These loads provide a good starting point for designing a new steel-cored AP round for the 5.56mm.To gain some idea of the effectiveness of each projectile's core, we will examine their respective length-to-diameter ratios, gleaned from images of the sectioned bullets. Unfortunately, I was unable to find any reference material for the Russian 7.62mm BP projectile. The rest are as follows:
M2: Between 4.1 and 4.6 calibers, depending on sample
M61: Between 3.3 and 3.7 calibers, depending on sample
7N22: 5.2 calibers
DBP-10: 4.9 calibers
Because the M2 and M61 cores have such uncompetitive L/D ratios, we will forgo scaling those projectiles down, and instead focus on analogues to the 7N22 and DBP-10.
Scaling the 7N22 projectile to .224" caliber, we get a weight of 59.9 grs and a length of 1.020". Doing the same for the DBP-10, we get a weight of 60.7 grs and a length of 1.015. So far, both bullets seem very comparable, with the 7N22 projectile having a somewhat higher ballistic coefficient.
Bullet seating will be calculated both for the intended depth of the original 7N22 ammunition, and to fit within 5.56mm's OAL, respectively. The former produces a figure of 2.488" overall length.
The 5.8mm caliber has two versions: one intended for rifles, and one intended for DMRs and machine guns. The former has an overall length of 58mm, while the latter has an overall length of 61mm. The case length is 42mm, giving the each bullet 16mm and 19mm of exposure, respectively. We will use these figures, adjusted for caliber to 15.19mm (.598") and 18.03mm (.710"), as well as the original figure for 5.56mm of 12.70mm (.500") to give us overall lengths for each.
Utilizing the Powley Computer, we can determine performance for all five configurations:
|Fig. 1: 5.56mm with scaled down 7N22 bullet and the same relative seating depth as 5.45mm|
|Fig. 2: 5.56mm with scaled down 7N22 bullet|
|Fig. 3: 5.56mm with scaled down DBP-10 bullet and the same relative seating depth as 5.8mm heavy ball|
|Fig. 4: 5.56mm with scaled down DBP-10 bullet and the same relative seating depth as 5.8mm light ball|
|Fig. 5: 5.56mm with scaled down DBP-10 bullet|
|Fig. 6: A hypothetical 60gr steel-cored 5.56mm AP load|