I started a 2nd character with a melee build, and thought it was worthwhile to put together a list of all the ammo I've acquired to use as a guide for future characters. The character is level 25, and has used a fair number of fusion cells, and a few Molotov cocktails and frag grenades, but nothing else. No ammo has been bought or sold. I collect all ammo that I see, and I don't farm.
Every playthrough will have a fair amount of variability, but the relative percentages should be approximately the same each time. Some ammo only shows up after you reach a certain level threshold, obviously. This isn't a complete list, just a list of everything I've seen as of level 25.
EDIT: The numbers below don't include ammo looted from shotguns.
Fallout 4 Ammo Loot Count - Level 25
----------------------------------------
.308                     709
.38                     4488
.44                       222
.45                     2246
.50 Cal                 169
10mm                2074
5.56                     381
5mm                      17
Railway Spike           9
Shotgun Shell       556
2mm EM                    0
Cryo Cell                 13
Flamer Fuel           271 (893 additional fuel picked up from the Forged quest @ level 26)
Fusion Cell          2000 (est)
Gamma                 114
Plasma Cartridge   144
Fusion Core            36
Mini Nuke               12
Missile                    43
Cryo Grenade            4
Frag Grenade          46 (est)
Molotov Cocktail     64 (est)
Nuka Grenade           2
Plasma Grenade        2
Pulse Grenade           5
Bottlecap Mine          6
Cryo Mine                 8
Frag Mine               35
Plasma Mine             3
Pulse Mine                9


 Why can't i upgrade an assault rifle to fire .50's? 
