To double the effective range of any bullet, you need to do one or more of several things to increase the ballistic coefficient:
1. Increase the sectional density. You can do this by either increasing the weight of the bullet or decreasing the diameter. Since we’re going to assume that we’re not going to decrease the diameter, we have to increase weight.
2. Decrease the bullet form factor, which is a mathematical way of saying “make the bullet longer, ‘pointy-er’ and blend the transitions from the nose to the body, and put on a boat-tail rear end for the trans-sonic transition...”
3. At the same time you’re increasing mass while sitting on top of the same cartridge, you’re going to quickly reach a point where you cannot achieve the same muzzle velocities with the heavier bullets as you can with the lighter bullets. For something like a .223/5.56, you will see your Vm go from about 3,000 fps to
Here’s a chart of .224 pills and their Bc’s:
http://www.extremeaccuracy.net/Scripts/prodList.asp?idcategory=57&curPage=1&sortField=description
OK, look at p1 and then the VLD pills on p2, especially the 80 and 90gr pills. You’ll see that the Bc’s for the VLD’s are twice that of the 55 to 60gr spitzer-style pills.
So such bullets exist. Why not use them?
Well, they’re so long that you either have to seat them well down into the case, or you can’t fit them into the magazine. If you seat a bullet down into the case, your case pressures go up, meaning that you have to back down on your powder load (and your velocities) in order to get the brass to contain the pressure.
4. There’s another problem with heavier & longer bullets: For heavier bullets, you need to speed up the rifling twist. For longer bullets, you might need to change the chamber throat. If you then switch back to lighter/shorter pills on the same rifle, you will over-spin your bullets (possibly causing fragmentation before they reach the target) and you’ll likely have accuracy problems from the longer leade (the jump from the case mouth to the rifling lands in the throat).
Can doubling the range be done? With a whole lot of caveats, “Yes.”
Can it be done in a rifle where the magazine length is a restriction on overall length of the round AND feed well enough for full-auto fire? Probably not. There are plenty of guys shooting 5.56 at 1000 yards/meters with VLD pills and doing a very good job of it, but they’re not shooting issue battle rifles, nor are they shooting full auto (or even burst fire).
Going back to the OP: Is putting steel into the core instead of lead going to increase the effective range? What mathematics I know of external ballistics says “no.” You need to get heavier, not lighter, bullets, longer bullets (in terms of calibers) and pointer bullets to increase the range. Then again, I don’t know what they might have done to the powder charge under the pill. A longer barrel with slower powder would help increase initial velocities with constrained pressures, but I don’t see that happening on issue rifles either. The trend is to shorter barrels, not longer.