May I ask as to the scale and scope of the PMCs? I'll be happy to make recommendations. It looks different if they're a 20,000 man "Corporate Army" versus a battalion of vets who shouldn't go home (looking at you former SS types) versus a company of scruffy adventure seekers. In terms of small arms, this is what would be out there:

Bolt Action Rifle In all flavors and varieties. Enfields, Mausers, Springfields, Moisin Nagants, Arisaka There will be fifty years of these built up, of varying grades and types. Cheap, virtually impossible to jam (though you can still misfire bad ammo), and usually chambered for a round that will make you stay down if hit. In many cases, state armies will still have this as a mainstay weapon. Of note, many are service rifles - they'll still be quite accurate for a good shooter, but the modern perception of them being precision systems wasn't there when they were built. The technical accuracy will be excellent, but for many - especially older ones - the iron sights will be notch and post systems, the ergonomics...not a priority...and things like trigger weight more intended for, well, service, than for precision engagements. So handing one of these to a local peasant won't make him a marksman. But on the other hand, he can probably get his hands on one.

And so can your mercs. No, it's not optimal for a jungle. It does not throw walls of lead. It is probably too heavy and long. The range is wasted. But, you know, the Japanese, Chinese, Burmese, and British beat the hell out of each other in jungles using rifles just like these. So "sub-optimal" is a far cry from "won't work."

Semi-Auto Rifles The Garand is probably the trope namer on this one. Despite it's clear utility, Semis were basically leapfrogged for most nations. They went in to WWII with bolt actions, maybe had a few 'self loading" or "autoloading" rifles than were less common than the classics for industrial scale and engineering reasons (the German Army in WWII, for example, only built about 400k Gw43s ), and eventually went to assault rifles in the late 50s with very few stops. (The British held onto semis until the 1970s as the exception.) So you're at the dying days of the SA battle rifle. But there will be plenty of them to go around if you had a WWII-like event and one side was US-like. Slightly less technical accuracy, but honestly by being a product of recent modernity would have the most advanced "common" sights, and very few people need to see in a firefight if bullets are landing with 1.5 inches or 2.25 inches of where they aimed. Due to their single real source at industrial scale, odds are that if any force has these as a mainstay, it is with some backing or at least tacit approval from one of those superpowers.

So why would you have it in a jungle? Well, if your merc company was not in the business of custom tailoring it's weapon load to every contract or just hasn't adapted to years of jungle fighting, this would be considered the state of the art for a battle rifle. Accurate, superior rate of fire to most people's bolt actions, reliable. Hard to beat as a general purpose weapon.

SMGs These will be popular. They will generally come in three varieties.

Cheap and light ones like the sten gun, made for mass production. They throw lead. They don't weigh much; really, theyre about the lightest "main" weapon you can carry. They're super handy and, being made from mostly stamped parts are really, really cheap. Sure, the rounds are usually lighter - but when you're both either blasting bushes where you can't tell if you hit anyone anyhow, or hitting targets inside a house or on a trail...does it matter? Up to your players to decide, though the first time they hit a man twice and he's still standing, they may reconsider. Also if you do need to have a firefight across a football field...accuracy will suffer.

Reliable bullet hoses. Looking at the PPsh series. Famously tough. A little bit heavy. And, as the Americans learned in Korea, easy enough for a peasant to use to gain fire superiority real quick.

More to follow.