Revolvers have stuck around for a long time, well past the point that they are for the most part obsolete, even today. Revolvers remain popular for several reasons, with the two most prominent being the Rule of Cool and reliability. Revolvers look cool, they feel cool, and cool guys use big slabby phallic revolvers. This is the tool of a gunslinger, a sharpshooter, a wild west cowboy. On a more practical level, they are very simple, and very toughly made, so that breaking one is a feat in and of itself. The open construction lets debris fall out, no springs to wear out, and making one jam usually requires a Three Stooges level of incompetence or the ammo itself is bad.
Power armor revolvers are exceptionally heavy pistols firing enormous rounds, in the vicinity of 20mm. These are functional cannons that have short barrels and a revolver feeding mechanism. Being large and robust, these guns can be handled and reloaded by a man in power armor, and at close range, have the ability to damage other power armor foes, or similar hard targets like robots, reinforced doors, light vehicles, and so forth. The power armor trooper's arm and shoulders absorb the recoil of the weapon, and a normal human can certainly pick up the 20 lb pistol, but firing it outside of an armor suit is a solid way to dislocate shoulders, break arms, wrists, elbows, and depending on where the gun goes after it is fired, broken bones in the face, concussion, or even death.
Pistols aren't going anywhere in the CE. For a power armor trooper, having a PA scaled pistol is always going to be a popular option. As with the revolver, these are 12mm+ weapons and are semi-automatic short barrel pistol grip light cannons. The appeal of the pistol is as a close quarters weapon, such as inside buildings, ships, etc where a long rifle is infeasible. For non-military power armors, such as security forces, pistols are going to be a more socially acceptable alternative to carrying power armor assault rifles. Likewise, such weapons are more ammo efficient, and the users are less likely to go Rambo in a bad situation.
Large ammo is expensive, but power armor pistols tend to be geared more towards efficiency that armor busting stopping power. These weapons use 12mm or 50 cal ammo, which while still devastating is a step down from the 20mm cannon rounds revolvers use. There are plenty of pistols that do fire these larger rounds, and they are just as popular as revolvers. The cannon pistols are easier and faster to reload, and hold more ammo than revolvers do. These weapons have the same human user limitations as revolvers do.
3. SMGs and Automatic Pistols
Sometimes volume of fire is desirable, and that is where submachine guns and automatic pistols come in. Functionally similar and sometimes larger than conventional power armor pistols, power armor SMGs are cannon scale buzzsaws. These weapons are powerful and compact, but tend to sacrifice a bit of range for their rapid fire capability.
SMGs are favored by special forces operating in confined spaces and urban areas, as well as being a popular sidearm for power armor Reflex Teams who are likely to encounter bugs or other cosmic horrors when attempting to close a DFE. Facing an armored cockroach nine feet tall is less frightening when you have a Beretta 99 13mm automatic. Non-military strike forces also favor SMGs because of their high rate of fire and stopping power combined with the fact that they are not six foot long assault rifles.
The shotgun is an archaic firearm, large bore, low rate of fire, short range. The power armor version of the shotgun embraces these things. Power Armor shotguns are massive, being 0 or 00 gauge, firing rounds the size of small beverage cans. This makes these weapons heavy and almost impossible to fire without assistance, usually a bipod, shooting brace, or other mechanical assistance. They were completely designed to be fired by a man in an 8 foot tall robot suit. The low rate of fire is mitigated by the fact that the range and spread of these guns is terrifying.
Why is the shotgun being made for power armor? In special dont punch holes in the hull situations, pellets and polymer shot aren't going to bust a hull, coolant system, or other vulnerable mechanical systems. In urban areas, most walls will stop these projectiles so that a power armor squad unloading against a rogue faction inside a highrise don't kill everyone on the floor with overpenetration. In the case of DFEs and bug hunts, having access to enormous slugs, dragon's breath, and flechette rounds can clear a room of creepy crawlies, deadheads, bugs, and whatever else might be around. With the little guys sucking wind and their own version of blood, the power armor troopers are clear to see what is still standing for their more focused attention.
5. Grenade Launchers
Oft maligned as n00b toobs in games, grenade launchers are going to be a popular option for power armor troopers in the CE. The previously bulky weapons are just a power armor pistol grip away from already being the right size for troopers. Upscaled versions could give troopers access to 80mm, 120mm, and larger grenades. Gas grenades, stun grenades, concussion grenades big enough to drop a small building, armor busting grenades, there is room to make a 30 out of just power armor scale grenades.
Grenade launchers are going to be valuable tools for power armor teams who are in assault and DFE roles. The ability to project a large amount of explosive power at relatively short range would be a tremendous asset for fighting similarly equipped foes. The ability to fire gas, smoke, flares, and other assorted ammunition can be a huge asset, and potentially an unexpected one.
6. Anti-Material Rifles
Anti-material rifles are intended to destroy vehicles, and other equipment rather than the people using them. This goes back to WWI when anti-tank rifles were a thing. The increased strength and recoil absorbing capacity of a power armor suit brings the anti-material rifle back into the game. These are very large single action weapons firing large high velocity rounds, eight and nine foot long rifles for men in armor suits. These guns are intended to destroy power armor suits, aircraft, light armor vehicles, shoot through buildings, and even punish light and medium mecha with precision shots.
The most popular anti-material rifles are magnetic or chem-jet (hybrid magnetic/conventional propellant) weapons that mix moderate sized ammunition, in the 35mm-50mm range to extreme velocity. These weapons are fully intended for hard targets and overpenetration is a major feature or flaw, depending on the shooting situation, of the round. These guns have to be fired from prone or kneeling positions, and the trooper using them cannot shoot them from the hip. The entire suit is required to act as the shock absorber for the gun. When going against large targets like armored combat vehicles and mecha, the shooter can make called shots to roll for critical hits, using their small size and accuracy of their weapon to bypass the armor of the target. This is typically damaging leg crits, disabling tracks or wheels, and otherwise being a menace to vehicles a scale above their own.
Full on long barrel rail guns are also popular, if expensive options.
7. Bazookas and RPGs
The carry capacity of the power armor trooper is impressive enough that a dedicated heavy weapon was upgraded for their use. The rocket launching bazooka and rocket propelled grenades of Petroleum Era warfare were reintroduced. The new systems saw the size of their explosive warheads increase tenfold, giving power armor troopers the ability to carry their own artillery into battle. These flying bombs deal artillery splash damage in combat, are devastating effective at taking out power armor suits, destroying bunkers, and seriously damaging mecha. How can a rocket bomb carried by a trooper seriously damage a mech? Mecha are generally built to fight other mecha, so their armor plating and protection is generally laid out to resist attacks from a foe roughly their same size. They aren't designed to have that armor protect them from a human sized foe shooting at them from five feet off the ground.
RPGs and bazooka type power armor weapons are going to be far more common in second and third world arsenals, where their cheap production and ease of use, at the expense of their operator, is an attractive option versus the high expense of trying to fight technologically or numerically superior foes. Groups like Amerikka Command and the Noddites are going to use them with sophisticated warheads, and some supporting nations will have them as a garrison level deterrent. Unlike many of the above weapons, these are generally not used in containing DFEs. Anything crawling out of a horror pit requiring a power armor sized bazooka to injure is likely going to require mecha to actually be killed. The Fat Man/Davy Crockett shoulder fired nuke would fit into this category.
Additional Ideas (1)
The .787 is a slab sided cinderblock of a pistol chambered for a 20mm round (.787 caliber). It can damage power armor at close range and can be used for called shots when used at point blank range. At medium and long range it is more of a nuisance weapon. Against conventional infantry and thin skinned vehicles, the .787 is a mauler. These are commonly seen as side arms and secondary weapons for security and police patrol armors. The gun is also an effective club when it is out of bullets.
The 20mm SMG is a popular mercenary and security force weapon, capable of laying down a barrage of 20mm shells in fast, if not particularly accurate, fashion. The weapon is ammo hungry, and far too heavy for normal infantry to use, but it is an absolute buzzsaw against autons, devastating in close quarters combat, and otherwise a really loud obnoxious way to make an entrance.
The 1.18 Revolver is frankly a terrifying weapon. The revolver is power armor sized and fires a 30mm cannon round, and can hold between three and six rounds, depending on how large the gun is. Unlike most .787s, the 1.18 can fire specialty munitions. Most of the time, troopers who pack 1.18s pick high explosive and armor piercing rounds, for fun. The revolver is massive, and most power armor users have to use both hands to control the it.
There are a large number of pistols, submachine guns, machine guns, and assault rifles chambered for the formerly popular .50 round, the 12.7mm heavy machine gun round. Formerly useful, this gauge of round has proven itself to be less than effective in power armor versus power armor engagements, and as such has been largely abandoned by most militaries. Large caches of guns and ammo remain in this configuration, meaning that there is a ready supply of them for third and forth tier military forces, wastelanders, Silk-roaders, low end mercenaries, gangs, and otherwise neer'do'wells.
The allure of 12.7mm guns is that most all of them can be used by conventional infantry, mounted on civilian vehicles, and otherwise be used in large numbers. While not effective against modern armor and equipment, they can still readily chew up normal infantry, shred through encounter suits, bring down robotic foes if enough ammo is used, and increasingly, these weapons are being upgraded to full automatic and link fire capability where sheer number of rounds fired bring down light and medium targets.
The pinnacle of .50 weaponry is the actually fairly terrifying 12.7mm minigun. This weapon is quite a bit slower than the larger GAU-8 Avenger used by the A-10 Warthog, but the ammo drum is limited to the carrying capacity of the power armor trooper, (and a fully load GAU-8 is just over two tons). The 12.7mm Rotary can reach speeds of 1800 rounds per minute, and if loaded with armor piercing rounds, can turn a surprising number of foes into scrap.
The original Nordenfelt was a 19th century attempt to create a rapid fire weapon by having a large number of barrels and chambers fixed to a platform. It was surpassed by automatic receivers and gatling guns, but the design was renovated in the CE, using relatively common Neo-Soviet 23mm cannon rounds. The power armor .905 Nordenfelt is a forearm mounted weapon, where three to seven barrels are fixed to the arm of the power armor and flexing the fist causes the guns to fire. They all fire at the same time and cause a surprising amount of damage, when firing armor piercing rounds.
Variant Nordenfelts are common. The concept of mounting multi-barrel weapons on a bracer can be applied to almost any weapon, provided the builder has the time, resources, and will to make it. There is no reason to say that there aren't laser nordenfelts, or plasma versions. Such variants would require extensive power supplies, but gyrojet and chemjet guns could add heavy low rate of fire punch for a relatively low investment.
Often considered the grandchild of the .22, the .866 is a 22mm non-specialized round. The .866 Gardener is a double barreled rifle with a rotating receiver. The rifle can keep up a high rate of fire for a prolonged period of time, as the alternating between barrels allows them to cool without reducing the rate of fire. These guns, while not the most effective against power armor, are very useful in urban and other close quarters situations where their good range and rate of fire allow them to clear out the area.
dakka, lots of dakka
Not related to the previous revolvers, the 1.57 fires a 40mm grenade round. The 1.57 revolver fires 40mm grenade rounds, which are high explosive by default, but it can fire a large variety of special munitions, including gas, chemical, flashbang, and other more exotic rounds. These weapons are less common, as they are fairly bulky, and heavy.
A 50 mm shotgun, the 1.96 is a purely anti-infantry weapon, but an absolutely devastating one. The most common versions are semi-automatic and have bullpup configurations, and rely on the armature of the power suit to handle the recoil of the weapon. Firing a 1.96 by hand would seriously injure any human foolish enough to attempt it. 1.96s can fire specialty munitions like solid slugs, and the frightening and highly popular dragonfire round, causing the gun to emit an impressive fireblast. These can be used to clear rooms, set fires, and generally cause havoc and mayhem.