Some rough guides are: 9mm seems pretty common and they drop in piles of 50 for standard shots, mid/late game a glock should make a decent ‘walking around gun.’ The ammo that the SKS takes drop in piles of 80 (cant remember the type), this can make it a good general purpose rifle. Find three stacks of ammo and you are good for a fair while.
So yeah, generally most bullets above .22 will do the job, you want to save the really high powered stuff and anything with good armor pierce for turrets and nastier enemies, but generally the gun you want you use in a more ‘day to day’ fashion is the one you have the most ammo for.
I generally grab all the ammo I can find above .22 ratshot and store it in my truck. Ill also go for 1 gun for each type of ammo if I can. Also grab all the gun mods you can get your hands on, they are small and you never know when you will need one.
For guns, for pistols im generally looking for the one with the highest damage bonus, recoil and dispersion arent generally a huge worry. Rivetech pistols are all well and truely top tier, keeping one in a holster as a backup handcannon for hulks and the like is always a smart play even if you only have limited ammo for them. As an eg. im partial to the 1911, ammo is pretty common, it has a decent damage bonus and can 1 shot many enemies with a bit of skill, it has a small mag at 7, but with a spare mag mod that problem is minimized a bit.
Pretty much any shotguns beyond a single shot is great but they are very loud and fairly large, ammo also takes up a fair bit of space. Ill generally use a shotgun if im looking to clear out a mall or something, then go back and drop it off so I can go through and loot.
Rifles I put into 2 catagories: Low powered training rifles and High powered ones for hitting hard targets. Low powered is something like a .22 or 9mm, you are pretty much going to use this to up your rifle skill to a decent level, they will kill stuff, but it will take more than a few shots usually.
High powered rifles are best used for things like turrets and clearing labs. .308 and 30-06 are both nice heavy calibers. You want something with high damage, low recoil and then hopefully low dispersion. As an eg, I love the savage, it has nice high damage, heavy caliber (.308) and low recoil. Only a 3 shot mag, but generally Im hoping to 1-2 shot single targets like turrets with it.
I also pretty much always take trigger happy, so ill only use a weapon with burst fire very very late game when I have 1000s of spare rounds. SMGs I find are only really useful for clearing out schools and malls where you come up against large hordes of lower tier enemies. They are also great for clearing out spores near fungal beds, but beyond that, im going for a high powered single shot gun.