Well, the models and the fluff, as well as the rules that support the game, are all three different animals. They overlap, for sure, but then again I just finished reading a chapter in Savage Scars where a regular Tactical Marine had three Melta Charges hanging on his belt. Not only that Tac Marines can't get melta bombs rules-wise, but to have three model-wise would be very inconvenient.
So that said, the models we are granted do not represent the actual fighting gear in all its detail. For example in earlier editions of marines, they came with pouches for ammo and such, but these are pretty much gone now. Haven't bought a tactical squad in ages, but a command squad a month back and saw no pouches.
Though "realistic" having a bunch of pouches on Marines really breaks the aesthetic. One or two is fine, but a realistic amount of ammunition would clutter the models.
Bolter mags can't feasibly have any more than 15 or so rounds in a sickle. Being that each round is .75 calibre, a 30cm long magazine can only hold 15 rounds. Maybe a few more if the sickle effect is good. Maximum 30 if one would assume superior double-stacking. But that's it. Physical limitations. You can't have .75 calibre ammo come endlessly out of a single bolter clip. Not even the AdMech can make that happen without warp portals and precision teleportation that the Imperium is not capable of.
And for those saying that marines only fire 14 rounds a battle. Well, do assault cannons really rend battle tanks to pieces with only 4 rounds a burst? Really?
I have not seen any fluff representation where an assault cannon fires a controlled burst of 4 rounds and halts to save ammunition. It's even mentioned in the rulebook that ammunition is scarse for the infantry variant assault cannon, but not so much for the vehicle ones. It's safe to assume that the number of shots weapons fire in the game do not represent the actual numbers of projectiles or beams released by these weapons in "real" combat.