Nice write up and well done on your first deer! You never forget your first hehe!
I must admit muntjac are notorious for ending up gut shot - They are small, twitchy and prone to sudden movement when you least expect it meaning you can often end up with the shot not quite going where you expect. Or indeed leading to a miss!
Add on to that the fact that the rumen is very thin and can sometimes rupture just from the impact of the bullet even if the shot does go in the right place. You've got an animal not much bigger than a domestic cat and you're whacking it with a s*dding great thirty cal lump of lead at the thick end of 2000mph - Thats gonna do some damage!