If more barrel twist relates to stabilizing longer rounds (1:7 twist meant for stabilizing military tracer rounds, etc.),  why do super-long .50 Caliber bullets get approx. 1:15 twist rates in their barrels??

Seems counterintuitive...

The .50 bullet is not that long comapred to its diameter. It is not proportionate to the 5.56.

Does bullet length/width proportionality have anything to do with this though?

It's all proportional. An 18" naval projectile from the Battleship Iowa is several feet long. That doesn't mean that it needs one turn in 6.5 inches of barrel travel.

Think mass,length, speed, and octave of the bullet design.

A 55gr FMJ stabilizes well at 1/9, but the 55gr tracer is a longer octave/length and needs the extra twist to stabilize the bullet.

The other thing to consider, in regards to Mil-spec of barrel twist and bullets, is once the bullet reaches it's target, a slower twisting bullet will start to tumble, and separate at the cannel, verses a faster twisting round that will not tumble and punch straight threw.  Granted that a faster twisting bullet may be a bit more accurate, it the performance of the round on the target that is more critical.

Just food for thought.

Troy,

Evereything I've read from the U.S. Army ballistics folks would seem to indicate that a bullet which is only marginally stable in flight does indeed tumble when impacting a soft/liquid medium such as flesh.

I can't find any other explanation why the 5.56 round that we used in Viet Nam, with 1 in 12 barrel twist was so much more lethal than the larger 7.62 NATO.

The rate of rotation required to stabilize a projectile is determined by the density of the medium through which it travels.  For example, the twist rate for .223 bullets that would be needed for stable flight in water would be measured in MILLIMETERS rather than inches per revolution.

Once a bullet enters flesh,  it doesn't matter if it was marginally stable in flight before impact or if it was highly stable with excessive rotation, either way it's going to do about the same in flesh and that is to tumble and in the case of the .223/5.56,  it is to also at least break in two and go off in two different directions, or continue to break up into tiny bits of hot shrapnel.

Either way,  the massive disruption of a .223 bullet at usual velocities as it impacts flesh results in a more dramatic wound cavity than the in-and-out straight hole that is the more likely result of being hit with a .30 caliber FMJ bullet.  

Regardless of twist rate,  5.56mm bullets have been pretty well demonstrated to be more effective at doing massive damage to people than .30 caliber bullets simply because the .30 caliber bullet has to penetrate more tissue before it starts to fragment and it is also a more physically robust bullet to begin with.  

Why do varmint bullets work better on small animals than big game ones?  Because the varmint bullets do their disruptive (disintegrating bullet) damage within the relatively short path of travel through a small animal.    Same thing applies to people,  because a chest shot offers a target that's on average perhaps one foot thick as the bullet travels.   5.56 will come to a stop in a chest shot, after fragmenting.  .30 caliber needs a longer penetration depth to do its worst work,  so it just leaves a hole instead of a crater.

Obviously,  you want the bullet to do all its dirty work within the victim's vitals.  A bullet that fragments within that critical distance is going to do the job quite well,  and one that simply zips on through without fragmenting is not going to be so effective.

If you were shooting big game where the heart is located 18 inches into the chest from a preferred presentation,  then you'd be best to select a round that delivers the most effective fragmentation/damage effects in the 12 to 24 inch penetration range,  at least when talking about bullets that are intended to fragment.   But big game bullets are more likely to be designed around the mushroom concept.  Expand as big as possible and hang together, so as to create the largest possible wound channel.  Blowhing a two inch hole through the heart and lungs is a HIGHLY effective way of stopping practically ANYTHING.

And when we get into large, dangerous game,  suddenly we're dealing with another concept entirely,  and that is one of penetration, penetration, and more penetration.  Heavy, fast, solid bullets that are designed NOT to expand but to drill the longest possible wound channel (and also to break down massive bone structures) rule the day when dealing with rhinos, lions, cape buffalo, elephants, and the like.   Since we're now talking about a wound channel that may be ten feet long,  we're still talking about a LOT of physiological damage, even without mushrooming or fragmentation.    

I've come to understand a little of the design concepts behind a number of bullets, and it's a more complex subject than I would ever have imagined!

I guess the ultimate bullet would punch a large hole, mushroom for a while, and then blow up with devastating force, with the fragments exiting the other side of the target,  but those are four mutually incompatible performance parameters.

Penetration, expansion, explosion.  Pick ONE.

CJ

Troy, I stand corrected!!!!!!

Quoted: It's all proportional. An 18" naval projectile from the Battleship Iowa is several feet long. That doesn't mean that it needs one turn in 6.5 inches of barrel travel.

16"... the Iowa Class battleship had 16".