Most modern centerfire ammuntion you will find today is inside lubricated. It simply means the driving bands and lube grooves are inside the cartridge case. If you look at a .22 rimfire cartridge they are representative of heel based bullets. The bullets are the same diameter as the case with the driving bands and lube grooves exposed. The heel at the base of the bullet is a smaller diameter to fit inside the case. Having the lube on the outside makes it real messy to toss a few rounds in your pocket, especially on a hot day, and so, along with a few other reasons, the inside lubed idea won the popularity poll, while the heel based centerfire idea slowly drifted into obscurity.