It took an exacto knife, 3 crowns and 4 black bullet helms and some crazy glue.
Basically I decided to cut the base off a crown. It took me 3 tries to get the cut nice and straight, I had a lot of crowns, so I didn't care.
Getting the Bullet helm trimmed to nicely fit under the crown took about 4 tries and finally worked. The key was to cut along the line of the moulding in order to ensure a straight cut, the bullet helm plastic is very soft compared to the crown so it took me a few tries.
As for glue, be careful, too much and the two pieces won't sit right, too little and the base of the helmet will break off when you put it on the fig.
Make sure the glue is dry or you will never get it off the head. I tested my helmet on a smiley head before I risked the Admiral Head I use for my fig.
The helmet is now as stable and easy to take off as any other standard helm.
I only recommend doing this if you have either great skills with an exacto knife(I didn't) or many helmets you don't mind ruining. Cut right, the bullet helm top does look good on a mercenary.
After a long absence, I have returned. I can't wait to start building again.