Background: Shooting velocity is a critical determinant of competitive success in ice hockey, yet evidence for the use of weighted-implement training in high-level junior players is limited and the long-term retention of such adaptations has not been documented. The aim of this study was to compare the effects of off-ice shooting training performed with a heavy (260 g) versus a regular (170 g) puck on on-ice shooting velocity, accuracy and handgrip strength in junior players, and to examine the retention of these changes. Methods: Twenty male junior ice hockey players (18–19 years) were randomly allocated to a Heavy-puck group (n = 10) or a Regular-puck group (n = 10) and completed an identical six-week off-ice shooting programme (18 sessions, 100 shots per session) with their respective pucks. On-ice wrist-shot and snap-shot speed (radar; standard 170 g puck for both groups), on-ice shooting accuracy and bilateral handgrip strength were assessed before the intervention (pre-test), immediately after six weeks (post 6 weeks) and after a six-week retention period of normal on-ice training (post 12 weeks). Data were analysed with 2 × 3 mixed-model ANOVA with Bonferroni-corrected post hoc comparisons. Results: A significant Group × Time interaction was found for wrist-shot speed (ηp² = 0.61), snap-shot speed (ηp² = 0.78), left-hand handgrip strength (ηp² = 0.30) and shooting accuracy (ηp² = 0.24). The Heavy-puck group displayed substantially larger velocity gains at both post 6 weeks (wrist shot d = 2.97; snap shot d = 4.73) and post 12 weeks (d = 2.56 and d = 3.21, respectively). Left-hand handgrip strength gain was also greater in the Heavy-puck group at post 12 weeks (d = 1.40). A short-term cost on accuracy was observed in the Heavy-puck group at post 6 weeks (d = −1.21), which was fully recovered at post 12 weeks. Conclusions: Heavy-puck off-ice training produced large and durable improvements in on-ice puck velocity, with a transient and recoverable cost on accuracy, supporting its inclusion in the off-ice preparation of junior ice hockey players.