A combat guide was requested so I'm gonna try to explain the reasons behind my aim settings and give my two cents on the different weapons since I dabble quite a bit with virtual guns. It has to be said that I'm still not sure about a lot of the settings so I mess around with them quite a bit to see what works, and what works for me might not work for you. With that said, below is how they look at the moment. Also, feel free to post your thoughts, tips & tricks in this thread.
Lock-On Mode (On-Foot and Mount/Vehicle) are both set to wide. Only started using it the other day after DC's shout and have yet to notice any difference compared to normal, but in theory it should allow you to lock onto enemies faster as you don't have to turn all the way to face them directly. Downside can be that instead of targeting the enemy in front of you, it will target another enemy in the peripheral which wasn't an uncommon problem in GTAO. There's also the chance that I'm taking it too literal. There's not a lot of info on this setting.
Aim-assist (On-Foot): Again, not quite sure about this one. During story mode I turned it all the way down when trying out free aim to build up muscle memory, and I forgot all about it until yesterday when I turned it back up (which is default if I'm not mistaken). I felt like it helped but it's hard to tell tbh. Definitely didn't hurt so I'll leave it maxed out.
Aim-assist (Mount/Vehicle): This should without a doubt be set to max.
Look Sensitivity can be turned up quite a bit without overshooting and the picture getting too distorted when moving the camera. It allows you to look and turn around quicker.
Aim Sensitivity is a tricky one but very important for quick headshots. It really depends on what you are comfortable with and I'd suggest raising it bit by bit. Once you are comfortable with a new setting try raising it again. If you can't get the hang of it then lower it back down and you have found your spot. Another thing to keep in mind is that a high sensitivity will make it harder in other areas like hunting, shooting of horseback, etc. So it very much depends on what you value the most.
Aim/Look Acceleration: This is how quickly your cross hair accelerates when aiming from when you first move the analog stick and how quickly the camera accelerates when not aiming. I'd say it's the same as the above. It can hurt you in other areas.
Aim/Look Dead Zone: Assuming this is the same as in GTAO, dead zone is the time between when you move the analog stick and when the movement happens on-screen. So definitely turn that all the way down so there's no delay. No idea why this setting is even in the game.
Gonna focus on which weapons it makes sense to buy money-wise while still being properly geared up for a fight. Main goal is to not spend money on unnecessary shit.
Sidearms: This is purely going off the stats and might prove different in practice, but I'm looking to get the Semi-Auto Pistol. It's cheaper than the Mauser and beats it in fire rate and accuracy while only losing in the damage department. And since headshots are so important in auto aim, I'll rather take fire rate and accuracy over damage. A headshot is a kill no matter the weapon so it's better to have the higher accuracy to land the shot and the quicker fire rate if the first shot doesn't land.
Whether you've have started out with the free Volcanic Pistol or High Roller Revolver, I wouldn't recommend buying anything before having enough money for the Semi-Auto/Mauser. It's not worth the upgrade IMO.
Another option is to go with the Sawed-Off Shotgun and free up the long gun slots for a rifle/repeater combo. It only holds two bullets but with an off-hand holster you can rock a pair of these bad boys.
Recommend: Sawed-Off Shotgun, Semi-Auto Pistol
Repeaters: The Lancaster seems to be the weapon of choice in this class. Compared to the Litchfield it has superior fire rate, reload and accuracy at the cost of a little fire power.
Rifles: The Bolt is a must-buy. For now it's the best rifle in the game for longe range killing and big game hunting, and it comes at a decent price so there's no reason to buy a bit cheaper rifle first. It was the first weapon I bought and upgraded.
The Varmint Rifle is also a must-buy for hunting small game. Very cheap and works fine without upgrading it in any way. And with it's low recoil it's deadly with those headshots, both against the AI and in PVP.
As for the snipers it's not really a contest. The Carcano is by far the better gun. It doesn't unlock before level 50 but the Bolt can be equipped with a scope so you don't have to waste money on the Rolling Block while you wait for the Carcano to unlock.
Recommend: The Bolt, Varmint and Carcano
Shotguns: Since I've the Ultimate Edition I started out with the Pump-Action Shotgun. If I hadn't, it probably still would have ended up in my armory as a shotgun is a must-have in my book. It's considerable better than the cheaper options and also 300 bucks cheaper than the Repeating Shotgun. No reason to spend that much money when the superior Semi-Auto Shotgun unlocks at level 42.
Recommend: Pump-Action and Semi-Auto Shotgun
Bow: Mandatory! Great for stealth missions and just overall a lot of fun.
Modifications: I'd would say save your money and only buy upgrades for the weapons that will be the main ones in each their classes for a considerable periode.
Like in GTAO you can do the quick 180° turn, though it's not quite as effective as you keep on going in the same direction as before you turned around if you don't press down and then up on the left analog stick. The turn is done by holding down the right analog stick (without aiming), then quickly aim to make your player turn around. It's comes in handy for when you spot an enemy behind you on the mini-map.
Don't know if it has a name but let's call it the combat dive. It's common knowledge but some might have wrote it off completely because it's not the combat roll from GTAO. While it's not anywhere near as effective, it's actually still pretty useful in situations where an enemy gets the jump on you. It's not OP like the combat roll as the enemy will quickly be able to lock on again but it will be a tough shot going for the head and that can buy you time to get one off yourself. It has to be said that if you are already really low on health or the enemy is carrying a shotgun the dive isn't of much use as they can easily get off a bodyshot before you get back up. The dive is done by pressing square while aiming and pointing the right analog stick in the direction you want to jump.
The option to expand the mini-map for good is there so why not use it? It doesn't take up at lot of space on the screen and you don't have to press down on the d-pad all the time for a better look.
Also giving a better look is going for the third person view that is furthest away from the player. It gives you a bigger chance to spot enemies coming from the sides and helps you scan around corners more efficiently all while the aiming zoom remains the same as in any other third person view, though it still might take some getting used to.
Guess that is it for now. Might cover the ability cards in the future. I hope you'll find some of this useful. Again, if you have some combat tips & tricks of your own and/or thoughts on weapons, post them here.
Videos of the turn and dive: