Ah this an easy one. Most of the time you will find standard size shrink tube, 1/8, 1/4, etc. I think the 1/4 is the one that is a tight fit on the 6.5mm bullet. The next size I found to be to big. Here is what you do to get a tight shrink tube on the bullet. Spray a little window cleaner inside the cut shrink tube, then slide it over the bullet. The window cleaner will allow it to slide right over the bullet, hit it with a heat gun which will dry the window cleaner and give you a nice tight fit over the bullet and wire.