Forum being a neuter noun in Latin, it's appropriate to pluralize it as fora. But when words are adopted into English language they become English words, so it is just as valid to pluralize as forums. (The Latin form is valid by usage resulting from a grammarian movement to make English adhere to Latin rules).
And re: the OP, IMO no one lives long enough in AC to use much of a 100-round mag.
And re: the OP, IMO no one lives long enough in AC to use much of a 100-round mag.