Base burst size is 5. On a minigun, it remains at 5 with Full Auto (+2) and Long Game MK II (-2).
It then increases by 2,75 per burst (1,75 base on Long Game MK II, +1 from Mag Dump with 5 spec points into Extra Minigun Burst), for up to +33 at 12 stacks of Wind Up.
This adds up to 38 shots per burst, and it indeed works like it should at 100% combat speed, but changes at different values.
It increases to 39 at around 300% combat speed.
Then to 40 at around 450% combat speed.
Then it actually goes down to 37 at 500% combat speed.
Showcased in the video below, starting at 2:44:
https://youtu.be/orOh10ujUnE?t=164
