One of the big reasons to buy an assembled upper is that it’s not considered a firearm so you don’t need an FFL transfer to get one, so you can get the whole thing and mate it to a lower in a matter of seconds.
Another big reason is the convenience. An assembled upper, if it is complete, will come with basically everything you need to put together a functioning rifle in short order. If it lacks anything, it might lack a bolt carrier group (more on that in a second) but otherwise, it probably has everything you need.
That being said, and while it can be very convenient and save time if you build with an assembled upper instead of a stripped upper and a part kit (or just buying all the parts separately), some assembled uppers might come with parts that are better off replaced.
The following three are among the best candidates here. If you do start your next build with an assembled upper, consider swapping out the following for some better upgrades.
The Handguard
If your AR15’s assembled upper came with a Picatinny rail, consider swapping it for a KeyMod, or better yet, an M-LOK rail system.
Picatinny rails have been the standard for a long time, and they are good handguards, with tons of shooting attachments that are compatible with them (most standard shooting accessories are) but they are not without their shortcomings.
Notably, Picatinny rails are heavy, and not just heavy but very heavy when compared to alternatives like M-LOK rail systems. They also can snag on gear because of all the raised edges, and if that weren’t bad enough, some users consider them relatively uncomfortable to grip and shoot without gloves on.
The solution to this is to add Picatinny rail covers, but that actually takes an unfortunate situation and makes it worse. Those Pic rail covers will make the rail more comfortable to grip, they will add even more weight and on top of that, will reduce the heat dissipation that the handguard allows.
An alternative like a KeyMod or M-LOK rail system will offer compatibility with plenty of shooting accessories (and more and more by the year) and will be considerably lighter than a Pic rail system. Plus, they have lots of open space that encourages heat dissipation and they’re more comfortable and ergonomic to grip as well.
The Bolt Carrier Group
There’s a chance your assembled upper didn’t even come with a bolt carrier group, because they sometimes don’t, but even if the thing did, the chances are that it came with a mil-spec BCG, which means it will be phosphate coated.
Phosphate coated bolt carrier groups have a bit of tooth so they hold onto liquid lubricant well, and on top of that, some operators prefer the non-reflective finish because it is discreet, but the truth is also that there are much better modern BCG coatings than phosphate.
Some great alternatives are black nitride, titanium nitride (which has an attractive gold-tone finish), nickel boron, and DLC, or diamond-like carbon.
All of these bolt carrier group finishes are extremely hard and wear-resistant, and all of them insulate the steel underneath against corrosion.
Most importantly, they all exhibit excellent permanent dry lubricity, which means that less liquid lubrication is needed to keep them running smoothly. It also means it is harder for fouling to adhere to them, so it is easier to keep them clean.
Also, another alternative, which is not necessarily mutually exclusive to this, is a lightweight bolt carrier group that will offer snappier cycling and faster cycling time.
The Muzzle Device
Just like your assembled upper likely came with a Pic rain handguard and a mil-spec bolt carrier group with a phosphate coating, it probably came with a threaded barrel and a muzzle device. That muzzle device was almost certainly a flash hider.
A flash hider helps eliminate the muzzle flash upon firing the rifle, which in turn helps prevent the operator from losing the sight picture due to temporary blindness. It is a practical muzzle device but it also assuredly has its limitations.
A much better alternative is a muzzle brake, which you can likely just thread onto the muzzle (assuming compatible threads) after removing the flash hider.
A muzzle brake has ports which will alter how gasses are ported at the muzzle and can significantly reduce felt recoil and muzzle jump, all without affecting shot power at all.
The rest of this is that you can shoot longer with less fatigue and keep your sights trained on target for faster, more accurate follow up shots.
All in one, one of the best upgrades you can make to an assembled upper that came with a flash hider.
Shop Assembled Uppers and AR-15 Upper Parts
The good news about these AR-15 upgrades is that they can be made together. You don’t need to choose one or the other. You can replace not just the handguard but the bolt carrier group and the muzzle device all in one go.
Check out what we offer here at MCS Gearup and if you have any questions about compatibility before buying, get in touch with us at Sales@MCSGearup.com and let us know how we can help.