What range must a spell cast "on" a Shield Guardian have?

Question

The "Spell Storing" ability of the Shield Guardian says:

To do so, the wearer must cast the spell on the guardian. The spell has no effect but is stored within the guardian.

This seems to indicate you cannot cast, for instance, Fire Shield on the Guardian, because that has a range of Self.

This seems different than what is intended, as the wording for a ring of spell storing is:

This ring stores spells cast into it, holding them until the attuned wearer uses them....Any creature can cast a spell of 1st through 5th level into the ring by touching the ring as the spell is cast. The spell has no effect, other than to be stored in the ring.

The use of the word "into," which does not seem to indicate any range, versus the use of the word "on," which seems to mean the guardian is a target of the spell.

Which is the correct interpretation, grammatically, versus what is the RAI?

Answer 1

Your reading is correct: the Shield Guardian must be the target.

The phrasing "cast spell on [x]" is used consistently throughout the PHB, and always refers to [x] being the target of that spell. Here are some examples:

Nystul's Magic Aura:

If you cast this spell on the same creature or object every day for 30 days, placing the same effect on it each time, the illusion lasts until it is dispelled.

"Breaking up your move" (PHB 190):

For example, if you have a speed of 30 and a flying speed of 60 because a wizard cast the fly spell on you, you could fly 20 feet, then walk 10 feet, and then leap into the air to fly 30 feet more.

Animate Dead:

To maintain control of the creature for another 24 hours, you must cast this spell on the creature again before the current 24-hour period ends.

Therefore, the Shield Guardian can only store spells that can target it, which precludes spells with a range of "self".

It's worth noting that the Shield Guardian is a creature, and is thus a legal target for spells that target a creature, whereas the the Ring of Spell Storing is not a creature. This difference might account for the difference in wording between the two.

Answer 2

The Ring simply functions differently and has no bearing on how the Shield Guardian functions. The similar names are irrelevant and don't override the actual descriptions of the different functions.

A Shield Guardian must be targetable by the normal spell targeting rules for its Spell Storing ability to proc. Spells with range of Self can't target the Shield Guardian and therefore can't be stored in a Shield Guardian.