Bullet length/weight & barrel twist - discussion

[NC-rifle]

I am looking to have a discussion on bullet length/weight and barrel rifling twist rate. Since my experience is with AR-15 0.223/5.56, I picked this forum to post.
It is interesting that so many choices of bullet weights are available in factory ammo. And it is my understanding that when it comes to barrel twist, the bullet length is really the parameter, but the factory loads typically list bullet weight (which is a function of length, shape, material (lead core, steel tip, all copper, etc.).

To get things started, here are a few bullets I pulled from factory ammo and my measurement of the bullet length (sample size of 1). Note the 62gr steel tip (M855) is longer (0.900-0.920) than the 62gr lead core (0.807-0.811).

Although AR-15 barrels are made in many twist rates (1:12, 1:8, 1:7, etc), lately most 16" to 20" barrels are 1:9, 1:8, 1:7.

 

[NC-rifle]

I was surprised these two 62gr penetrator bullets were this much different in length. Not sure if it was my single sample and production tolerances, or if the conical shape of the bullets are different, or if the steel penetrators are different sizes.

 

[NC-rifle]

And the 55gr polymer ballistic tip of Hornady and Sierra.
Along with the 50gr V-max BT (Hornady bullet) used in Fiocchi

 

[AR10ShooterinNC]

The less dense the material the bullet is made of, the longer the bullet is generally going to be. Shape of the bullet will also play a big part in the length, round nose bullets will be shorter than spritzer pointed bullets.