I've heard of people tackling .50BMG rifles via plans, but the general consensus is that you need to have about $20K invested in machine shop tools in order to realistically build one. Plus MANY hours of labor. When this is all done and considered, the price of actually buying one doesn't seem so bad.
My personal opinion is that unless you really have a need to fire off rounds in a .50BMG in rapid succession, a good bolt action will be plenty enough fun to play with. To be honest, I would much rather shoot my model 99 Barrett than my M82A1 any day. I haven't tried the model 95 yet.
Building a semi-auto .50BMG from plans, while interesting to do if you have the metal working knowledge and machinery available, would probably be an all consuming project to undertake. It would be much more complicated than a straight bolt action, I would think.
Then there comes the time to pull the trigger for the first time, just to see what happens.
I've heard that one of the early Serbu bolt action rifles had a fault that caused a shooter some severe injury to his shooting hand when the case ruptured. I believe it fired when the bolt wasn't completely in battery position, but I don't remember the details any longer.
Good luck!