Use some MOLLE-compatible straps or tactical bungee cords. Just thread them through the MOLLE webbing and across your rifle. Generally, it’s best to run them horizontally across the back panel. That way, you’ll get better stability. Don’t forget to secure both the stock and the handguard area.
When it comes to a full-size rifle like the 550, I prefer to use a dedicated rifle scabbard that’s mounted to the side of my pack. Strapping it to the back panel just doesn’t feel right to me.
If you're planning to be out for long stretches, I'd seriously suggest hooking up a quick-release buckle system. It can be a real game-changer when you need to drop your pack in a hurry or grab your rifle fast without fumbling around.
