The sling is a massively useful piece of equipment. A long gun without a sling is like a pistol without a holster. You’re just not serious. Let’s talk about a few ways to use a sling, and what kind of slings you should (and shouldn’t) consider.
We’re going to focus on tactical use, because I’m a tactical kind of guy. I’ll touch on slings for high-power-type marksmanship competitions later.
What kind of sling should you get? You should get a quick-adjust two-point sling, unless you have a big reason to get something else. If your job is doorkicking, and you’re one of the first guys in the stack, you may find a one point sling is better for you. Everybody else should stick with the modern, quick-adjust sling. There are many out there. I happen to prefer the VTAC one. The Vickers one is good too.
The problem with the one-point sling is that it doesn’t stabilize the weapon on its own. Your carbine will swing, probably between your legs, whacking you in the knees and groin as you move. Fun. So you’ll need to put a hand on the gun to keep it stable. Hopefully you don’t need both hands for stuff.
Properly mounted, a quick-adjust two-point can be lengthened to facilitate transitions. Or it can be tightened to secure the gun and keep it out of the way. Given the two points of attachment, it will stabilize the gun fine on its own, letting you do other things with both your hands. It will also comfortably support the weapon, but let you get the gun into action quickly.
Let’s note that a quick-adjust sling requires a quick-adjust mechanism. A nylon strap is not a quick adjust sling. That’s good for carrying a rifle on your back and looking bored and not ready. Fresh out of the 70s. Have a c-ration with that, cheapskate.
I should also tell you that three point slings are stupid. I don’t know anyone who uses these anymore, probably because it isn’t the 90s. Just say no. Three point slings are a great way to strangle yourself. Or trip yourself.
Now that you have your sling, you need to figure out how to mount it. Some of this will depend on carbine setup, and some of this will depend on you. As a general rule, the further apart your attachment points are, the more support you’ll have. The closer the attachment points are, the easier it will be to manipulate the carbine in transitions. It’s up to you which you value more. QD sockets will help you adjust and figure out what works right.
For the forward attachment point, if you have a modern handguard with rails, keymod, or m-lok, you can get a QD sling socket that attaches to it, and you can move it around until you find what works. Problem solved. Don’t forget to loctite it down once you found what you like.
Further aft, you can get a receiver end plate with a QD socket. Most not-craptastic stocks come with a QD socket on either side. Again, the end plate socket will work better for transitions, and the socket on the stock will stabilize the rifle better. If you opt for the stock, remember to attach the socket on your strong hand side. I.e. if you are right handed, use the socket on the right side. Having the swivel on the outboard side will feel weird at first, but it will allow you to switch shoulders without the sling choking you.
If you’re shooting high power or similar competitions, you want a sling that’s going to help lock the rifle in place. These slings are loop slings, and they connect your support arm bicep to the rifle. The mounting swivel is adjustable so you can get your support hand in behind it. This will significantly increase the stability of the rifle once you master it. The sling should be tight, and is taking your muscles out of the problem of supporting the rifle.