Now with the addition of turrets it's a bit more dynamic, and a bit more of a thinking game. Here are my thoughts on how best to use turrets.
Before I mention anything else, I should say that you can construct your sentry turret on the Sulaco, screwdriver it to unsecure the bolts, then drag it onto the shuttle or drop-pod pre-assembled. This saves you plenty of time if you're going to use it to secure the LZ, and this way you can preconfigure the settings to your liking.
General Tips
Sentries target ANYTHING without an ID card. This includes survivors. If survivors are being brought past an area which contains sentry guns, turn them off. Conversely, if you are transferring survivors, use your radio to alert the engineers that they should disable their sentry guns for a moment. I have seen dozens of survivors killed by sentries at the LZ.
Feel free to chat up fellow engineers and ask where they're setting up their defenses. Two sentries can lock down an area infinitely better than one.
If a squad has no engineer, confirm with the CO, or that squad's leader, and ask to be let into their prep room. You can hack the engineering prep cell, take the sentry, and add it to your defenses. Don't do this without permission from someone high up.
Sentries now function like smartguns. They can not teamfire fellow marines who are wearing their ID, unless the safety option is disabled.
Sentry bullets can detonate welding fuel tanks. You can use this to set up traps, where a welding fuel tank is set up six spaces in front of a sentry, at a space where xenos are likely to come. If they run into the tank and the sentry shoots it, they'll be stunned by the explosion and the sentry will kill them.
Sentries fire armor-piercing shells. They will never bounce off alien or marine armor, always inflicting their full damage. This makes them highly effective against crushers and predators.
Sentries can be ordered for 200 additional points at requisitions. If you return to the Sulaco after deployment, check with requisitions and see if they have the points for one.
Ammo magazines may be ordered for 80. Since you get two ammo magazines per sentry crate, it's almost never better to order ammo magazines by themselves.
Sentries do not care about cloaks, and will fire through them.
Sentries have extremely small 1000-charge power cells by default. For every sentry you're bringing to the field, bring one high-capacity power cell to swap it out.
When slashed past a critical damage threshold, sentries will start to 'let out sparks and smoke'. Watch for this message and back up, because the sentry will shortly explode, with approximately the power of a frag grenade, in a 3x3 matrix centered on the sentry.
Settings
What do I toggle to kill stuff best? An age-old Colonial Marines question.
Burst-fire or single fire?
Turrets have a single-fire mode and a burst-fire mode which fires three shells at once. Both of these settings fire with the same interval. The single-shot mode is perfectly accurate out to seven tiles, but if a xeno triggers it and then steps out of the way, the bullet can travel past 7 tiles to hit any xeno in the path. This works identically in burst-fire mode.
In burst-fire mode, however, shots have a small spread (not inaccuracy, like with other weapons - the shells always hit if they touch a xeno.) At the maximum targeting range of 7 tiles one or rarely two shells will often miss the target. This setting eats up ammo more quickly, but please note: burst-fire mode has superior DPS to single fire mode at all ranges. The only downside is the additional ammo consumption. I do not consider this, personally, to be a large problem - you get 600 shots per sentry and can order more at requisitions.
Burst-fire mode is essentially the only way a turret can crit aliens - single fire mode does not surprise and overwhelm with its damage, and can not keep aliens off barricades. I recommend you always leave burst-fire mode on.
Conical fire or 360 degree fire?
Sentries may be set to fire in a cone in front of them, stretching out to 7 tiles, or in a circle around them, stretching out to three tiles. Please note that these ranges include diagonals. By placing your sentries so that enemies have to approach along diagonals, they will have to walk twice as many tiles to attack the sentry, since players may not move along diagonals, while bullets can.
Sentries set to 360-degree mode only fire when an enemy enters the three-tile perimeter. At first this may seem to be a sheer downside, but if you're using burst fire it can actually be used to your advantage. On burst-fire mode, shots almost never spread out within three tiles, so the turret will always hit the full three-round burst on anything they target within that range.
While set to conical-fire mode, sentries can attack from a full half-screen-length away (ie. the sentry will only just become visible to aliens when it fires at them.) This is good if you need more breathing room, but do not leave sentries in this mode unattended unless they are covered by other sentries set to 360-degree firing mode. Even if the sentry is surrounded by walls, remember that aliens can melt those and freely attack your sentry from behind. To change a sentry's firing direction, simply wrench it.
Always leave sentries in burst-fire mode if they are set to 360-degree targeting. There is no downside to doing so.
Static placements
Here is an example of an ideal multi-turret static placement of sentries. The bottom and top sentry are set to 360-degree firing mode, while the middle two are in conical mode, being wrenched to fire at the left and right as needed. Note that the top and bottom sentries are placed so that they can watch over the middle two conically-set sentries, and that their diagonal firing lines extend deceptively far, allowing all four sentries to attack aliens venturing near the barricades.
The barricades block crusher charges and prevent spit from harming the sentries (yes - spit hurts sentries. A lot.) Always place barricades facing in the directions you expect alien will try to spit or charge from. Always place barricades at least one tile away from the sentry, otherwise aliens can walk right up to the barricade and slash the sentry through it. Leave magazines and spare power cells within the barricades for convenient storage.
Always place both tables and barricades at the corners of sentry emplacements, since you can not construct two barricades in one tile, in order to fully block those tiles. Having both a table and barricade makes it so that ravagers cannot just charge through the tables to get to the sentries, and makes aliens have to slash stuff longer.
There should be at least one entry point, typically on the side of the emplacement you gauge will be least prone to enemy attack, where you can climb across a table to refuel or repair the sentries. Leave this spot without a barricade - just a table. Or use a false wall, but beware that aliens can hide behind this when it's closed.
Setting up concealed mines within the barricades, or at vulnerable points at the perimeter, can be a horribly effective tactic in further reinforcing your emplacement's defense. Note that explosions damage sentries, however.
Mobile placements
Sentries can be moved around by turning them off, screwdrivering them, pulling them around, and reversing the process to re-anchor them. This can make the battlefield a scary place for aliens, who never know if they'll be bumbling into a freshly-established sentry nest that wasn't there thirty seconds ago.
If you have spare sentries once the crucial areas are locked down, use the spares wisely. Identify front lines, get to them, and establish safe-zones for the marines with your sentry. Keep the aliens on their toes and box them in. Static, fortified sentry nests are how you establish no-go zones for the aliens, but these ever-changing placements are how you thin out the alien numbers.
