It is possible to draw a card even when it is not your turn. This is not cheating, it is a distinctive feature of a card we have and following the game rules and game design.
The M4A1 is the key card.
The US M4A1 has the unique ability that you will draw it from your deck as soon as one of your cards is being discarded from your hand by your opponent.
The M4A1 is a Special-rarity card, which means you can have up to two of these cards in your deck.
When your opponent makes you discard, you will instantly draw these card(s) into your hand out of turn. If your hand is full, the units will be discarded as per the rules.