McCallum was named in the World Rugby Under-20 Championship Scotland U20 squad for 2015.
Despite progressing through the age-grade systems as a loosehead prop, he made the switch to tighthead prop and graduated from the BT Sport Scottish Rugby Academy, joining Edinburgh Rugby on a two-year contract.
He debuted for Edinburgh Rugby as a replacement in the 45-10 defeat of Treviso in October 2016, before making his first start for the club in the 59-17 victory over Timisoara Saracens the following week.
January 2018 saw McCallum being included as one of four uncapped players in Scotland's 40-man squad for the 2018 Six nations tournament.
The following year, he signed a two-year extension which will see him remain in Edinburgh until at least 2021.
In July 2021, he signed a short-term contract with Glasgow Warriors.
(LS July 15, 2021)
Career
 |
Newcastle Falcons Tighthead Prop |
2023 - present |
 |
Scotland U20's Tighthead Prop |
2015 - 2017 |
 |
Scotland Tighthead Prop |
2018 - 2019 |
 |
Glasgow Warriors Tighthead Prop |
2021 - 2022 |
 |
Barbarians No 4 Lock |
2022 - 2023 |
 |
Edinburgh Rugby Loosehead Prop |
2016 - 2023 |
 |
Worcester Warriors Tighthead Prop |
2021 - 2023 |
