Weaponry

Weaponry has advanced quite a bit from the 2020's in America, things like Coil guns, plasma rounds, SMRT capability and various other advancements being made that dramatically raises the reliability and capability of most fire arms. That being said, not everyone has access to these more advanced fire arms. Many are still running far older models.
For instance wastelanders still use a lot of RPG's, M16's, AK47's and Uzi's instead of the advanced variants that the nation's military, Companies and otherwise get access too. This has made much of the country resemble more of the Militia's once seen in foreign places like the Middle East or Africa except now in America. With Sophisticated military forces slaughtering them through their use of advanced technologies.

Design

In the 2020's most weapons were designed to wound the enemy and not kill them. This was found to be unfavorable during WW3 due to the sheer size of enemy combatants and many sides came to prefer whole sale slaughter rather then patiently chipping at the enemies supplies. This means that in the 2070's injury is seen as a secondary thing to death.
This created the idea of the Plasma round, a round that'll not just severely wound you but detonate a small portion of your body hoping to kill you. If it fails too the gamma radiation form the plasma dispersal can cause radiation sickness, Death and cancer if left untreated which it often can in mass combat situations with a high fatality and injury rate.
Killing off as many people as possible, this has resulted in many weapons in the 2070's to be far more lethal with far less care being put into the welfare of your enemies. Meaning that people on average wear far more armor meaning that it is not too uncommon to be using AP rounds against people. The UN's restrictions of warfare have also completely eroded away after their dissolution.
Meaning that lethal lasers, biological and chemical warfare are completely legal and are a normal presence in most armies both state and Private.

Gun Rights

In the US, it is no longer legal to own any type of fire arm for any reason, however as to be expected the enforcement of such a law has proven disastrous even more so as Police began only protecting the rich and powerful. So you can now get a gun from basically anywhere with zero registration, background checks and given the more lethal nature of fire arms in the 2070's.
Gun crime is not just worse, but far more bloody. But many view it as a spectacle and even go so far as to fund such crimes for the show. The probation of fire arms, just like the probation of alcohol in the 1920's has completely backfired and only made things worse then they've ever been.

Advancements

SMRT - It means smart, it's a quick way of saying something that track enemies. In the 2070's most SMRT weapons use pinger rounds rather then other forms of IFF because when hacked, it doesn't affect wider IFF systems. The Pinger rounds when shot carry with them sensors that detect near by enemies including the one shot. SMRT rounds working best with the enemy shot.
Pingers exist in around every 4th-7th shot allowing the soldier to ping multiple times a SMRT mag. There is also another alternative that is widely used and that is the Pinger scope. The pinger scope uses Augmented reality to high light enemy positions from when they're last seen. Using the AI in the gun, SMRT rounds can track and attempt to follow the target.
SMRT also has flaws, it works less in foggy environments and is vulnerable to electromagnetic interference and can also be messed with in bad terrain. That being said, SMRT still increases the likely hood of hitting a target more then enough to justify it's existence.
Multi-Scope - A Optic that can switch between multiple visions instead of being locked to just the one or two. Multi-scopes or multi Optics or Mult-ops are commonplace amongst advanced militaries however it is still difficult to use a Eeotech at longer ranges so you will still need to equip longer range optics to fire more reliably at longer ranges.
Coil- Most guns these days use augmented firing mechanism, or Electro assisted systems with some just using straight up electro magnetic means only to fire a bullet. A Coil is laced with in barrel and powered by a Micro battery that is normally nuclear charged. These coils are made of Ultra- conductors named Silver 2, a synth version of Silver that allows far greater flow of electricity that makes the bullet much, much faster and as a result, hit much, much harder.
The primary reason most militaries prefer the Assisted systems is in the event of disabled electronics (Which can be very common sometimes), then the Combustion firing mechanism will still work, not totally crippling a combatant's ability to fight during such situations. This does mean that guns are still loud but they're not as loud and often have iconic zapping sounds while they fire, in midst of the boom.
Modular Designs - Just about every advanced gun can have various different attachments, barrels, mags, and otherwise put on them very quickly with simplified designs that make most guns very economical despite seemingly lookin the same. They'll all much easier to disassemble and thus repair and all try to use similar mechanisms of staying together to make mass disassembly and assembly a far cheaper and faster process.
Plasma Rounds - As explained before, Plasma coated rounds now exist, often burning white. Turning most tracers white but sometimes armies will use different gases to make the Plasma burn a different colours. Such as the US uses Red still and China as well as Russia uses Green still, referencing their old BluFor and OpFor dynamics and systems.
Plas rounds all carry sodium that contains most of the plasma during it's flight, this means it doesn't just drop everywhere and coat everything in plasma, or pump out ridiculous amounts of gamma radiation before it hits the desired target. It's quite effective at chewing armour but AP Plas rounds have more energy put into them and tend to be far more explosive. Really chewing at armour when fired upon with AP Plas rounds.
Enviro Optimization - In general most modern day weapons have been optimized for most environments. This is done via modular designs. Meaning a soldier can switch out various parts in their fire arm to make it operate better in jungles, snow environments, underwater or in the vacuum of space. Obviously if you take a M4A2 specked out to handle desert environments and then run into the snowy mountains of Romania, you'll have problems.
The environmental reliability of firearms is now much more dependent on their user and choice of equipment rather then gun just being unsuitable full stop.
3D Printing - In the 2020's more and more fire arms were being produced with the 3D printers but in the 2070's everything is made with it. 3D printing is so reliable and efficient that it's made things like weapons attachments dirt cheap and easy to make and use. All around decreasing the overall cost in all things fire arm. Meaning that a soldier could get cutting edge rifle with cutting edge attachments with cutting edge synth materials for a mere fraction of the price it would take to give a 2020's soldier that same treatment.
Recoil Control Technologies - RCT, this is an umbrella term adopted sometime during the 2020's to 2030's that refers to piece of tech designed to control technology. The AN-94 design in 1997 was revolutionary for it's moving barrel. However it failed to become standard across fire arms due to the complexity of the tech and the cost of making such a barrel.
Since 2041 3D printing and it's continual advance as allowed the AN-94's moving barrel design to be reevaluated and now most barrels on modern firearms are moving barrels. This means that when a soldier fires his M4A2, the barrel will kick back, absorbing the recoil. This allows the soldier to fire in burst while appearing to fire in single fire. Every trigger pull leaving 3-5 bullet holes for what appears to be far less effort.
As a result Single Fire can feel like shooting a laser, with recoil being so severely reduced.
New Attachments- One new attachment is the Round Counter, it tells soldiers how many rounds are left in their rifle without the need of Cyberoptics and also offers basic debug information. Another is the new Grenade Launchers, with increased range and ability to fire all various types of explosives adds a new layer of versatility to the riflemen.
There is also sensor attachments that allows soldiers to detect nearby enemies without Cyber optics, meaning they can se normally but also see where everyone is.
Nanomer - In the 2020's Polymer was becoming the future of fire arms materials but in the 2040's a new revolutionary material was invented. Nanomer, A cheaper more dependable material that is enhanced down to the nano scale. It also comes with some recoil cancellation of it's own and is quite heat resistant. The big problem with Nanomer is how light it is.
Sometimes if a soldier is not used to Nanomer guns, they may forget where their gun is pointign and negligently discharge themselves or others. The number of attachments and pieces of gear people add to their guns does add weight but guns are no where near as burdensome as they used to be.

US Military Weapons

Carbines

M4A2 - The advanced variant of the M4 model, which includes the Mk.13 -mk.16 models and specifications. The Prime M4A2 Carbine is a popular rifle within the military, having being a general improvement over the M4A1, being capable of finally replacing the M16. M4A2's flexible design across all of it's variants allows the user to attach new attachments and even barrels or other parts according to their needs.
This includes SMRT capability, with ping rounds to ping enemies with and use Bullets that'll attempt to track and harm enemies on the move or behind cover. The most cutting edge development for the M4 is that with the right customization, one can automatically switch the side the bullets come out of. This has resulted in an increasing number of soldiers using the M4 for CQC including coupling it with the short barrels, making it easier to use in more compact environments.
G36k-1 - Another popular Carbine for the US military, produced by Maxwell Industries, using the technology of Heckler & Koch to create a modern variant of the G36K with G36C-1 also being another popular variant for the G36 rifle. Most of the short comings of the rifle being ironed out.
AK12/AK15 - A NATO Variant of the rifle that came into use during WW3, now produced by Maxwell Industries. It has been updated for the times and modified further to better work with NATO attachments however depending on the barrel used, Can use Opfor and Blufor rounds very easily. Can also be customized to use Opfor attachments.
HK416 Mk.2 - An new and improved version of another iconic H&K weapon, still used a lot in the army, almost as frequently as the M4A2 edging out the the AK series rifles and are more popular then the G36k-1. It comes with all the same Upgrades as the M4A2 but has better coiling and thus making it's bullets faster. It was redesigned by Maxwell Industries.
SIG Sauer MCX Mk.2 - New and improved, often used by Special operations operatives and still takes the 300.Black out Nato round, redesigned by Maxwell Industries.

DMRs

M15 EBR - The Updated M14 EBR, The M15 has twice the range and damage output of it's predecessor with superior Recoil compensation with a specially designed compensator by Maxwell Industries that further reduces Recoil. If you use the Bi pod, it turns the M15 into a near laser. The Primary problem with the M15 is insufficient SMRT capability, SMRT Pinger rounds only really working in limited conditions from a certain range of 300 metres.
Meaning that many designated marksmen only use SMRT if the conditions are right or if they're forced to engage targets at a closer range.
M18 - A similarly named rifle designed by Hotoka, it is a DMR with far more robust computing power meaning greater SMRT Capability, including a better capability of handling advanced scopes that outline targets on the IFF. That being said it has far less in terms of accuracy being a slightly improved version of the M14 Enhanced but it's a nice alternative for Marksmen who favor information over accuracy.
M110 SASS Mk.2 -Redesigned by Maxwell, the M110 has been updated for the modern battlefield and is a very good balance between the M15 EBR and the M18's capabilities.

MG's - LMG's

M250 - Designed in 2050, the M2 has become a far more economic system with even fewer jams and greater heat capacity with more effective bi pods and recoil control. Allowing Gunners and auto gunners to dish out longer volleys of fire with bigger ammo mags that can really suppress the enemy, if you add the destructive capability it'll also chew through many notions of cover and leave enemies in a cone of destruction they can't escape.
It is a combination of M240 and the M249 based on the load out you've picked for it, defines if it is a general purpose or just a LMG.
Browning M3 - For the longest time the Browning M2 has stayed the same for years, It has finally been updated with new materials, better recoil control, more efficient bipods, SMRT capability, Advanced coiling and much more. Making the Browning M3 the choice machine gun emplacement for the US Military.

Sniper Rifles

M25 - The Upgraded M24 that come with new recoil cancellation technology including advanced scopes that aid snipers in making their shots. The M24 being the prime Sniper rifle of the US snipers back in the 2020's and the M25 is still the superior sniper rifle in the 2070s though it is not wholly without competition like in the 2020's.
Barret 50 Cal M50 Mk.2 - Designed in 2050 by Maxwell, the Barret 50. Cal vastly improves damage output, recoil control and general use. The Bullet goes far faster thanks to advanced coiling in the long barrel that enables a 120% increase in speed and subsequent damage output. Meaning that the Mk.2 is not just the optimal rifle for light vehicles but is also a choice sniper rifle for heavy vehicles.
SMRT attachments and advanced scopes with extended mags, making a 10 shot rifle, makes it a very capable alternative to the M25, however it's high kick can still be a problem and it still kicks up a lot of dust and muck when it fires. It is also still impossible to silence as the large electromagnetic boom is still very loud thoguh no where near as loud as it's predecessor.
Sako TRG Mk2. - Designed in 2034 by Maxwell, the SAKO TRG Mk.2 brings this classic sniper rifle into the future that has comparable range and recoil control to the M25 that makes it a popular alternative for more an economic and smaller sniper rifle.

Shotguns

Maxwell Auto shotgun - A specially designed Auto Shotgun, that takes the concept of a Auto shotgun and makes it useful on the battlefield. The Shotgun has the capability of controlling it's recoil making burst follies a reality. It is still difficult to use on full auto without robust implants but it is a powerful CQC weapon that uses special Slugs that can really shred heavy armor and cover, meaning Soldiers using it can clear entire rooms without even entering it and likely not have to worry as much about the durability of the wall their shooting.
Benelli Mk.203 - Designed in 2030, the new Benelli carries over the durability and use of it's predecessor but comes with coil upgrade that significantly ups the damage output of it's slugs including a improved Autoloader that gams less and more efficiently feeds slugs into the gun.
Mossberg 590A3 - An updated Semi Auto shotgun with updated parts and capabilities. It also has the best SMRT capability, allowing special forces to clear corners and buildings far easier then they were once able to. -Still favored by Delta Force.

Hand guns / Revolvers

USP Match Mk.2 - A popular 9mm hand gun that thanks to it's coil and plasma round upgrades, hits much harder then it's predecessor that can be SMRT capable if given a SMRT Attachment.
M10 Beretta - The M9 has been a staple hand gun of the US military for decades but the M10 is it's newer generation replacement designed by the Maxwell, sharing some similar capabilities as the USP. It is still very popular but not as popular as it once was.
Glock 40 - The Glock 40 designed in 2040 by Maxwell is a advanced variant of the Glock series, latest in the line and just as simplistic and versatile and flexible as it's older variants but with the standard upgrades. What makes the Glock 40 unique is the Compensator Generation 7 that minimalizes almost wall recoil. Making the Glock 40 the most accurate and easy to use Hand gun in the US military.
Making it the most accurate hand gun in the Military's, the problem is supply. The Glock 40 is so popular that their routinely out of stock, forcing some soldiers to select other options instead.

Launchers/explosives

FIM-67 Stinger - Designed in 2067, updated from the 45 Stringer made in 2045. The 67 is a very vast, heat seeking Aircraft missile that can have laser guidance or be manually fired depending on the customization. Designed to fire various versions of the same missiles that includes the Acid Spitter, that'll coat air craft in highly corrosive acid that'll damage the haul severely.
Designed to force the enemy to do expensive repairs rather then out right destroying it. There is also the option of shooting such missiles into exposed aircraft missile bays to either disable or if lucky, detonate the missile. There is other missile types available much like the acid spitter.
FGM-240 Javelin - The Updated Javelin missile, designed in ww3 to give the US an edge, this Javelin is now more then just a Anti tank weapon but a broadly anti vehicle weapon if given the right missiles. Launching at far faster velocities making APS's effectiveness against the javelin more unreliable. This launcher, wielded by an explosives team can sink entire battle ships with few well placed strikes.
It is also has built in SMRT capabilities within it, allowing it to cleverly guide the missiles to it's targets.
M7 MAAWS - The M7 was designed in 2069 as the integration of various other US recoilless rifles, able to serve various purposes outside of just being Anti vehicle. The M7 depending on it's missile can be also use for Anti personnel, Anti Mine, Anti Electronics and anti fortification. Some missile types requiring a quick change of missile barrels.