OK, I know this is a subject that has been beaten to death over and over and over but what I'd like to do is look at these two excellent rounds both individually and together. I won't waste your time with my opinion.....I'm going to waste your time with REAL SCIENCE and FACT! To begin with the 5.56 62 gr travels at an average speed of 3100 fps @ muzzle and has a muzzle energy of 1767 Joules or 1303 ft./lbs. The .223 Remington (civvie equivalent of the 5.56) 62 gr (Hornady FMJBT, so we can get down to more of an apples for apples comparison) travels at roughly......are you ready? 3000 fps @ muzzle and a muzzle energy of 1738.1355999999998 Joules (sorry if math hurts your eyes) or 1282 ft./lbs. These are ONLY two examples. You will find a wide selection of ammo in this caliber with varying bullet weights and velocities but for the purposes of this discussion we will stick to the examples given. The round has excellent accuracy, is relatively light weight and cycles well under normal conditions. What this round lacks is punching power. During the Vietnam war the initial run of M-16s chambered in 5.56 were found to be less than adequate. The round would not punch through thicker trees but the enemy AK-47 would. Much of the fighting was up close and personal and the 5.56 is so small and moves so fast that at close range it went straight through and did not always cause the expected damage. The reason is that the round is meant to hit flesh which slows the front end down and causes the bullet to tumble and yaw leaving a path of destruction in its wake. If by chance the round struck bone, it would bounce around causing a lot of damage. In theory, this works great as long as your target isn't 10 feet in front of you. At close range the 5.56/.223 tends to just go straight through and out. Backing up a little, During the battle of Dien Bien Phu, the Garrison of French Foreign Legionnaires were VERY fond of the M-1 Carbine because of the close quartered nature of the fighting. One Legionnaire reportedly stated the reason that the M-1 Carbine was so well liked was because its .30 caliber pistol round would cut the Viet Kong in half. That was 1954, long before the M-16 made its debut. Moving back to the present, we have other examples of the limits of the 5.56. Afghanistan gave us several reports of enemy fighters being hit squarely in the head and they did not stop, they just kept on fighting. Unusual yes! but it does happen. Our boys in Vietnam also felt the round lacked "knock down power". Many times soldiers would, if given an opportunity, give up the new M-16 for an older M-14 because the .308 round had a much better punch and the rifle itself simply performed better in the jungle environment.
There was an attempt for awhile to build 7.62x39 ARs. The reason was and is simple. Larger bullet doing more damage. For our example I will use the 7.62x39 123 gr. bullet with a muzzle velocity of roughly 2200 fps and 1791.8388379999997 Joules muzzle energy or 1321.61 ft/lbs. Some of the 7.62x39 ARs worked well but the bulk of them had cycling problems. In short they weren't always as reliable as their owners would like. Another issue was the non-standard 7.62x39 AR bolt. One great use of those bolts is in the .50 Beowulf because it has a rebated rim that is the same size as the 7.62x39. The 300 Blackout aimed to solve the feeding issue by cutting the existing 5.56 case down and re-necking it to accommodate a .30 caliber bullet (Pictured above) . It would use the same Bolt and Magazines with no loss of capacity. The obvious change was the only required change....the barrel. The 125 gr. 300 Blackout has a muzzle velocity of 2100 fps and will deliver 1659.18736599 Joules of energy. Grain for Grain of bullet weight, the 300 Blackout will provide an almost joule for joule performance compared to the 7.62x39 but with two big differences. The first is that the 300 Blackout cycles flawlessly in a properly constructed rifle meaning that the barrel length and gas system are properly selected for the type of Blackout round being used. Let me explain further....The blackout has two popular flavours, supersonic and subsonic. Supersonic will operate perfectly in standard 16 inch barrels and may be used with or without a suppressor. The sub-sonic rounds are designed to be used with a suppressor and generally work best with that added back pressure. We'll talk about sub- sonics later. The second and maybe more important point is The 5.56 at 500 meters has 394.5 Joules or 291 ft/lbs of energy, the Blackout will have the same energy at 700 meters from a 16 inch barrel, However the max range according to U.S. Army standards of hit probability for the M-4 (max distance at which there is a 50 % probability of a hit), means that the effective range of the 5.56 in an M-4 is 500 meters and the 300 Blackout....460 meters. So science and NOT opinion tells us that the 300 Blackout at 500 meters is packing more energy than the 5.56 NATO at 500 meters so the choice is up to you which one you think is better. SO What can we deduce from the above information?
1. Both Rounds are very good. Accuracy for both rounds is also good.
2. Both have their particular talents, situations where they perform better than the other.
3. The 300 Blackout will deliver a lot more energy to a target at longer ranges.
4. The Blackout has nearly the same EFFECTIVE range from a 16 inch barrel as the M-4.
5. In CQB situations, the Blackout may offer an advantage with a larger slower moving lump of lead.
6. 300 Blackout subsonic is quieter when suppressed than a suppressed M-4
7. In REAL WORLD applications it would appear that either round will get the job done so it will, in many cases boil down to personal preference BUT if you're thinking logically, in real world applications, the odds of you needing to shoot anything beyond 250 meters is going to be mostly unrealistic. Anything that far away will probably not require engagement.