.308. Looked again. Looks like I don’t have to run in open. Depends on my weight I think
Even if you get it wrong, the guys running the event are good about adjusting your categorization later. On my signup form last year, I put down civilian as I didn't realize my muzzle brake would lump me into open division(some rifles with factory brakes can run civilian, those with non-factory brakes can't). Anyway, I had a muzzle brake that I didn't realize would put me in open and it was no big deal, they just updated the paperwork for me. In my mind, the categorization was just background paperwork: The event was awesome, I really enjoyed myself and I'm looking forward to this upcoming match.
Just make sure you are prepared for any weather. Two years ago there was steady rain on Saturday. By the end of the day, I recall there being at least one person who was worse for wear after spending a full day laying out in a wet grass field. I was lucky that year as I packed rain gear and was comfortable throughout the event. Then, last year there was a lot of sunshine and those who didn't take measures to deal with the heat were again worse for wear. So, bring lots of water to drink, keep skin covered and use sunscreen on your skin that can't be covered.
Some of the more seasoned guys can give you a better run down of things that are useful to bring, but here(in no particular order) are some other things you might want to bring:
Shooting mat. It doesn't have to be fancy, a camping or yoga mat will suffice. Or, just lay on the grass if you don't mind the risk of getting wet/cold.
Spotting scope. There are usually one or two stages were a spotting scope can come in handy. Just need 1 per team.
Food. I don't recall having enough time to run off and buy lunch, so I was glad I brought food.
If you are participating in the Friday site in day, 2-way radios are needed if you want your target changing partner to read off your shots from the berm. Not really critical at shorter distances if you have a decent scope, but by the time you are working on your scope dope for 400/500/600, it will be up to your partner, who will be stationed down behind the berm, to tell you where your shots are landing via radio. (Bring spare radio batteries, one fellow had his batteries crap out.)
Check the course of fire and make sure you bring enough ammo.
Most people bring at least 2 magazines(10 rounders typically). Then, there are some people who are masochists and top feed their Remington 700's(You know who you are).
I like to bring a write-in-the-rain book where I keep my scope dope numbers. Others like to tape their dope sheets inside the lens cap of their scopes. Lots of recipes for addressing this point.
Many participants bring carts to haul around their gear. Some use converted baby strollers, others use yard-work carts, some build their own from scratch. Then, there are always those guys who travel light and just throw their stuff in a pack and then throw the pack over a shoulder for the walk back between stages.
Bipod and/or sandbags. I use a front bipod supplemented with sandbags.
Hat for sun or rain
Finally, don't forget to bring a good attitude and pay attention to the safety rules.