Yes - and there are alternatives
Monster Manual p 120 describes the typical giants' "big leather shoulder sacks", dispels the myths remaining from 1e/2e and then says:
In truth they usually hold a few battered and smelly personal items,
a supply of throwing rocks, some less than fresh rations and some personal trinkets.
The key sentence for Draw or Sheathe a Weapon is:
If your weapon or weaponlike object is stored in a pack or otherwise
out of easy reach, treat this as retrieving a stored item.
If the shoulder sack is considered to be a grimy handbag/manbag (as appropriate) slung over the master shoulder and hanging down the non-master side of the body, it is not "out of easy reach", although given the infinite variations on equipment carriage it will always be the GM's call. While finding one specific item in such a bag might be time-consuming (we have all seen people going through their bags for ages looking for that missing receipt or loyalty card) plunging a hand in to grab the first big throwable rock out of a supply of same is pretty simple. In some instances it might be appropriate for an action to be taken opening the sack on the first round of combat only (eg where it has been laced shut while climbing or in inclement weather).
If there was a better-resourced stone-throwing giant who is stocking different ammunition types to throw (silver, cold iron, barrels of pitch) then they can afford to get a custom sack to let them locate particular ammunition by touch, just as many adventurers have sheaths, Ehlonna's Quiver etc in D&D and soldiers have load-bearing equipment today.
Alternatives to rapid fire
While there are numerous variables, rapid fire may not be the best way to boost damage. For GMs who really want to dish the hits out to their players there is the Hulking Hurler prestige class (Complete Warrior page 40-42). The Two-Handed Hurl Trick "Meteor Strike" will deal quite impressive damage to a single target in a single round - base damage + 3 x Strength modifier (1 x Str mod for standard, 2 x Str mod for the trick). This can be further boosted if Ranged Power Attack is taken as the last trick in the prestige class.