30 C.F.R. § 75.1320 – Multiple-shot blasting.

(a) No more than 20 boreholes shall be fired in a round unless permitted in writing by the District Manager under § 75.1321.
(b) Instantaneous detonators shall not be used in the same circuit with delay detonators in any underground coal mine.
(c) In bituminous and lignite mines, only detonators with delay periods of 1,000 milliseconds or less shall be used.
(d) When blasting in anthracite mines, each borehole in a round shall be initiated in sequence from the opener hole or holes.
(e) Arrangement of detonator delay periods for bituminous and lignite mines shall be as follows:
1. (1) When blasting cut coal—
  1. (i) The first shot or shots fired in a round shall be initiated in the row nearest the kerf or the row or rows nearest the shear; and
  2. (ii) After the first shot or shots, the interval between the designated delay periods of successive shots shall be at least 50 milliseconds but not more than 100 milliseconds.
2. (2) When blasting coal off the solid—
  1. (i) Each shot in the round shall be initiated in sequence from the opener hole or holes; and
  2. (ii) After the first shot or shots, the interval between the designated delay periods of successive shots shall be at least 50 milliseconds but not more than 100 milliseconds.