Complex sentence: The only difference between the AK-12 and the AK-15 is their caliber--the AK15 seems to be a backup should the Russian Army switch back to the 7.62mm caliber.
Split into simpler sentences: The only difference between the AK-12 and the AK-15 is their caliber. The AK-15 seems to be a backup if ever the Russian Army switch back to the 7.62mm caliber.