The two cases [303 Brit & the 308 Win] have identical capacities, so that should tell you what the real difference is....zip! The limiting factor for the 303is the rifle it is chambered in [Lee-Enfields tend to be a bit "springy"], and some will bring up the fact that it is a "rimmed" case, which can be a detriment to feeding from certain magazines. In a front locking bolt action like the P14's, the 303 can be loaded to equal the 308 without any issues. I have a couple of these, and have found that 308 data works great. the .003" bullet diameter difference is a non-issue. One thing that will plague the reloader is the lack of a really good bullet selection for the 303 British, whereas the 308 has an endless variety available. The bottom line is, within 300 yards, no game animal can tell the difference of a 180 grain bullet at 2500 or 2650 fps. If your 303 is working well for you, keep using it. If you feel you need an upgrade, then buy what suits your fancy. Regards, Eagleye.