30 caliber 180 grain boattail spitzer point bullet. According to my well worn Sierra manual, this bullet fired from the .308 Win. , leaves the muzzle at 2600fps (maximum load) and fired from the .300 Win. Mag, leaves the muzzle at 3000fps(maximum load). Remember now, according to the Army "experts", this extra 400fps in velocity equals 50% greater effective range.

What does "effective range" mean? Glad you asked. It means the range where the bullet no longer has enough velocity to deliver a lethal blow to a man. What is the minimun velocity required to deliver such a blow? Glad you asked. Some experts say 400fps others say 600fps and still others say something in between. For our purposes it doesn't matter, we are concerned with the DIFFERENCE between the two cartridges which stays the same no matter which minimum velocity is used. I used 600fps in order to save myself some calculating. Using the Sierra ballistic table I tracked the velocity loss for both cartridges down to the lowest number above 600fps. Here is what I found.

At 1800 yards the bullet fired from the 7.62/.308 Win. is moving at 628fps, if I took it to 1900 yards the velocity fell below the magic 600fps. Because I'm lazy I figured everything in increments of 100 yards, this makes the maximum effective range of this cartridge 1800 yards. If the Army "experts" are correct, the .300 WM will be effective at 2700 yards. Not quite. According to Sierra's ballistic data the "SUPER DUPER BAD GUY FLATTENING .300WM is effective out to a whopping, wait for it, 1900yards. Keep in mind folks, that this difference in effectiveness only comes into play at ranges past 1800 yards. At any distance under that, where 99% of sniper shots are taken, there is 0% difference in the effectiveness of these two cartridges.

Why do I care? Glad you asked. I'm weird in that I prefer that my "experts" actually know stuff. Oh yes, and there is the $4000 per rifle that the Army is going to spend to make this "improvement". The bad news for the Army sniper is that he's going to get a rifle with significantly increased recoil and noise, and negligibly increased perfomance. The good news is, due to the bulkiness of his new ammo he won't be able to carry as much of it.

Note to those who want to get REALLY technical: Yes the faster bullet has a slightly shortened flight time thereby VERY slightly reducing the effect of a crosswind. I maintain that this minor advantage is offset by the loss of the 7.62NATO/.308 Winchester's inherent accuracy.


King Selfish said...

I really appreciate you posting this. Now I don't have the most boring entry on this blog. Just joking, don't shoot me.

My understanding of ballistics is roughly the same as my understanding of quantum theory, so I've decided to accept on faith that you and your sierra reloading manual know what you're talking about. I suppose the people who are deciding this issue are the same ones who decided that a clunky 9mm pistol should replace a battle tested .45

One thing we know for sure, the .308 is an outstanding pirate killer.

vulcandeathgrip said...

I caught the live version of this back when I was shopping for my new rifle a few months ago. I couldn't decide between the .308 or .300wm so I asked the archduke his opinion (or maybe he just gave me his opinion), anyway what ensued was a half hour anti-.300wm dissertation. he convinced me to go with the .308 and I'm glad I did, partly because I knew that he had quite a .308 ammunition surplus and no rifle to shoot it.