Ahoy, welcome, and no. No, no, and no. Airsoft uses 6mm plastic BBs in the 0.2 - 0.5g range, at about 1.1J auto, up to 2.5J for bolt actions. Please, please don't show up to show off some "home defence" gear that's legally spurious.
Airsoft sniping is fun, but we can all put a crosshair on a target and pull a trigger. After that, it comes down to how well your toy gun is shooting, and the vagaries of weather and airsoft physics. You can forget holdover at airsoft ranges - the way airsoft hop works means that your BB will go pretty much straight, then plummet sharply.
I don't mean to come across too harshly, just to be clear that in airsoft most of your shots are going to miss, and that you're there to take hits in good cheer, not to make them. If you come with that attitude, you'll be welcome anywhere and you're likely to find folk who are happy to help and support you. People have played airsoft in wheelchairs, or motorised 4x4 or 6x6 mini-tanks.
Whether you can compete is going to be entirely dependent on your local site. Indoor CQB is a good shout, you can have a great time playing a slow walking game and covering choke points, and playing with a light SMG/PDW or pistol is viable. In woodland, you'll find a much wider variety of sites. Some of them are flat, dry and compact with loads of cover, some of them involve lonnnng slogs through swamps to engage and respawn. So please don't be disillusioned if you start out playing at one of the latter sites - other alternatives are available.
Give it a try, you'll find that airsofters are a very welcoming bunch. Just ask someone about their guns and see if you can get us to shut up.