Currently, the M6A2E1 in-game is restricted to firing non-explosive APCBC rounds only, which is inappropriate for a tank rated at BR 5. This results in abysmal post-penetration damage. Additionally, its weak-spot red-ringed sections can be penetrated to detonate the ammo rack by the Tiger E, while the M6A2E1 suffers a lengthy 15-second reload. Granting it explosive-filled APCBC ammunition would better equip it to combat the King Tiger.
After all, it uses a 100mm-thick T-1E1 hull topped with composite base armor of 140mm plus another 40mm. You can’t expect a 1943 vehicle to have all-around armor capable of stopping the King Tiger’s 88mm rounds. It’s just an experimental prototype, so having weak spots is totally reasonable.
This is Enlisted, not War Thunder. Experimental tanks cannot use better ammunition than production tanks; otherwise, they wouldn’t be event reward vehicles, but premium vehicles that require payment.