If I were going to do this, I'd weigh a loaded round, file the tip of the bullet to the desired length, and weigh it again to determine how much weight you've removed. If it's not too much, and you wanted to shorten a bunch of them, I'd use my El Cheapo Harbor Freight chop saw to get the job done.
The BIG BUT is that I don't think I'd do it at all, but that's just me.
Sometimes you can get away with playing with fire, other times you get burnt. Think about it.