I think it's a limitation of the way OpenXCom does things, but that's just my own feeling.
I'm sure you can run some FPS game that has multiple (graphically represented) projectile trajectories at the same time on your computer (even if just something like the first Unreal Tournament), so it seems unlikely that it has anything to do with your hardware, more with the way the game does things.
If you try to shoot the buckshot at short range, I am pretty sure it works since the trajectories are shorter so less intensive for OpenXCom to handle. This is also the best range to use the buckshot since you are more likely to score multiple hits on an alien that way. Over long ranges, explosive ammo is the way to go.
