In case people haven't seen them. Here are the old summoning rules from BL 3.3

BL1.1.3 Summoned Units
Certain units may be summoned to appear at the start of a Chaos formation’s action through the
use of sacrifices. Sacrifices are purchased for a formation before the battle (see the army lists
below).One sacrifice purchased for a formation can be ‘spent’ to summon daemonic units to the
battlefield at the start of that formation’saction, before the action test dice roll is made. The
sacrifice that is spent allows you to call D6 summoning points worth of summoned units to the
battlefield (the number of summoning points it costs to summon a unit will be listed on its data
sheet, but as a rule of thumb Greater Daemons cost four summoning points and all other units
one summoning point each). If the formation includes a Chaos Sorcerer or Khorne Champion then
you roll 2D6 whenever you spend a sacrifice to summon Daemons, rather than 1D6 as would
normally be the case. Note that you only receive the bonus dice for having an Chaos Sorcerer or
Khorne Champion if you first spend a sacrifice. Once a sacrifice has beenmade, summoning
points must be used to summon daemonic units into play immediately. If for any reason any
summoning points generated by a sacrifice are not used, they are lost and may not be carried
over into subsequent turns.

Summoned units must be set up within 5cm of any unit from their formation, apart from other
summoned units that have already been placed (i.e. you cannot place a ‘chain’ of summoned
units). They may not be set up in the Zone of Control of an enemy unit or in impassable terrain.
The type of unit that can be summoned depends on the summoning formation’s faction, as
follows:
Undivided: Daemonic Beasts
Khorne: Bloodthirster, Bloodletters
Nurgle: Great Unclean One, Plaguebearers
Slaanesh: Keeper of Secrets, Daemonettes
Tzeentch: Lord of Change, Flamers

Summoned units count as part of the formation for all rules purposes a slong as they remain on
the battlefield. The only exception to the normal rules is that the loss of a summoned unit does
not cause a blastmarker to be placed on the formation. They are otherwise counted as normal
units, and are included when working out if a formation outnumbers an opponent in an assault or
is broken by blast markers,etc. Note that Daemons that are killed in an assault do count towards
the number of units killed by the enemy when working out modifiers tothe result dice roll.
Summoned units remain on the battlefield until the end phase of the turn in which they are
summoned. In the rally phase, after a formation has attempted to rally, the Chaos player may
spend one sacrifice to keep any and all summoned units that belong to the formation in play for a
further turn. If the sacrifice is not made then all summoned units in the formation vanish back to
the warp and are removed from play. Note that this may lead to the formation breaking if the
number of blast markers on the formation is greater than the number of units left in play after the
Daemons have been removed.

Any summoned units will vanish back to the warp when a formation breaks. In this later case a
sacrifice may not be spent to keep the summoned units in play. Note that sacrifices are not lost
when a formation breaks.

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Guns don't break formations. Blast Markers break formations.