Hey Boot you probably already figured this out, but if you didn't -
You have to make sure that the background color doesn't change at all. It has to be the exact same RGB values as the default BMP or it won't be transparent. So if you can do this with Corel, check to see what the default bmp background color RGB is, and make sure it stays the same after you change the gunsite. The gunsite color can be any color.
When you draw the gunsite lines you can't go all the way to the edge of the background color. YOu have to leave at least one pixel of the background color around the edge.
Also it has to be a 256 color bmp or it won't be transparent.
The default.bmp is 128x128 in size, but I've seen some that are 64X64. I'm not sure if there is a maximum or minimum size.
What I do is open the default BMP, make a copy of it, and then work with that. Or you can just copy the default.bmp file to another folder and open that.
Or, with Paint Shop Pro, you can make a palette of the default.bmp colors, so when you want to make a new gunsite from scratch, you just load the palette and the colors are right.