I didn't have to do this one-by-one. I went to the bench and selected the 9mm bullets to break down. I had 50 of them. It asked me how many to break down, so I selected them all. I then selected the 9mm bullets to reconstruct. It, again, asked me how many to build. I selected as many as possible (I think it was 44 - you do lose a few when you break them down the first time). The achievement popped instantly and that was my first time at the bench.