Based on the rulebook context:

• On page 18, it states: "Some cards improve your weapon proficiency and can be used as long as you have Bullets."
• It also says: "Store cards that give you resources can be exhausted once per round to gain the effect."

Analysis

• Store cards that give you resources are limited to being exhausted once per round for their effect.
• Store cards that improve weapon proficiency can be used "as long as you have Bullets," with no mention of an exhaustion or once-per-round limit.

Summary

If the store card improves weapon proficiency and requires bullets to activate, it can be used multiple times in a turn, as long as the player has enough bullets. There is no stated limit for these cards, unlike resource-generating store cards, which can only be exhausted once per round.

Source: page 18.

Source: Core Rulebook (pp. 13, 18, 28, 7, 8)