A blowback works sort of like a pogo stick, where the force of the bullet being pushed out the barrel is balanced by the weight of the bolt and the spring holding it closed; so, if you use a bullet that is too heavy or too light, the action will either come back too fast, beating the rifle like you were hitting it with a sledge hammer, or it will not open at all. Given that, I would say that you SHOULDN'T use any ammunition but the 180-grain factory standard in that rifle. In fact, I wouldn't fire that rifle AT ALL, unless you first have it fully checked out by a gunsmith who knows what to look for.