MK18?
@Johny_Baba why don’t we buy MK18 CQBR en masse in carbine competition? Or can you tell me which is the best AR-15 with best reliability, cost and accuracy? Is it CAR-816?
The only "MK18" trademarked by a company and available on market is from Daniel Defense, not Colt. Colt does have CQBR-pattern rifles they can offer if they want, but they're not called Mk18 on the market.
As of why we don't buy this or that - remember that its the competition & commercial bidding process that decides L-1, we have no way of knowing how much each bidder is going to tag per-unit cost of their offering at, so no way of knowing who's going to win. No one expected SIG to offer $990 a piece for the 716s, everyone thought IWI was going to win.
Any AR-15 made by a good company with the same type of gas system and same overall features of BCG (extractors etc.) will be roughly similar in terms of reliability & accuracy. Cost like I said is an arbitrary factor that any company can change at the last minute in order to grab the deal by undercutting competition. As such the retail shelf prices of said weapons on civilian market are not good markers to go on, they won't be the same in the competition.
A costlier gun on market may turn out cheaper in the competition if the company thinks they can afford to sell with a lesser profit margin - in the hope of further orders or foothold in Indian market. It becomes a strategic decision on the part of management.
But broadly, I will give you these markers to go on with regard to the advantages of two major variables in AR-15 design:
Gas system
Direct Impingement: broadly better accuracy & slightly lesser felt recoil & muzzle flip (no piston mass moving back & forth), weapon can be overall lighter & cheaper.
Short-stroke Piston: tolerates a much wider range of ammo types (even badly-made ones) by reducing the margin of variance ammo exerts on effective gas cycling, can run better with all types of suppressors, again by reducing variance exerted on cycling, can keep the chamber clean(er) for longer, requiring less cleaning/maintenance of chamber.
Barrel
Carbon steel/Chromoly: tolerates bad/high-pressure ammo better, is more durable/stronger than other barrel materials, lends itself very well to accuracy. Can be lighter.
Stainless steel: more resistant to rusting & corrosion than carbon steel, can tolerate varied environmental conditions much better. Better potential for accuracy than chrome-lined.
Chrome-lined: Even more resistant to rust/corrosion and more tolerant of environment than Stainless steel, can make the inside of barrel potentially maintenance-free in most conditions. Not the best choice for accuracy.
Note that all the advantages of one translate into disadvantages for other (comparatively).