A commonly advised precaution in setting up AR15 optics is to avoid mounting directly on the top rail gap, where the upper receiver’s picatinny rail meets the handguard’s picatinny rail. The two components do not form a connection solid enough to guarantee the security or accuracy of a mounted optic when locked in place on the top rail gap. Additionally, you run the risk of the two rails shifting over time which won’t break your optic or it’s mount but will likely cause your zero to be inaccurate as well as cause the mount’s bearings to fail over time.

Using a monolithic AR upper design alleviates the issue, turning the usual spot of the top rail gap into available real estate by integrating the handguard and upper receiver into one solid piece. The monolithic upper entirely avoids the interference from shifting handguards over use and minimizes the labor in building out an upper receiver. This free float barrel design can be attractive to those who are looking to mount a substantial optic setup with repeated zero retention as an added bonus. The key is less moving parts and less failure points.

Why are Monolithic Uppers not as Popular?

There are disadvantages to the monolithic AR upper design that has held it back from taking off since its inception and development decades ago. For most users of the AR platform, modularity is king. By pursuing a monolithic upper receiver build, you will be foregoing a great deal of options for your firearm.

Popularity of various handguard patterns have changed as different locking designs will fall in and out of style over time. Quad rails affectionately referred to as "cheese graters,” were the standard before there were any Keymod rails. Then Keymod fell out of fashion because of a US Naval study that deemed M-LOK rails as king. While M-LOK currently reigns in the market, this may eventually change as well.

What Are the handguard options for AR monolithic uppers?

For monolithic uppers to have become the next big thing, it would have needed an update to their receivers to meet the demand of ever-changing handguard designs. From a business standpoint, this is a big obstacle to overcome and is overall a hard sell for manufacturers of monolithic uppers. Making the handguard integrated into the receiver means you’ll have to be content with what you have in terms of the handguard. Otherwise, you’d have to pass on the monolithic upper in favor of an upper receiver that can take interchangeable receivers. Some prefer a slimmer handguard or different contours, which simply aren’t always available options with monolithic uppers.

Barrel Options

As barrel nut and handguard mating assemblies improved, the proprietary locking systems of monolithic uppers gradually lost their edge. Monolithic uppers' extensive use of proprietary barrels further added shovels of soil to the hole they were in. If you wanted to change your 5.56 NATO build to a suppressed 300 BLK, monolithic AR upper owners are out of luck. Caliber changes via barrel swaps are unavailable as are adjustable gas blocks to facilitate running suppressed.

Why are there so few companies making monolithic uppers?

Actually, there are few brands that go the extra mile for monolithic uppers. Typically, these can only be brands that are confident in their upper enough to trust that their customer base will not be inclined to modify their uppers far from their stock configuration. For Lewis Machine & Tool Company, they have the brand image and quality to trust that when someone buys an LMT rifle, they likely want it to remain with LMT rifle parts overall; and this confidence extends to their flagship Monolithic Rail Platform system rifles.

Are Monolithic Uppers More Expensive Than Standard Uppers?

As monolithic uppers have to start out life as a much bigger piece of aluminum, the machining cost of monolithic uppers is higher than the typical upper receiver and threaded handguard combination. This was especially relevant upon the release of monolithic uppers like the Colt ACC-M (Advanced Colt Carbine-Monolithic) monolithic upper, as the Colt ACC-M ran at a base price higher than most Colt rifles at the time. With Lewis Machine & Tool being one of the few companies known for going all-in on monolithic uppers’ production, they can definitely be considered expensive as LMT’s quality does not come cheap. Overall, you can expect them to cost more than standard complete uppers as well though it will depend the current supply and demand of the market.

Best Monolithic Upper

These are some of the best monolithic uppers you can get in today’s market. Comparing these products will also give you a better understanding of their differences in form, function price and perhaps even value.

Colt Monolithic Upper

The Colt AR 15 monolithic upper featured all of the classic Colt rifle aesthetic: a chrome lined barrel, A2 birdcage, classic quad rails. The upper came in two variants: Colt LE6920 and Colt LE6940. The former is equipped with a fixed fin front sight while the latter uses a foldable front sight. While both are discontinued now they originally went for $1200 to $1400 at retail value. As mentioned before, this is a hard sell for Colt on the commercial market when their other line of non-monolithic complete rifles went for the same MSRP. If you can find any of these colt monolithic uppers for sale today, be prepared to pay for collector’s value.

LMT Monolithic Upper

The Monolithic Rail Platform (MRP) is LMT’s front runner monolithic upper coupled with their Modular Ambidextrous Rifle System lower (MARS). While buying LMT parts means you’ll be limited by their proprietary design, the quality of performance and abuse durability is unquestioned. The LMT monolithic upper alone begins around the $900 range. If you’re alright with paying $530 and up for a compatible LMT barrel then you’ll be quite satisfied.

Are Monolithic Uppers Worth It?

While boasting greater rigidity and durability, the primary benefit to monolithic uppers is simply the luxury of using all of your picatinny rail space without any concern for the bridge gap. Even if you take away the tolerance issue between the handguard and upper receiver, most people are more concerned with the tolerance of the upper and lower receivers.

You’d be hard-pressed to find a scenario where the increased strength of the monolithic upper matters compared to that of a conventional upper. It just doesn’t make much of a difference compared to a less expensive handguard and upper receiver combination. Handguard to upper receiver mating fittings are now much simpler than before and more than tight enough that the accuracy between an optic on a monolithic upper or the optic on a conventional upper makes little difference if at all.

If you plan to use a thermal optic coupled with another LPVO or just really really want to use that picatinny rail space that is normally a top rail gap, then definitely consider the monolithic upper. This is a niche where you have to decide whether or not it is worth your money.

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