The Watson .50 BMG is slow as molasses to operate, that is the biggest down side. The .50 BMG is terribly expensive to shoot, even milsurp ammo is going to run you over a $1 a shot and if you reload, if you want good reloads, you are going to drop a lot more money. I got the AR-50 and by the time I got the scope and all the goodies to reload match grade ammo, I had three times the cost of the rifle tied up in the .50 BMG total. There is nothing cheap about shooting the .50 BMG, don't be fooled into thinking you can shoot it cheap using surplus ammo, it is still going to cost you a lot. And forget about putting a cheapo scope on it. Figure at least on spending $700 to twice that amount on optics. The cheapest scope I would recommend is the Leupold VXIII LR series. Better still are the Nightforce or Mk 4. I have absolutely no faith in the budget priced Super Snipers by Tasco. Some hold up, many do not. The .50 B on the other hand, is not cheap to shoot either, but it is a hell of a lot cheaper to shoot than the .50 BMG. Optics do not need to be so rugged and you can shoot a lot more quickly with the semi. The 50 B is great for shooting 150 yards or less, while if you are not shooting the .50 BMG at ranges of at least 300 yards, you're just wasting ammo and making noise IMO. Why use a long-range rifle for shooting at short range? The 50 B will not cost you an arm and leg to get going either. Dies, bullets, brass, and powder are much cheaper and you don't need a special press to load for it. For the amount of powder you shoot in a .50 BMG, 220 gr to 235 gr, you can load 4 or 5 .50 B rounds.
Just something to think about.