there are some people who play the 'front to back' method with barrages, the templates tell you how many dice to roll, but casualties are removed front to back as per normal shooting..... I'm not sure if this was in the rulebook at some point, but it does prevent barrage 'sniping' and means tyranid players don't have to adopt strange shaped formations to prevent the precious synapse-warriors being shot from miles away.....

there are some people who play the 'front to back' method with barrages, the templates tell you how many dice to roll, but casualties are removed front to back as per normal shooting..... I'm not sure if this was in the rulebook at some point, but it does prevent barrage 'sniping' and means tyranid players don't have to adopt strange shaped formations to prevent the precious synapse-warriors being shot from miles away.....

To fire a barrage declare take a barrage template and place it on the table where you want the barrage to land. You are allowed to place templates over your own units, or units from several enemy formations, just so long as you get as many units from the main target under the templates as you possible can. All units –friend and foe alike – under the templates are attacked.
Next refer to the data sheet to work out the total number of barrage points. The whole formation fires at once, so the number of barrage points for each weapon added together to find the total number of barrage points. When you have worked out the total of barrage points refer to the barrage table below.
The barrage table tells you the hit roll required to hit each unit under the barrage template. Roll to hit all units (friend or foe) under the template with the appropriate to hit values. If formations other than the main target formation suffer any casualties then they receive one blast marker for each unit lost. In order to speed dice rolling we recommend rolling to hit all units of exactly the same type together, and then removing any casualties from those closest to the enemy first. For example, if you ended up with an Imperial Guard Commander, six Guard infantry stands, and three Fire Support stands under a set of bombardment templates, I’d roll once to see if the commander was hit, then roll six dice all together to see how many of the Infantry stands were hit, and three dice altogether to see how many fire support stands were hit. You get the idea, I’m sure!

Note, this is the one occasion where the normal "front to back" mechanic is not applied.