In general, calling the Bushmaster ACR weapon is not entirely correct. The fact is that such a name is applicable only to the civilian version of this product, and since more people have access to civilian weapons, the name of the weapon has accordingly taken root. The weapon variant for the military has the designation Remington ACR, and initially the machine gun was generally referred to as Magpul Masada ACWS.
Let's try to understand this confusion.
How did the machine Remington ACR (Bushmaster ACR)
It all started with the fact that Magpul decided to improve the design of the M16 to facilitate the maintenance of weapons, but everything grew into a more serious project, at least this version, which was followed in the company.

It is possible, and this is only my guess, and not a fact. Magpul already had significant achievements in terms of new weapons, but there was not enough financial resources to complete the work. In this regard, I repeat once again, it may be that the money was taken to update the M16, which is a very promising direction in terms of return on profit, but this money was sent to complete the work on the new machine. And once again I repeat that this is just my guess.
Anyway, after 4 a month after the start of the modernization of the M16, a new Magpul Masada ACWS submachine gun appeared (if you translate an abbreviation, you get something like an “adaptive combat weapon system”). Or rather, not even an automaton, but a whole range of small arms consisting of 2 submachine guns chambered for 5,56x45 with 368, 257 barrel lengths and the designation Carbine and SQB; sniper rifle for the same ammunition with a barrel length of 457 millimeters and the designation SPR; automatic machine chambered for 7,62X39 with the designation AK. A manual machine gun chambered for the 7,62x51 cartridge appeared a bit later, although it can only be called a machine gun within the framework of the national classification of weapons. All this could easily be transformed from one to another with the help of interchangeable modules of which the weapon consists.

In the civilian market there are options for weapons with barrel lengths 318, 268, 406, 457 millimeters. Moreover, the amount of ammunition for which the weapon can be adapted, namely the cartridges 5,56x45, 7,62x39, 6,8x43, 6,5x39, is significantly expanding, at the moment .300 AAS (7,62x35) has also been added to them. Given the popularity of weapons, this list may expand. It goes without saying that for the civilian market all weapons are deprived of the possibility of automatic fire.

Thus, for the civilian market and for the army, weapons are offered by two different companies and money goes to different wallets, only one wallet in the right pocket, and the other in the left one.
It should be noted that the machine itself is well proven in Afghanistan, that is, with significant temperature drops and not the cleanest conditions.
The design of the machine Remington ACR (Bushmaster ACR)
Returning to the issue of upgrading the M16 for the 4 month, it should be noted separately that the final result can boast of an automation system different from the M16.

Despite the fact that the bolt and gas block of the weapon are represented by separate modules, each of them is easily disassembled for repair and maintenance. The same applies to the trigger mechanism.
There is a very interesting fact about the trigger mechanism of this weapon. This is the only element that distinguishes the civilian Bushmaster ACR from the army Remington ACR and is presented as a separate module. That is, in order to obtain military weapons, it is enough to replace only a separate module, which in the case of global hostilities can be useful if there is a shortage of weapons, which is unlikely of course, but still.

What is really quite useful is the presence of various options for the machine guns, the standard butt, by the way, is folded up and does not interfere with firing from a weapon, although the fire mode switch closes on one side. Given the widespread use of light armored vehicles by the US Army, it becomes unclear how a weapon with a fixed butt can be in service.
If we talk about the modularity and ease of operation of these weapons, then it should be noted that the designers coped with their initial official task - the weapon can be completely disassembled and assembled without additional devices. Even the barrel can be replaced “on the knee”, while at medium distances it will not be necessary to shoot the weapon - the deviations will be minor, even though the aiming devices are located on the receiver and forearm.

Often, you can find information about the incredible accuracy of this machine, comparable to the accuracy of a self-loading sniper rifle. Conversely, there is no secret in this, such a result is achieved by fitted details of the mechanisms of the weapon, and most importantly thanks to a high-quality trunk, whose channel is chrome-plated. It is even stated that the thickness of the chromium layer is three times thicker than that of the M4, which, in theory, should have a positive effect on durability. Speaking in numbers, this is the 0,5-1 corner minute by single shots. But these are the numbers that give the owners of these weapons, the manufacturer does not say anything about it.
Pros and cons of the Remington ACR (Bushmaster ACR)

The main disadvantage of this machine is its cost. Of course, for each individual module, you can distinguish individual negative aspects, but the beauty of modularity is that if something is not pleasant or not comfortable, then this part simply changes to another and as a result you can assemble a weapon not only suitable for specific tasks, but also fully appropriate personal preferences arrow.
Conclusion

If we draw any conclusions on the design of the weapon, then we can say that there is nothing supernova in it, just the weapon is made qualitatively, not from scrap metal, with high accuracy and normal quality control.
It is impossible not to touch on the topic of modularity of weapons. For some reason, for most people, even the possibility of changing the trunk is absolutely unnecessary for the machine. And indeed, no one will carry with them interchangeable trunks of various lengths, much less change a long barrel to a shorter one when on the move, when you enter the room, for greater maneuverability. It is even doubtful that someone will change the same barrel immediately before the fighting, adapting the weapon for a particular situation. The main advantage of the modular system is that weapons are much cheaper in mass production.

Now imagine that there is one receiver, which is completely the same for all three classes of weapons. There is a trigger mechanism that is the same for a machine gun and a machine gun. There is a bolt group which is suitable for both the sniper rifle and the machine gun. So the list is endless. As a result, instead of doing two or three different parts, you can make one the same.

Even training in the maintenance of weapons, which is not fundamentally different from each other, is an indisputable plus of a modular system.
So modularity is not a huge amount of various unnecessary additions from the fact that money has nowhere to go. Modularity in a firearm is the ability to count and save money.