My newly acquired .223 XP100R has a front sling swivel stud but does not have one in the rear. I guess the front one is intended for a bipod. I'd like to install a short sling. Any tips? I don't know if the synthetic stock is hollow or not. Are there short slings made for this type of gun?

Yes, that McMillan stock is more or less hollow. I believe that it is glass fibers laid over a foam core (or something like that). Depending on where you want to place the stud, you could drill a hole large enough and then epoxy the stud in the stock......