Energy comparisons are useful, but aren't the full story.

Energy is only one component of what makes a bullet successful.

Let's give an example using .300blk. From a 10.x" barrel, with the 110gr Barnes you are getting about 2,200fps, and with a 147gr FMJ you will get around 1,900fps.

Plugging those velocities into JBM shows the 110gr Barnes as having 565ft/lbs of energy at 300 yards, and the 147gr having 700ft/lbs of energy at 300 yards. Yet we know that the 147gr FMJ is not nearly as effective of a bullet as the 110gr Barnes.

Best to look at energy as the means in which the bullet is able to perform it's work. That work can be categorized as penetration, expansion and/or fragmentation, as well as potential for tissue damage. But that ability to do work (energy) only matters in relationship to the materials available and the engineering (or design) of the bullet. The 147gr bullet has more material and more energy, but you can't say that it's going to do more damage to tissue then the 110gr Barnes, with less energy and less material, but a better design.

So comparing energy numbers isn't really ever an apples-to-apples comparison.

A better way of looking at it, in my mind at least, is to start the process with an identification of what you want the bullet to be able to accomplish at particular distances out of a particular barrel length, and then from there identify what caliber and bullet combination meets that requirement.

For example, to me a PDW weapon is one that is concealable, quick to deploy, should be accurate enough to 200+ yards, have solid terminal ballistics (ie. be able to penetrate at least 12" of 10% gel) and since a PDW is likely to be used in or around vehicles, perform even if the projectile hits an intermediate barrier (auto glass, sheet metal, etc).

Both 5.56 and .300blk have rounds that are capable of that.

So the question becomes which one does it better?

There are more than a few gel tests on both and from there we can get the following info:

The 5.56/.223 62gr Fusion (out of a 16" barrel) will have a max expansion of around 0.535", min expansion of .352". Average expansion of about .443". A shorter barrel will affect those numbers.

The .300blk 110gr Blacktip will have a max expansion of around 0.782" and a min expansion of .491". Depending on the test, the average seems to be in the .585" - .636" range. Those numbers come from 8"-10" barrels.

The .300blk has a larger temporary stretch cavity, which isn't an end all, be all, but it's something to notice. It also penetrates deeper.

The 5.56 will lose initial velocity faster as you go to shorter barrels than the .300blk, due to the power burn rate.

For those reasons, I keep a 10.3" .300blk (and am considering a 8.x" .300blk). At the same time, due to cost and availability of the 110gr blacktips, I also keep a pair of 11.3/11.5" 5.56 SBR's loaded with Federal Fusion.