Join Date: Dec 2004
Location: Boulder, Colorado
IMHO, if you don't absolutely LOATH cleaning, a DI system is better. The new DI uppers, from my experience, don't have the crazy cleaning and lubing requirements they used to. Not sure if its looser tolerances, materials or what??? They will get super dirty, but don't start having jamming, feeding, cycling problems as early like they used to. Maybe I'm just lucky on my newer rifles. Of course, the longer you go between cleanings, the more time you should expect to spend getting all of the CD off of the action parts.
Also, don't listen to anyone who tells you that cleaning the gas tube is necessary. The pressures and heat in the gas tube are sufficient to burn and blow any residue away from previous shots. I've gone countless (excess of 25k) rounds without spraying or cleaning my gas tube.
The DI system, as noted, is fully standardized, where piston is not. This can even include BCGs, along with the expected release valve and piston. Also, adjustable piston systems may need to be tuned based on the load you are shooting. Even a slightly mis-tuned piston system will be slower to cycle than a DI, even if it cycles reliably.
So you save some money, and some weight, and lose some potential drawbacks to the piston system. To answer your question, the gas block is replaced by a gas relief valve, the gas tube by a piston, and often the BCG, or bolt carrier is different between DI and piston. I haven't checked in a couple of years (since before I traded my last piston rifle in), but they were $400-$500 more at that point.
Id also like to suggest getting a complete upper. While parts-count-wise they are less complicated than a lower build, you have to worry about getting the gas system aligned and pinned, barrel aligned and torqued correctly, and check head-space and bolt seating. You MIGHT save $50, if you are lucky, on a $800 upper. Not worth the ~6.5% savings, which you would, if doing it correctly, easily spend on specialty tools.
On barrel, I wouldn't go over 1:9. 1:9 will shoot into the mid to high 60's gr. If you want to shoot heavier, go to a 1:8 or even 1:7. But don't expect a 35 or 40gr dog "poofer" to shoot well. Fast twist rates can cause wobble on lighter bullets. I like the 1:8 for the compromise. If you are a typical AR shooter, a 55gr FMJ will be 95%+ of the ammo you send through it, since it is so cheap. I have found the 1:8 shoots these better than the 1:7, though my 1:7 experience is limited compared to 1:8 or 1:9. On barrel material... chrome will give you better longevity, where a good SS will give you a hair better accuracy. Your choice there, both are good.
On buffer tubes... all commercial buffer tubes have 6 positions, whereas true mil-spec have 4. Maybe someone produced otherwise. It is a metal tube! It will not fail on you, one place where Mil-spec is irrelevant IMHO.
For chamber... unless you want a precision rifle, go with a 5.56 or a .223 wylde. The .223 wylde is a bit better than 5.56 IMHO. It more or less cuts the difference down the middle between a 5.56 NATO and a .223 Remington. It still shoots .223 Rem and 5.56NATO. You will find with a 5.56 action, .223 Rem cartridges will not be as accurate, especially in the cheap loads. This is because there is extra "slop" in the head-space. I've personally found that super-premium .223 Rem fires great out of a 5.56, but the super cheap stuff is below mediocre. For most shooters, .223 Rem is cheaper and much easier to find without special order than 5.56, so most of the "plinking" is with .223 Rem.
For the stock, I also like Magpul MOE stock. They have one for Mil-spec buffer, and one for commercial buffer, just buy the one that matches what you have. if you want a precision rifle, definitely use a fixed stock... but it sounds like you want adjustable, and the MOE is the best per dollar here for me. They are all made from various plastic polymers. I haven't seen any in metal. Never heard of one failing due to UV, cold, chemical exposure, so to me they are all the same material wise.
For Bayo... what you describe is the standard. But others have produced different specs that still have a Bayo lug. Nothing I've ever wanted, but I've seen them. Of course the mid-length gas system and the 16" barrel is the most popular, so expect to find more parts available, and more options like handguards / quad-rails available for such a setup.
Don't care about a shrouded-pin carrier (the shrouding exists on the carrier, rather than the pin). As stated, most are, so they can be used in a select-fire weapon. They add a small bit of safety, but such a small amount it is negligible. Also adds a small amount of reciprocating mass, to aid in reliable cycling, again negligible. If you plan to cycle lots of rounds, the BCG is one place NOT to skimp on costs. But buy it anywhere that makes quality components. BCGs and Barrels are mixed and matched all the time.
If you do build your upper... make sure to get a gas-block alignment tool, and at least a go/no-go head-space tool (or a full head-space gauge), a set of barrel-blocks and a punch set. Might be worth buying a bottle of whiskey for an experienced friend or acquaintance to check your work, or throw a gunsmith $50 under the table for a quick once-over before firing. On subsequent builds, you will know that your first one was done correctly, and might not want/need it to be double checked.