I'm not sure exactly what you mean by "semi-tactical".
The slings that are often seen on battle carbines like the AR that are called tactical usually incorporate a capacity for very rapid length adjustment. This type includes the popular Blue Force Gear Vickers sling and the Viking Tactical VTAC sling. Both come in padded and unpadded versions. You can find a bunch of videos on-line demonstrating their use. These slings are intended primarily for weapon retention, allowing the carbine to be dropped from the hands but retained against the body. They are generally worn over the shooting hand shoulder and diagonally across the back. The quick adjust feature allows the sling to be rapidly lengthened if there is a need to quickly transition the rifle from the strong side to the weak side shoulder.
For the type of use that I think you are entertaining, this type of sling would probably not be required. Most two point slings can be set up for carry or use as a hasty sling so long as they have some capacity for length adjustment. If you want something to better stabilize the rifle when shooting, especially when sitting or prone, you might want a loop sling. This is a sling that has the capacity for form a loop that goes high up on the support arm and is usually tightened on it. My favorite for this purpose is the leather model 1907 sling, but good ones can be expensive and they are a little heavy and complicated. A good and cheap alternative is the USGI web sling that was supplied with M1 rifles during the latter part of WWII and Korea, and on M14 rifles and M16s in Vietnam. You can find them in cotton or nylon. Good ones can be found here:
http://www.ammogarand.com/amm1gaodcowe.html
The spring steel "J clip" allows the rear of the sling to be quickly detached from the butt stock sling swivel. A loop is then formed at the center of the "H ring" or triglide that is slipped over the support arm and tightened. The sling length can be very quickly adjusted using the cam lever.
If you want something a bit more modern looking, but still reasonable in price, you might look at the Magpul RLS (rifleman's loop sling):
http://www.magpul.com/products/magpul-rls-sling
The video shows how it works. The sling keeper slider allows a loop to be cinched down snug on the support arm, or the sling can be used like a Rhodesian sling with an open loop. The only disadvantage of the RLS is that the distance from the loop to the fore-end swivel can not be quickly adjusted as it can be with the USGI or 1907 leather sling.
You might also consider a "Ching sling" for rapid rifle stabilization but these require adding a third sling swivel on your handguard. I have used all of the above mentioned slings but have never used a Ching sling. I have heard them praised by hunters who need to sling up very quickly for an off-hand shot:
