Corn-Picker,
As a member of the "Buy a Mil-Spec TDP compliant AR" fan boy club I have nominated myself to write a huge word fort trying to convince you to buy a better AR/M4 pattern rifle than you might have otherwise done. I realize this will all be for naught, and that much like Sisyphus I am bound to the hell of watching this effort roll down hill at the end only to being again.
Now when I say buy a rifle that is as close to the TDP as you can get, I really am implying that you want to look for certain materials and assembly methods that will ensure reliable function and a long service life. You don't have to slavishly adhere to the .mil TDP and get an iron sighted M4. However there are some features of that general issue carbine that you do want, and should settle for no less than.
I will now attempt to explain what those features are, and why you want them. Others who are even more knowledgeable can feel free to jump in and fill any gaps I leave.
We shall begin with the TDP (Technical Data Package) and you: So you realize that you are not going to be the baddest mo-fo of all time kicking in doors after fast roping out of a Blackhawk or Osprey, and shooting some savage in the face at the appointed time and place. Welcome to reality for 99.9999999% of us even those with military experience, hell even for those of us with combat experience. So you are probably wondering why in the hell you should give a rat's-a$$ about getting an AR-15 that meets a bunch of mil-spec techno-jargon, you're looking for a range toy or something to shoot small furry rodents with, not a combat gun!
Well the first thing we need to go over is why a TDP exists, simply put the TDP exists because over time and experience a large organization that own tens of thousands of M16's and M4's that get used and abused, has figured out what works and what does not. Lucky for us, we can also learn from their experience, and benefit from it without going through all the pain in the ass it took to get there. The TDP has call outs for everything from dimensions, materials specification, production methods, and assembly methods. What it all adds up to when followed scrupulously is a highly functional, accurate, reliable, durable fighting rifle/carbine.
So, being an intelligent fellow and doing some research before you buy you are probably wondering "what is actually in the TDP for an M4 carbine or M16 that I can actually look for, and how does it realistically benefit me for what I want to do with this firearm?" Well here we go I will try to give some succinct answers.
1.) The bolt carrier group. This is in my opinion the single most important assembly that the TDP calls for that more often than not will ensure a functional rifle or carbine. Sadly it also seems to be an area where a lot of manufacturers take short cuts.
We will begin with the bolt itself, the TDP calls for this part to be manufactured from Carpenter 158 steel alloy. Once it is machined, shot peened and heat treated the TDP also demands that the bolt is individually high pressure tested and magnetic particle inspected. This of course costs money, and many manufacturers do not bother, they batch test instead: if one passes all the parts pass. Unfortunately the bolt on an AR pattern rifle is not the strongest or most robust rotating bolt known to mankind. The lugs are small, and over time and many rounds they start to crack at the base. Figure 20,000 rounds of service life and you should look at replacement of the bolt. Given the critical need for a strong part here, I think it is worth the extra $$$ to get one that is individually tested. There are also some bolts being made of newer stronger alloys than Carpenter 158, so there is room for improvement here over the TDP. You also want to see a tool steel extractor with the current extractor spring, hard rubber spring insert, and black rubber donut. A lot of makers cheap out here too, and erratic extraction and ejection can be the result.
Moving on to the bolt carrier what we are really looking for is a hard chrome lined interior for durability and ease of cleaning, full profile full auto style carrier for smooth cycling, and a properly staked gas key. The gas key staking in particular is vitally important for reliable function, and infuriatingly is often over looked entirely or done in a half assed manner. This is also easy to verify visually when you break the rifle open and pull the bolt carrier group out. A properly staked gas key will look like this:
http://s6.photobucket.com/user/Alph...ch/COLT_l_LMT_M_LMT_R-stake-L197-2-1.jpg.html
As you can see there is a lot of metal moved into and over the tops of those hex head bolts. They will not back out. If the gas key doesn't look like that, then put the rifle down and walk away. Chisel marks across the top will do nothing, and light staking from the sides that don't move enough metal also can loosen. When these bolts loosen (and they will over time even with a light firing schedule) you will develop a gas leak, and the rifle will start to short stroke or fail to cycle due to inadequate gas pressure in the bolt carrier group. That is what the problem will manifest as, the other issue is that the loose gas key will be misaligned and beating the crap out of your gas tube well before that prematurely wearing it down to the point you may be losing gas pressure there as well. So you'll get to make two repairs you didn't want to. Plus you'll end up being frustrated as hell with this damn gun that doesn't work right.
The last bit I will mention is that the exterior of all these parts will be called for to have a phosphate finish. There are other finishes available, many of them are excellent, some are frankly crap. All of them should be lubricated regardless of any claims you don't need to lube the rifle. Phosphate does a good job of holding lubricating oil and protecting against corrosion so basic mil-spec is just fine here.
2.) The barrel assembly. Here is where you will also pay more money to get a mil-spec quality barrel. There is no way around this, since CMV 4150 ordanance grade steel is tougher to machine, harder on tooling, and slightly more expensive to buy as a raw material than more common 4140 CM. The same can be said for 410 stainless steel versus the much more common 416 stainless. So with that in mind why spring for the more expensive barrel? Namely for durability, the reason those steels are specified on the M4/M16 (CMV 4150) and the MK12 (410) is that they hold up to a more severe firing schedule than lesser grades of steel. Now you might not plan to ever run your barrel that hard, but over the course of your lifetime you may still accumulate many rounds through that barrel. If the barrel you buy today continues giving good accuracy for a lot longer but only cost a bit more, which was the better deal? Plus you might just find that a semi-auto rifle is fun to pew-pew-pew with at a rapid rate on occasion, for which the tougher steel will be handy.
The other issue with the mil-spec barrels of the CMV 4150 variety will be that they are typically hard chrome lined in both the bore and chamber. This will greatly enhance the durability of the barrel, as well as aid in reliable feeding and extraction since hard chrome is a slick surface. Plus the hard chrome usually makes for easier clean up too. In the past many have felt that a chrome lined barrel will be inferior in accuracy to a non lined barrel, and that may be true but if my chrome lined Colt barrel is any indication they don't give up much accuracy if any. Probably an increment so small most shooters will never notice within the effective range envelope of the .223/5.56mm round.
Another worthwhile barrel finishing method worth considering is salt bath nitriding, also referred to as nitrocarburization, or Melonite. This is a method where the barrel is immersed in molten salt, and through the magic of heat and chemistry the surface of the steel to a depth of several thousandths of an inch is case hardened to a ridiculously hard surface that is extremely wear resistant. Plus it finishes the barrel inside and out in a nice uniform black color, whether it be chrome moly vanadium steel or stainless steel. I have two barrels finished in this method and they are excellent, and durability is supposed to be very very high. So this is an alternative to the TDP that can be considered an acceptable alternative/just as good/improvement.
While on the subject of the barrel, now is a good time to talk about the gas manifold/gas block/front sight post... whatever. The TDP calls for this to be pinned in place, and for the barrel to be phosphated underneath where the front sight post is installed. The important functional bit here is the pinned gas block/FSB. Many manufacturers cheap out here and use set screws, or clamp on manifolds. They are not as secure as a pinned manifold, period. particularly true if the hand guard doesn't cover the manifold to protect it from bumps and thumps.
The last issue with barrels that I will harp on is the fact that the 5.56mm NATO chamber is probably your best bet when buying an AR/M4. This chamber will handle all commercial and surplus ammo, and frankly when applied to a good barrel doesn't give much up in accuracy compared to a .223 Remington chamber. Plus the NATO spec chamber will be more reliable in adverse conditions, heat, and a heavy firing schedule. I have heard good things about the Wylde chamber but have only passing experience with them. Tight match chambers belong on target rifles at Camp Perry or in high power matches, they have no place on a general purpose semi-auto.
3.) Upper and lower receivers. I don't have a lot to say here. Make sure they are made from a forging or billet, forging is most common. Make sure they are forged or machined from 7075 T6 aluminum, and are Class 3 hard coat anodized. The upper receiver should have M4 feed ramp cuts that are machined before anodizing. The upper should also have a forward assist and a dust cover, sometimes the bolt carrier might need to be persuaded to seat all the way into battery, maybe you fall into a creek or something coyote hunting and the rifle gets all gunked up. Beyond that as long as the parts are in dimensional spec, you should be good to go.
4.) If you will be using a collapsible stock make sure the castle nut is properly staked in place on the buffer tube. Some morons have been known to use Loctite, which is bad and will make changing the part out if desired a royal PITA. Plus it won't hold the buffer tube in place as well as just staking the damn thing in the slots provided for staking. Plus Loctite will gum up the threads back there. You'd be surprised how many don't do this right. Also on the subject of buffer tubes, this should be made of 7075 T6 as well for maximum strength. Here I am not hell bent on it since the M4 makes a lousy cudgel, but should you need to butt stroke an uppity zombie or some nonsense you'll want the stronger buffer tube and more secure attachment staking provides.
5.) Recoil buffers.... sigh.... in this day and age why anyone would still put a CAR buffer in an M4 clone is beyond me. These rifles tend to be pretty well gassed to run on a wide variety of ammo. To smooth out the reciprocation of the bolt carrier group and reduce wear and tear reputable manufacturers and the military use a heavier buffer than the old CAR buffer for the collapsible stocks. This is super easy to verify by breaking the gun down when you inspect the bolt carrier group, just look at the face of the buffer. If there is no marking on it it is a CAR buffer and probably should be heavier, heavier buffers are marked with an "H" for heavy, or "H" followed by a number the, the higher the number the heavier the buffer. Fortunately this is also easy enough to remedy by buying a heavier buffer and installing it.
6.) Hand guards. Yeah the TDP is really easy to improve on here. Get a free float tube or rail of your choice that is of good quality and solid mount up. Hardly anyone sticks with the TDP non free float hand guards/rails.
7.) Trigger. I admit that I hate GI triggers with a passion. They suck. The only thing I will give them is they are reliable and generally safe. I am a huge unrepentant Geissele Automatics fan boy. They make the best 2 stage AR trigger on the market today for any price, and offer it in a variety of configurations and pull weights. Plus they are the only provider of a two stage trigger that has managed to get their trigger certified for military use as part of the SOPMOD package for the M4A1. Their design is safety certified by the Crane Naval Surface Warfare Center.
Enough of my ranting. I probably forgot a **** load of stuff like details on gas port sizes, and the best grade of steel for trigger and hammer pins. Or the best muzzle device. Someone else can fill you in on that.
I'll simply leave you with a photo of what a 16" chrome lined Colt M4A1 contour barrel can do with good ammo from a bench rest at 100 yards with a Nightforce 3.5-15X50 mounted and my dumb ass running the bang switch.
The carbine in normal configuration without the Hubble Space Telescope sitting on top.
