The .50 Cal has the exact same damage ratings and hit box modifiers of the R700. The only difference between the two guns is that the .50 Cal fires Semi-Auto. Any other difference about the stopping power of the two guns is purely a perceived one.
Well, the R700 far more accurate than the Barrett .50 Cal, but I was complaining more about the fact that it should one-hit KO people just like it does IRL, to make up for its amazing lack of accuracy (which is also a load of crap). I mean, I do realise it's a game, but to have a hunting rifle like the R700 beat the Barrett .50 Cal in terms of accuracy is just weird.
For my money, I will take the M40 every time. More accurate, only one worse hit box modifier, and the highest damage rating of any gun in the game if you put an ACog on it (for some inexplicable reason).
I actually think that Infinity Ward confused the R700 with the M40A3 - the R700 is less powerful, so that should be the first weapon you unlock, and the M40A3 should be where the R700 is. That said, in terms of accuracy, zoom speed, firing rate and steady speed, the M40 does beat the R700, but I prefer to use a UAV Jammer instead of Stopping Power, and Bandolier instead of claymores.
That said, I don't ACOG snipe with the M40. I just don't like being killed with one, because it is exploiting a weird programming glitch, but also because I use an ACOG M14 and I'm trying to unlock ACOG for my Dragunov. I prefer semi-auto at close range.