Krav Maga has severel good ways to take long guns from others, even if their gun is slung 'assaut' style.
Same goes for disarming handguns. I've been taking it for a year to supliment my TKD and I can say it's well worth it.
Good skills to learn. And by learning how to disarm, you understand more of how to keep retention.
“We can evade reality, but we cannot evade the consequences of evading reality” Ayn Rand