Normally, a creature is limited to the number of attacks it can make as per its base attack bonus plus whatever abilities give it extra attacks, and, if it has the right weapon available, the creature can use each of its attacks to make either a melee weapon attack or a ranged weapon attack.
However, an unchained summoner's eidolon is further limited in the sheer number of attacks it can make by the table Eidolon Base Statistics: The Max. Attacks column of the table
indicates the maximum number of attacks that the eidolon is allowed to possess at the given level. If the eidolon is at its maximum, it cannot take evolutions that grant additional attacks. Attacks made with weapons, including those granted by a high base attack bonus, are counted against this maximum.
Thus were the eidolon's summoner to pick for her eidolon the 2-point evolution arms and those "[a]rms… have hands, [those arms can] be used to wield weapons, if the eidolon is proficient," but simply having those arms doesn't entitle an unchained eidolon to make more attacks. The unchanined eidolon remains bound by the chart's Max. Attacks column.
In other words, if the eidolon has a high enough base attack bonus and enough extra arms with hands, all of its attacks could be attacks with appropriate ranged weapons, like a pistol or a thrown dagger or a shuriken or a crossbow wielded in one hand, but also the unchained eidolon must possess sufficient Max. Attacks as per the chart to make all of those attacks in the first place.