Any of the holes can be drilled bigger.
For the acrylic (plastic) one, avoid using a drill (it’ll drill too fast and can melt it). Drill the holes by hand (use locking pliers to twist the bit). If you’re really going bigger, drill one size at a time or the bit will get stuck. With some of the plastic bullets, the holes do not line up and boring them out is necessary for the bullet to function the way it’s supposed to. Finally, take it apart and blow it out really good.You can have your bullet fine-tuned and customized in less than five minutes!