5.45x39 Russian surplus ammunition (7N6) was the most affordable ammunition available to shooters until it was recently blocked from importation. It has been imported into the US for many years without a problem.
The basis for the ban is that it is "armor piercing" ammunition. It does NOT meet the criteria for armor piercing ammo.
If you match the BATFE's very broad definition with the ammo, you will see that it doesn't fit. It wasn't designed for a pistol, it's a rifle cartridge. It doesn't have an AP core, it's mild steel. The pistol in question isn't a pistol in conventional terms. It's massive. The projectile isn't even designed like armor piercing ammunition.
Please have someone who actually has knowledge and an interest in firearms look at this.