At start of turn 1 roll a D6. On a 2+, mark down the number as the interference level. If you roll a 1, re-roll until you get a 2+.
From turn 2 and onwards, can call 1 Barrage for each commanding officer whose squad includes a comm-link. Roll a D6 and if you equal or beat your established interference level you may call in the barrage.
See Codex Imperial Guard Pg. 71 for further instructions on dropping Barrage
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