I usually spend my time over in the Mentalism section, but I came over here to share some experiences I've had with the Muscle Pass and the coins used.
I've done the MP recreationally for about two years and I typically use an English Penny. I chose to use that coin for two reasons: 1) It's lighter than a half. 2) It's thinner. I've found that thinner coins are easier to pass and grip. The vertical jimping on halves does little for me as far as grip. Then, while at a local surplus shop, I found a small square of aluminum. For ten cents, I couldn't resist. So, I got home and milled out an aluminum coin the exact dimensions of an English penny. When I weighed the penny, it came in at 8 grams and the aluminum, 2 grams. The aluminum coin absolutely flies out of a Muscle Pass and can be polished to be as reflective as chrome. The resonance is also pretty cool sounding; it's got a nice ring.
So, if you've been saving up to get a few Walking Libertys or something similar, you could easily, and cheaply, fashion out a few aluminum halves.