Changing the camo outfit on your Barbie doll doesn't do squat.
If you want to mod a carbine to make it work better, don't start with an institutional compromise some large government bureaucracy committee engineered.
From the basis of the M16 - in order of priority, meaning this is important, and each item depends on the previous being more important. Too many get this backwards.
1) shorter barrel - it's a carbine, which makes it 16.1". Shorter requires a BATF permission slip, and since this is an all around workhorse, not a CQB specialty item, that's good enough.
2) caliber - you lose too much velocity with the 5.56, improving power makes it more than a varmint round. Make it legal to hunt - and effective. In a carbine, the 6.8SPC effectively does that, and is also why it's the #1 alternate AR caliber.
3) upper - A3, of course, for mounting optics. Forward assist? not really needed. It's just that you will get one cheaper with it rather than without, so for the added utility of quietly chambering a round hunting, ok. That is actually it's only value. There is no military application for that, no one leaves the wire unless they lock and load, on safe. The shape is relatively unimportant.
4) optic - an all around workhorse
carbine is NOT a long range varmint or sniper. 2 or 3 power at most, preferably a red dot. They allow shooting both eyes open, acquire a target fast, are more accurate in fast shooting. For back up, standard issue irons. They won't break, are always ready, don't need extra time fiddling them into use, won't loosen and fall off, and deliver more value for the less expensive price. A cut down surplus M4 carry handle rear, and fixed front sight work as good cheaper than some fancy rail mounted BUIS for $100 more.
5) furniture - adjustable rear stocks are for armor plate wearing combat soldiers. They need the different length of pull, civilian shooters don't. Period. The disadvantages for an all around workhorse carbine are more parts, expense, and things that go "Cronk!" at the worst possible time - within hearing of your target. Use an A1 or A2 and save the money, it even gives a better cheek weld, making it more accurate. Far more comfortable in hunting weather, too. All around means winter use, something far too many ignore. That goes for handguards, too, if the barrel is milspec 2MOA, use them, not the institutionally imposed quad rail. It adds nothing to barrel accuracy, what causes a shift in the POI is using a long distance range sling. Wrong application. Don't. It's a red dot sighted 16" gun, not a three position International target rifle. Grip? Whatever. It's not a handgun, you support the weapon from the shoulder and handguard, all the firing hand needs is a rest to oppose the trigger pull. Don't overthink it.
6) Trigger - dead last and least important. CARBINE, remember? Get a trigger travel adjustment screw, take out most of the slack, your trigger will be 80% relieved of creep and grit right there. That makes it an all around workhorse, a crisp, close trigger with a field weight pull that won't go off getting it out of the deer stand or tripping over brambles. And, yes you do, I know for a fact some of you hunt with the safety off. A 2 1/2# target trigger is a known hazard in the field. If it wasn't, it would be milspec. It's not.
Most of the dress up recipes on the net have nothing to do with really making the AR an all around workhorse, it's just an OMG.com blog about the neat dress up gear someone charged on their credit card.
You eventually learn the more boring it looks, the more likely it's effective. Tricked out bling guns can't do an all around job - they have too many competing parts that the operator never learns how to use. Remember, you practice until you can't do it
wrong.
