Re: 50 cal bmg
I have to recommend joining the FCSA so you get access to the member's forum. IMO the FCSA is full of a bunch of beauty queen prissy sissy drama, but you can draw all of the info you need to get you started with your first year of membership there.
Personally I would (and did) hire a gunsmith to build you a single-shot on a trusted-to-perform action like the mcmillan or BAT actions. Have him stick it in the stock you prefer - Manners offers a nice stock. and outfit appropriately with your favorite scope, bipod, brake, barrel, etc.
There is so much research to be done to decide on the appropriate build options you really need to do significant research before hiring the build done. There are a lot of other things to consider than you would in a normal rifle build. Dropping to a repeater action will decrease accuracy more than it would in a lesser caliber, there are a TON of different chambers since there is no SAAMI spec and you need to know how much time you want to invest in reloading/shooting before you decide on a chamber. You wouldn't want to have to turn the neck of all of your cartridges if you just want to plink - likewise you wouldn't want sub-par accuracy if you expect the absolute most out of your rifle and yourself and don't care how much time you spend prepping brass, etc.
If I was going to just buy a box-stock rifle it'd be either the AIAW50 if you can afford it or the TAC-50.
Edit: I don't think I can recommend the windrunner anymore. Apparently they've dropped quality considerably from the rifles they were producing. This was a recent change, but until they re-prove themselves I think I'd keep my distance.
