Paul Cox and Matthew stared at this and talked about different things, trying to find the perfect way to compress the magic inside the gun without affecting the weapon.
They both came to a consensus around this: They needed to find a way to create a small bullet just as the magic was being created. Basically, they needed to make it so that, instead of compressed bullets, there would be some sort of highly resistant material that could compress them at the right moment, just before firing.
That would avoid the cost of producing the bullet since it was not always going to compress the magic but would only do so for a moment. So it was cheaper and wasn't really hard to make. However, the issue was that they needed to add several mechanisms to the inside of the weapon, so the size might go up.