When you talk about velocity, and how much is enough for any caliber, you also need to consider the intended purpose and especially, bullet construction. Apologies to those who already know this, but it's not common knowledge and I think some might be interested.
If you're looking for a range toy, or in general to only shoot paper targets, then caliber and bullet construction doesn't matter except to tell the guy at the bench next to you why you're obviously smarter than him. If you're looking for a defensive weapon, then you're getting into terminal ballistics. This is not always the same thing as terminal ballistics for hunting, either- the goal is to stop the threat, not to kill with minimal time tracking or minimal tissue damage.
Bullet weight for rifles isn't everything- there's a lot of anecdotal evidence out there to show the old 55gr Vietnam era .223 bullets performed a lot better than the current 62gr green tip does. Some would chalk this up to the lower velocity of the heavier bullet, or the lower velocity of the shorter barrel on the M4 compared to the old M16. This ignores the difference in construction. The 55gr is a standard lead core full metal jacket. When it hits, it tumbles and fragments, causing a much wider wound path than it would if it remained solid, and usually causing all the energy to remain in the target.
By comparison, the green tip is a split core, with a steel penetrator in front of a second lead core, all with a full metal jacket. This was developed because the Army expected to be facing Soviet motor rifle divisions in Europe, and wanted to punch through their standard issue steel helmets. Because steel is less dense than lead, the whole works is even longer than a similar weight solid lead core, and this required a faster twist in the rifling to keep it stable.
The combination of longer bullet and faster twist discourages the bullet from tumbling when it strikes a soft target. The reduced deformation of the steel penetrator in the front half further reduces the bullet's ability to fragment, and the combination of these effects is what causes overpenetration and reduced ability to stop the enemy. Fire the result out of a short barrel and it's no wonder we only issue 10" barrels to Rangers or SWAT guys, because most of us can't guarantee we will place a bullet accurately enough to work under those conditions. Of course, SWAT around here uses a 69gr .223 Matchking. At close range, this still retains enough velocity to fragment.
There are some bullets you find on the reloading shelf that were intended for .308win and .300mag velocities, and just plain won't work in blk. You shoot them at a deer or hog, and they act like a solid, or like a green tip- they just go through the whole animal like a laser beam and don't put them down. Barnes has released a special version of their TTSX with a different color tip that is supposed to expand at blk velocities, and I would guess if you fired one out of a .300mag the front half would fragment like crazy.
I should say that terminal ballistics is kind of a black art- no matter how a bullet functions in gel tests, or how many deer/hogs you drop, sometimes a bullet does something completely unexpected. There are many people and deer who have fallen to a .22lr, and there are people who have survived being shot through the heart at close range. As a general rule, though, you can pick your bullet to match your gun and your intended use, and if you do so correctly, you will have much better results.
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Almost forgot- what DRIFTER may have meant was that you lose less velocity with a short barrel on a blk than on a .223, which is correct and may not matter at all depending on what you are trying to do.
Looking at barrel lengths of 16" and 10", chosen because they are common numbers, Hugh's chart shows a drop of 394fps for .223 and a drop of 224fps for blk. This matters for terminal ballistics as the bullets drop out of their design window for fragmentation or expansion, which is of course different for every bullet.
For myself, I have a 16" .223 and an 8.5" blk, and I enjoy both. Neither is as much fun as the 16" .308, though.
