I should do this while it's still fresh in my mind.
Mandatory safety and reliability mods:titanium firing pin, one-piece gas rings, anti-walk pins
Mandatory accuracy mods:Demon Tactical expanding rear takedown pin
Recommended accuracy mods:Commercial target trigger, or "break in" of existing parts using diamond hones and Nanolube diamond-bearing oil, break-in of bolt carrier components with Nanolube
Recommended convenience items:ambidextrous safety, extended charging handle, oversize magazine release, Magpul pistol grip, Magpul telescoping stock, Magpul enlarged trigger guard, nylon buffer cap for recoil buffer, "anti-twang" plastic disk on rear of buffer spring, safety covers for exposed pieces of rail
"Luxury" items:new forearm, or extra rails, QD sling swivels, improved flash hider/muzzle brake, Magpul BAD (bolt assist device)
Personalization:funny/insulting engraving on dust cover, receiver pins, etc
Optics:backup iron sights, appropriate optics.
I cannot overemphasize the importance of not skimping on optics. All the other tweaks are intended to let you get the most out of your choice of optics. The GWOT has shown that rugged, precise combat optics are the key to turning a good weapon (M16 system) into a great one. Enhanced speed and accuracy results in a more effective, and confident, shooter.
A $30 chicom scope is better than nothing, but not much. It has no place on a $1000 weapon, other than as a placeholder until some super bargain in professional-grade optics comes by. One of the great benefits of having rails on the rifle is to experiment and upgrade optics, or other accessories, such as lights, lasers, and ghod-knows-what-else is hung on the rifle. A rule of thumb is that the scope should cost no less than half the cost of the rifle itself, making it the single most expensive, and critical, component.
The most common types of sights for this type of rifle will either be 1x red dots, or 1-4x variable scopes. If you're building a 5.56mm or 7.62mm "designated marksman" rifle, 3-9x or better (and more expensive) scopes are appropriate. A couple of my Remington 700 PSS rifles have scopes that cost more than the rifle, but that's for the realllly long shots.
Any decent optics on a tweaked M4-type rifle gives you an order of magnitude advantage over some POS turd-world AK clone. You don't want a disposable weapon designed for disposable troops when, with a bit more work and cash, your M4 gives you something a professional uses to gain any and every advantage that can be had.
I may have missed a few minor items, like a "Hello Kitty" color scheme, but you'll come up with your own ideas as you work your way down the list. The M4 clone is the '57 Chevy of carbines, and it shows in all the flexibility and customizing items, wise or dumb, that you can put on it. This is not to denigrate many of the other fine, and pricey, weapons in this category. This is just a way to take advantage of a common rifle that offers great benefits for the least amount of cash. You can do some version of all these tricks on a H&K, Sig, or other high-end rifle. But in this instance, we're working on a generic '57 Chevy, and not an exotic Ferrari or Bimmer.
One piece gas ring, anti walk pins, noveske flash suppressor, dueck canted BUIS, magpul stock and mag pul trigger guard I have. still looking for nanolube locally so using militech products from waldorf maryland (near my old navy EOD school)...
I have 4 different AR’s , 2 are HBAR sodpoodle / coyote sniper rigs and one described above is a NFA SBR. Last is a NIB Bushmaster that is being worked this winter for reliability and with also be a NFA SBR at some point. I think it’s upper will go into the spare parts bin and I’m looking to build it as a “piston” gun. (comments sought). I have used the Hk 416’s and love em thus far as they stay clean and cool for 3 gun matches and pucker factor phire phites.....:o)
My HBAR’s have Timmney drop in “skeletonized” and competition triggers with HART barrels. One is in 6.5mm Grendel as I was building a rig to try Col. Jeff Cooper’s 20/20/20 challenge — 20 rounds into a 20-inch circle in 20 seconds or less at 1000 yards..... it was an expensive hobby for about six months of range time and lots of reloading.....:o) BTW that is my most accurate semi auto “mouse gun” rig.
Grateful for your time as always..... gonna be a good year at the bench, building, overhauling and reloading for next springs fun.
Stay safe !