My question is do you need to engage the safety on your primary weapon in this case call it a AR15 when transitioning to your secondary weapon which will be a handgun. So for whatever reason you need to transition from your primary to the secondary do you always engage the safety (if it's possible) or is it acceptable 2 leave it be and just transition to your secondary weapon the handgun?

What's the context?  If it's empty and life or death it wouldn't matter, training class up to the instructor, or in a competition it would be whatever the rules are.

This.
In most cases, if you sling, barrel or bench a rifle and it's not on safe, it's a penalty or a DQ for the stage.

In a self defense situation, I don't think anyone is going to put up too much of a stink about it unless you let one fly in the wrong direction.

Yes.

And it’s a good habit.

Make it a habit to sweep the lever every time you transition. If it's ready to rock it will go to safe and if it's empty it won't matter.
You won't be any faster if you don't do it, but you sure as shit know you fucked up if the rifle goes off when it gets hung on something when it's hanging from your single point

Well I don't want to shoot myself in the foot and/or dick, so I'm gonna say yeah.  You do you though.

Reason its more important for rifles than handguns is nothing covers the trigger when you sling it.  Things can catch the trigger like mag pouches, bungee cords, straps, etc.

You do it on an empty rifle for good habits.  Recent training curriculums also teach engaging the rifle safety when reloading too.

Thanks for the answers and explanations!
The context for the question was basically real world application not about any match or class although id,imagine any class is going to teach to real world circumstances.

I figured the answer would be to always engage the safety when slinging your rifle to go to your handgun and I wasn't trying to get to detailed as in a malfunction or on a empty chamber. I was asking as if the rifle was up and running with rounds still in it and needing to go to your secondary.

I'm creating a range trianing program for myself and on the things I'm not 100% sure on I wanted to ask you guys what the correct thing to do is and this was one I wasn't sure about. The research I did didn't give me a clear definite ruling on it so I turned to you guys cause there's a ton of knowledge here and I trust it.

So again thank you very much for your guy's help and explanations on why the safety should always be engaged when possible

I always do (if possible, some malfunctions can prevent the safety from rotating to the safe position on an AR) and I think it's good practice.  Once you get in the habit, you don't even have to think, it just automatically happens.  I even safe my ARs before changing mags.

The only reason I would transition from a rifle to a pistol is the rifle is down and a threat so near I dont have time to reload or resolve the stoppage.

If you have access to training classes in your area, I’d suggest you start there instead of enacting your own protocols. Reason being and not assuming your skill level or background, there will always be some more knowledgeable and more experienced out there. There is also the power of observation, so if you pick up a bad habit early on, it can be diagnosed and hopefully corrected quickly. Once you get more comfortable you can continue your program independently. Best of luck, cheers.

I tell students to attempt to sweep the safety on during a rifle to pistol transition. Obviously, it won't engage if the hammer is forward, and it's not really a safety issue if the rifle is in an inoperable condition. However, I've seen guys transition after short stroking the trigger believing that their rifle malfunctioned. As soon as they release the trigger, it resets, and they now have a live operable rifle dangling from the sling, likely during movement, where the weight of the rifle alone will discharge the trigger if a piece of gear catches it. Therefore, I train students to attempt to sweep the safety even knowing it may not happen in a real world close engagement. This also maintains training consistency with engaging the safety whenever the eye comes out of the sights/optic.

Always engage the safety.  If your not engaging targets safety on.  Only exemption is a malfunction.

I subscribe to Frank Proctors idea  (first place I heard it) that if your eyes leave the sights the safety goes on. That covers ALL of it. When practiced it won’t slow you down at all.

Picking up a rifle and seeing that the safety was off during the entire course of fire including movement is butt puckering for me.

Break cheek-weld, weapon on safe.

That includes weapon transitions, reloads, malfunctions, and movement.

If you can't, then don't.

Train as you fight, fight as you train.  Yes, I've always engage the safety to its second nature and no longer think about it.

CD