>>348950592
I'm Oculus so idk if it's different on other things, but you can vacuum them up by grabbing one and then pressing Down on your joystick (or touch pad??) on other ammo of the same size. 

Up on the joystick is drop a bullet or put it into your belt slot. 
You can stack them by dropping them that way into your belt slots too, then grab a pile from your belt all at once like in that video.