556 over penetrates (especially here in the US where everything is drywall) and you risk killing others besides the intended target in “house use.” Buy a shotgun.
556 = small, light weight, flat trajectory, low recoil, easy to manage with burst or full auto. Combined with SS109/M855 is makes a big mess inside a human because of the instability and high velocity.
Non-elastic organs (Heart, liver, kidneys, and spleen) are ripped to pieces inside a human from the shock wave. The slug itself begins to rotate and after 7 cm penetration is on its side, unlike 308 which tends to punch nice clean holes through people because it does not begin to do so until 18 cm of penetration. The little 223 often comes apart and exits with two holes since the M855 ball has a penetrator tip and lead core back which usually split in two when the slug is rotating. Bottom line: 223 makes a huge mess of humans.
Nothing can replace good shot placement. If you hit the brain or spinal column the dude no matter how big or tough or what he’s doing, will drop. If you hit someone with a 50cal in the leg, as it happened at a TCP with us, the guy will sit there with his leg essentially blown off, but alive and conscious. A bigger bullet often does not bring better results and you can see that when you hunt. With my little Marlin 308 I drop big game, and someone else might shoot the same animal with a 338 (8.6 x 70) and this animal suffers. If I take someone and have them kneel, shooting them into the back of the head, a 22 will do the trick 100% of the time. If I shoot someone in the lower abdominal region with your 6.8mm he will be in agony, but most likely alive. Where the bullet goes will make the difference!
223 is a general purpose round. It depends on the ranges youre shooting at and what youre engaging. Against a human from 0 300 meters its nice to just be able to put the dot on the target and know that the bullet will only be 2 inches bellow, above, and then bellow line of sight in that range span. 223 is adequate to kill humans, that simple. On burst or auto you can control the weapon. Even when taking quick aimed shots you have an advantage because your recovery time is so little, practically none. I could easily go to the range and shoot with less than one second between shots, and every shot is actually an aimed shot, because 223 is like a kids cap gun. Because of the almost toyish feel when shooting its not an intimidating weapon that can easily be learned on even by those who might be intimidated with something like a G3. I can carry lots and lots of little bullets, and thats nice, because they dont have legs of their own and I had to carry them. 223 is not something you go bear hunting with, but it will do well with hogs or humans. 223 might not be the best choice when hunting dall sheep in Alaska on a mountain side at long ranges, but it will do perfectly well against a human from say 10 150 meters which is what over 90% of all your engagements will be at.
The 223 cartridge is a balance in performance aspects that while not perfect in all situations, all the time; is more perfect than any other cartridge most of the time. Its a round that was designed for a specific target in mind, operating at a span of ranges that a soldier might work at, and incorporating other aspects such as transportability. If having slightly more range that might only pay off in 1% of the realistic engagements means I have to carry a weapon that is heavier 100% of the time, I didnt make out on the deal! So while 223 is not perfect in every situation, it works very well most of the time and has a good balance of characteristics for meeting the purpose of what its intended use is, a general purpose cartridge for killing bad guys.
My point was that M16 doesn’t always = 5.56mm - I should have been more explicit.
I now use .223 for varmints - I can hold to a dime sized group at 100M - the bipod and glass helps as does the handloading ; )
For any social situations at the house I have a 45 ACP that is always handy..... Intrestingly I have seen 90 gr 5.56 loads advertised at 'surplus' Must hit like a buzzsaw at close range - it has to upset almost at the muzzle....
When I was on active duty, I prefered to carry the AK-47 (Romanian folding stock version) to the M16A2 or M4.
I found that the folding stock on the AK was safer and easier to handle during Fastrope and airborne ops. If you fastrope with an M16, there was a tendency to be butt-stoked by the weapon when you landed.
I found that the 5.56 round would be deflected in light foliage or moderate winds, while the 7.62 would not be affected as much. The 7.62 round is also easier to apply "Kentucky Windage" to since the splash of the round is easier to observe at medium-long ranges.
As far as the M4 is concerned, I hated it. Yes, it was a small package that was eay to handle, but that came at the expense of accuracy. We were issued the ACOG sight and that was a real boondoggle. Zero at 25 meters, confirm at 100 meters, and reconfirm at 250-300 meters only to find that the point of impact had changed at the greater ranges. I preferred my civilian AimPoint sight to the ACOG. The troops today have so many gimmicks attached to their M4's that it negates the weight saving benefits.
All in all, I preferred the AK series for the close in, heavy underbrush area's or CQB. The M16 was good for longer ranges found in desert area's. Both weapons are good when used within design parameters, or they wouldn't be as prolific as they are.