I'm the last generation issued the A1, 20" 55gr. I shared my father's, first generation issued A1, opinion. To light and absolutely needed all 20 to get up to tumble speed.

I like the ballistics of the 6.8 better as an all arounder to what I know of the 55gr. I know nothing of the 62 or 77 grainers other than the A2 the line units were getting was heavy.

Convince me that a short 10-12 inch pistol and modern 5.56 is the way to go vs a 16" 6.8.

Ammo cost and availability is already point one. I'm needing to know performance at all around ranges not the 1000yd gong but reasonable if you can see it ranges. Ideally performance that can do in deer sized critters over most ranges and bigger, hog, at closer range. Well maybe not the big hogs.

.223 for the ammo issue them other calibers might just be fancy clubs some day. NATO rounds are always the best bets.

I think this is the video for you to see the difference barrel lengh has on 223/5.56

6.8 is a bigger case and to large extent there is no replacement for displacement.

A short barrel (less than 16") 223/556 is a very situation specific tool. It aint worth a hoot for hunting anything and is made to be a good compact platform that can be maneuvered easily such as in and out of a vehicle or close quarters stuff. It still has enough energy to be reasonably effective at shorter ranges like less than 100 yards. I am referring to the 10.5 to 12 inch barrel versions. Also if you add a suppressor on a 10.5" it is still compact and easily handled. We are not limited to only fmj stuff as civilians so the whole max velocity for a fmj stuff is a null arguement. Just load nosler ballistic tips or pretty much any mushrooming bonded bullet and it is still effective at shorter ranges. Most 223 bullets stop expanding around the 1800-2000fps mark so know at what ranges your varmint bullets become fmjs.

In a 16" gun only loses about 200fps from a 20"barrel and is plenty effective but a little more compact. Thats why they are the most popular barrel size. A 20" barrel will always be the most effective and it you are using irons you have the largest sight radius and will shoot it a bit better than shorter ones. Most people use magnified optics or red dots so 16 or 20 doesnt matter much with accuracy. I have all 3 sizes and if stuff hit the fan i would reach for the 20" first probably and then the 16". I would leave the 10.5" for truck or house use. Also the muzzle blast gets pretty extreme when you get under 16" barrels.

For what its worth a 20" barrel using hot 55gr fmj loads will penetrate an ar500 steel body armor plate at short ranges. The shorter barrels will not come close. Velocity is still king in the energy equation.

Now if you really want a short barrel AR then 300blk is king in that department. It was meant to be fired from a short barrel, like 8 or 10". I have a 8.3" 300blk that i hunt with supersonic 110gr barnes tac-tx and heavy expanding subsonics. Killed a few deer with it, even though its not really optimal in my opinion.

I also have a 458socom AR that is more of a toy than anything, but it is devastating on short range hunting (less than 150 yds).

The other new super cool calibers that come in the standard AR platform are stupid IMO. They arent so much better than you can custom load a 223 round to do a similar thing that i have ever considered them. Plus factory ammo for them is expensive and not always easily found.

As far as ammo, I have a few 55gr stuff but i am slowly eliminating it from my stockpile. I only load 60-62 gr stuff and the only ammo i have bought in recent years is IMI m855 62gr green tip. M855 gets a bad rap i think. I have some m855 lake city stuff and it is horrible, shoots large groups (2 to 3") and has horrible standard deviation numbers like over 50 on my chrono. The Israeli IMI stuff has great numbers on my chrono and shoots decent groups, 1-2". My handloads which are nosler ballistic tips 60gr and speer gold dot 62gr all shoot less than 1" from both of my main rifles.

The only surplus ammo i buy is IMI when its available.

I have loaded lots of other bullets in the 55gr to 62gr range and nosler and speer are my favorites when price vs performance comes into play for what i use that caliber for. I have not got on board with the heavy 77gr stuff.

I’ve killed everything from deer to 250# hogs with a AR15 (5.56) 16” barrel anywhere between 10-180yds. Just like anything else, shot placement is the deal breaker. I’ve shot 62gr Fusions for both deer and hogs and even 55gr FMJ’s on hogs. Most pigs I try to shoot where they’ll run off and die, but most seem to drop anyways and end up having to be hauled off. Night time shooting I want them dropping on the spot...yeah I’m a grown man that’s more afraid of a hog than snakes and gators.
IMHO I believe an AR15 (5.56) and even a AR10 (7.62) is the most kid friendly deer rifle around. My boys both used the AR15’s then graduated to AR10’s to reach out a little farther with a little more punch.