I'm away from the PC right now so I don't have any pictures to demonstrate what the title is referring to so I'll be as in-depth as possible.

I made a duplicate pistol to test out some new magazine + projectile properties, and I increased the magazine from 12 to 18. To match this new size, I created a new weapon meter. However, the meter does not begin to deplete until AFTER I have fired the first 6 shots (begins on the 7th shot, the 1st of the original 12). I've checked and double-checked that the meters values aren't wonky, and there are definitely 18 rounds loaded. What am I missing?

You have to make a new hud that has slots for all of your 18 rounds. Other wise the default is to not start removing bullet symbols until the number in the mag is smaller than the hud support. If it didn't do that, the hud would show the magazine as empty, but you would still have 6 rounds in the mag you could shoot before you had to reload. I am not familiar with the process of making huds, but there are tutorials on how to make your own. As a temporary workaround, you could use the hud of another weapon that holds more than 18 rounds. The assault rifle or the needler hud could both work. The only problem with this workaround is that the game will always act like you should reload, and the hud will only display the max amount of rounds in the weapons mag.

An update: so for the last week I've given myself a sort of crash course on hud editing / creation, and have been studying everything from scaling issues (crosshairs appear 9/4ths bigger than they should) to alpha issues (CE seems to apply a curve of some sort to alpha values, which makes meters not work correctly). I'm extremely confident in the values of the new meter, and everything is as it should be.

I just did an experiment that was sort of what Nathan suggested. I changed the rocket launcher magazine from 2 to 4, and changed the sequence of its weapon meter from 6 (rocket launcher) to 2 (sniper rifle). So, now I have a 4-round launcher with a meter with 4 rounds that we all know depletes correctly! Which means everything should work right? Well, wrong! The rocket launcher definitely fired 4 rockets, but its new sniper meter didn't begin to deplete after the first 2 rounds. I have concluded that the bitmap is not the issue, there must be a value I've missed, or perhaps there is some code in CE that forces it to just do this to all default weapons?

SOLVED! Issue was the alpha multiplier value in the weapon hud. Thanks for the assistance.