Is this what you mean (the comments)?
if( entity->client && entity->s.eFlags & EF_MOUNTEDTANK ) {
// zinx - moved tank hack to here
// bani - fix tank bb
// zinx - figured out real values, only tag_player is applied,
// so there are two left:
// mg42upper attaches to tag_mg42nest[mg42base] at:
// 0.03125, -1.171875, 27.984375
// player attaches to tag_playerpo[mg42upper] at:
// 3.265625, -1.359375, 2.96875
// this is a hack, by the way.
entity->r.currentOrigin[0] += 0.03125 + 3.265625;
entity->r.currentOrigin[1] += -1.171875 + -1.359375;
entity->r.currentOrigin[2] += 27.984375 + 2.96875;
}