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The tensile, shear and compressive strength of all their tissues, including bone, skin, muscle, etc., is multiplied by a factor of 80. They are also very buff and so have a large body volume.
How resistant would this person be to conventional firearms? What kind of rounds would be necessary to score a reliable kill against him?
I find one reference to skin having a tensile strength of ~30 MPa. If it were 80x stronger, that would be 2400 MPa, which is in the same ballpark as kevlar with ~3600 MPa. As well as being weaker than kevlar it is also thinner than kevlar body armor tends to be... skin is generally 1.5-2.5mm thick, whereas body armor is often 6mm or more.
Your peeps would certainly be a lot more resistant to damage, but they won't be able to just walk into a gunfight and expect to escape unscathed. Low velocity projectiles might just be an irritation... it isn't clear how effective they'd be at crushing or bruising if they failed to penetrate skin, which they might well not. High-velocity rounds will certainly still affect them... supersonic rifle rounds and big handgun rounds are all serious threats, but it is a lot harder to guess at the effects of such things on the super-meat under the super-skin, so that could go either way. They might survive being shot OK, but they risk severe bruising which seems like to cause them a lot of pain and possibly limited movement.
Armor-piercing rifle rounds fired (eg. M993 or M995) seem like they should be capable of penetrating skin and at least some distance into the body underneath, and so are likely to be disabling if not necessarily outright lethal (but see below about medical issues).
I'd still expect them to drop when faced with anti-materiel rifles or machineguns firing similar rounds (eg. .50 BMG). These are slightly awkward things for regular humans to tote, but it should be possible to make a suitable "armor"-piercing explosive round that could be loaded into a readily portable 20mm grenade launcher (see the OICW, K11 or PAW-20 for examples of such things). These aren't quite conventional firearms, though some have been in production and general use. The specific requirement for a small armor piercing warhead doesn't really exist in the real world, but it isn't so farfetched that it couldn't exist in a world that had your super-folk in it.
A more serious problem than gunfire will be providing medical treatment. You may find that many kinds of pharmaceutical and surgical intervention are impractical or impossible without something that looks a lot more like a machine shop than a regular medical kit. I pity the physio trying to work on someone like this. Trying to remove fragments from a bullet wound might be very difficult, given the problems of dissecting or debriding super-tough bodies. Stitching wounds might be impractical, though gluing could work if they're not too super-strong. And so on.
Administering drugs like adrenaline or atropine might be quite challenging, given the material requirements of the needle, which means that under certain circumstances a regular human in decent protective gear might actually be better placed to survive their wounds or chemical attacks. All the super-strength in the world isn't going to help you once you've got a good lungful of poison gas, and chemical weapons (lethal or otherwise) seem like they'd be just as effective against your super-peeps as they are against regular ones.
Since it's not clearly defined what 80x tougher means, I'll make the easy comparison, one based on energy of the projectile. This is very fuzzy, so it doesn't really matter if we're talking about muzzle energy or impact energy to get a rough estimate.
I would also estimate, that anything able to cut, such as arrows or crossbow bolts with hard enough cutting edges, would likely be lethal at much lesser energy than 80x. The ideal shape of the arrow head would likely be different than normal hunting arrows of this type, probably much narrower to allow deeper penetration and better chance of reaching arteries. The exact effects of different projectiles would be impossible to know without experimenting with real ballistic material matching this 80x tougher human tissue and internal organs.
But let's get to the beef of this. 100 to 200 Joules can be taken as the energy range of non-lethal projectiles, based on random internet searching.
If we multiply that by 80, we get range of 8000 to 16000 Joules for non-lethal but debilitating projectiles. So let's say below that will not really slow these tough guys much, while above that can easily be lethal.
Now if we look at this page, the highest power gun listed there is rifle using 7.62 × 51 mm ammunition, with 3799 Joules of muzzle energy, Well below the non-lethal range.
Based on this simple energy comparison, it would not impart enough energy to be able to seriously injure your 80x more durable person. It would hurt like hell, and cause serious bruising and bleeding, but it would not cause a debilitating injury, unless it hit something like an eye, throat or a temple.
So, what would be enough? .50 BMG aka 12.7×99mm NATO machine gun round has energies in the range of about 18000 to 20000 Joules. So that would likely be at least debilitating with a single hit, and certainly lethal with repeated hits.
It is probably relevant and notable, that there are sniper rifles, which use this bullet (example). Those would be effective, though only at quite short range.
Just a data point, if it's of any interest: I've been out hunting game birds (pidgeons) and other hunters were in the area. They fired upwards, the bird shot came down on me. It was like grains of sand dropped from ten feet... completely harmless, unless one should happen to get in my eye, then I'd have to blink it out
