I have been using the Red Death exotic Pulse rifle gun since it was sold by Xur and have noticed that the amount of primary ammo I have in reserve drops at certain points, such as returning to a planet from orbit or reloading to a previous checkpoint after wiping i.e. I'm on the Moon with 435 in reserve, Go to orbit, return immediately to the Moon and now have only 288 ammo. I do this again and it goes down to 188. I've never seen it drop below having a full clip (27) and 183 in reserve. This doesn't happen with any other gun I use yet I've read no mention of this issue on any forums. If it helps, I have two pieces of legendary armor that increase Pulse ammo capacity in case that might be a factor.