Transitioning from a rifle or other primary weapon to a side arm can be a tricky process. The technique required is heavily dependent on the gear you use, different sling setups accommodate certain techniques better than others. A weapon transition should be simple and efficient above all else but when you transition is subjective to the situation. I try to stick to two basic techniques when I transition that I feel work fairly synonymously with both two-point and one-point sling systems while giving myself some options for the situational requirements at hand.
I typically pull my weapon to the side, when working with a team of shooters who can cover a hasty reload, so that I may continue to engage with one handed pistol shots until I can get my primary weapon back up and into the fight. That or I throw my rifle back so I can go at it with my sidearm exclusively for an extended period of time if that rifle reload is not going to be an immediate option; situation dictates. I also generally stick to a two-point sling set-up unless I am exclusively doing CQB; the sling I am currently using is a Blue Force Gear adjustable two-point sling. This set-up allows me to get the primary weapon away from my center line easily and get to my sidearm in a hurry.
When practicing the technique of transitioning you should focus heavily on smooth movement; speed will come with time and practice. Also on an unrelated note you should be practicing marksmanship and target acquisition while studying the concept of fire and maneuver above all else because that’s what wins gun fights. But I digress, focus on smooth being smooth like the Fonz when training. Break it down into steps if it helps in the beginning and work on a cadence for it if you must. As long as you can efficiently and safely move the primary weapon out of the way while almost simultaneously employing the sidearm, you’re good.
Above all else, use what works best for you and your situational requirements because we are all unfathomably different; even in the context of shooting alone. A weapon transition hinges on that fact paired with the kit your wearing, the weapons your using, the sling you have, and the situation you’re in; at least those are the big factors at play so base your needs off them. In the following video I will attempt to demonstrate the techniques I use and give brief explanations of them while dressed like a hipster.