If this helps.... you'll be shooting about 350-400 rounds.
Cost for 5.56= about $150-$200
Cost for .22lr= about $20-$30
People were shooting M1As, M1 Garands, AKs and ARs and there were several shooters that did not use a sling at all. A few shot a different rifle on Sunday than they used on Saturday, so bring both to the event and make a final decision there. This is NOT basic training and they will not send you home for not having a sling.
Be more concerned with the techniques you'll learn and less about hitting your target, this class is for learning techniques that will carry over to just about any rifle you'll ever shoot.
Now that you know the traits of the foregrip and front sight you can 'sling-up', just do it with consistent 'light' tension on the sling so stress doesn't transfer to the sight. (I wish I would have had this front sight data 2 weeks ago
)
Study the 6 steps to taking the shot and try to get some pre-event dry practice so you are ahead of the game.