New Army "Caliber Configuration Study"

[stanc]

There are many 7.62x54R bullets. Which one is the mild-steel core load, the standard PKM load, to which we should compare?

The one usually used is LPS Ball. It's what is most commonly found in the US, and AFAIK the type most likely to be encountered in places like Afghanistan.

I'm told the LPS was superseded in Russian service late in the 20th century by the ST-M2, which was in turn succeeded by the 7N13. IIRC, all three weigh ~148 grains, and look virtually the same. The main difference is internal, with the LPS and ST-M2 having a blunt steel core, whereas the 7N13 has a pointed (and hardened) core.

I'm not 100% certain, but because of the small air gap in the tip, I think the sectioned bullet is the ST-M2. Early production LPS bullets have a silver tip like the one on the far left.

MV and BC data is publicly available for the LPS, which is undoubtedly why it's used for such comparisons. I've never seen any numbers for the 7N13.

Regarding your point that a lead-free bullet, by virtue of its length, has a higher BC, is something I didn't realize. I thought material density also factored into BC, in the sense that if form factors are equal, the bullet with a higher material density has a better BC. Conversely, a bullet of a lesser density needs to be longer to have the same BC as a bullet of higher density.

That's mostly my understanding, too, except that sectional density (SD) is a factor in determining the BC. For example, the SD of two 150gr .30-caliber bullets will be the same, even if one has a lead core, and the other has a steel core. As you note, the lower material density of the steel-core bullet causes it to be longer. The greater length, combined with the same sectional density, effectively makes the steel-core projectile more streamlined than the lead-core bullet (assuming the same ogives and boattails on each), and therefore gives the former a higher BC.

This is why I postulated that a 108gr lead-free bullet in the 65G case might be too long to be practical to match 7.62x54R.

Yeah. While at dinner, I got to thinking about your earlier posts, and realized there would be a problem "going green" when the lead-core bullets a cartridge uses are already at or near the upper limit on practical bullet length. I don't know that there's anything to be done about it, though, other than accepting somewhat less ballistic performance than you would prefer...or going to a bigger caliber. Here are calculated bullet weight and BC figures for the 5.45mm 7N6 scaled up:

Dia. ----- Wt. ---- G7 ---- G1

.220" -- 53.5gr -- .168 -- .327
.224" -- 56.5gr -- .173 -- .337
.243" -- 72.1gr -- .188 -- .367
.257" -- 85.3gr -- .199 -- .388
.264" -- 92.4gr -- .204 -- .398
.277" -- 107gr --- .214 -- .417
.284" -- 115gr --- .219 -- .427
.308" -- 147gr --- .238 -- .464

( Adapted from http://forums.delphiforums.com/autogun/m...msg=5924.4 )