Necking down implies actually reducing the dimensions, you know - the render says 12.7mm. Change it, maybe?
Also, the round as it's drawn is still too small unless you were planning on the weapon being a third of a meter long. Obviously you're familiar with the ubiquitous (on the Internet) Desert Eagle. I invite you to look again and see how big a 12.7mm bullet is in real life - it's about the size of a grown man's thumb.
Necking down can mean that but often times it means that the case dimensions are the same as the parent case (in many cases) except that the neck around the bullet is smaller to accommodate a smaller diameter bullet, for pistol rounds this gives it a rifle round appearance. The dimensions stated in the picture are 11mm (diameter of bullet/neck), 12.7mm (the diameter of the case shoulder), by 32.6 (case length). In real life it is a wildcat cartridge that is a .50 AE round fitted with a .44 bullet, it's the same design concept as the .357 SIG being a necked down .40 S&W. I don't know what else to tell you, I agree that the bullet looks too small in the pic but it is what it is. I'm not going to change it (or any miss-spelled words) on this or any of my other old drawings for the simple fact that they are OLD and I have newer versions planed to draw up soon (I hope).
Some links about the .440 Corbon round if you feel like taking a gander. Info: [link] Pic: [link]