[Pirate]

For what you are asking you don't need advise from people on this site. For your intent and purposes just get whatever you feel you would like.
If you do want a sniper rifle I hope it is a REALLY big back yard (and hopefully don't have any neighbours for at least 500m for safety purposes) because otherwise one of your friends will be able to demolish you if they have even just a $50 fully auto crosman mini clearsoft gun from Canadian tire.
I'd even just suggest getting a Kraken for $150, you can get a rail and put a scope on it and shoot semi but when you get bored of sniping (believe me this can happen especially with small skirmishes of 4-5 people) you have the option of going fully auto.