Double time whether you volunteer or get forced.
Not always.
It used to be that if you volunteered, it was straight time.
If you were forced, it was double time.
The Company had always taken the position that the contract said "required" to work, and volunteers were not required, hence straight time.
@BigUnionGuy can probably tell you the year that a decision changed that practice, and made it double time for volunteers also.
ARTICLE 15-HOLIDAYS
Section 4
Except as otherwise provided in this Agreement, regular seniority employees required to work on any of the above named holidays shall receive double his/her regular hourly rate for all hours worked with a guarantee of eight (8) hours for full-time employees and four (4) hours for part-time employees. Also, no employee shall be required to work on Labor Day unless authorized by the local union.