i'd love to see a huge guide for sentry spots.

I don't pretend to know what I'm really talking about here, most of it is just rambling.

Mostly it comes down to common sense, what exactly do you want to do with the sentry?

It can be effective at simply guarding one door, so that it restricts where the enemy can push from without needing to pop early. To do that, you'd need to find an area that isn't very spammable from the push point, but they'll still need to get past in order to push.
I.e. Gully Riverside.

Another good use is a surprise play to force them to pop where they wouldn't need to.
This is ideal if you don't stick to a common position, but you'll also need to build it where it can't be seen too early. This is especially effective against Kritz, I guess. I'd personally only personally run this play against Kritz.

Also, no matter where you put it, it's really going to what to be able to aim at the point. This will be great at taking down a pesky scout attempting to cap in the action.

I always had the mindset that a sentry gun in a spot where teams would least expect would work well. Unfortunately, not many of those spots actually exist. Really, a sentry gun spot can succeed a lot from anywhere, so long as you have the team's defense set around it.

But anyway, here are a couple of spots I typically enjoy using on Badlands and Metalworks.

The Badlands spot is behind the point. It's a really useful spot at times. Not always, but often enough. Any scout trying to b-line to the point from bottom left is going to get wrecked. Any player on the other team who doesn't fight on top or basically right by the point will get wrecked (this is all as long as your team can point and click. I also said basically right by the point because it's either there or in a door way, and door ways are obviously a bad place to be stuck in).

The Metalworks spot is on last as well. It's under the bridge where the bridge and wall meet, on the side opposite of spawn (main door area, I guess). The only area they can see it from without being shot beforehand is far right. Cover that spot with a soldier constantly spamming and there won't be a reason for them to try and peek, let alone look for a sentry they don't know is there. I'm not sure of many teams who say, "be quiet for a second, let's see if we can hear a sentry", so you're probably good on that. This spot only works once, though it could POSSIBLY work twice, just not in a row. And holy shit, the spot can do tons of work. It allows your demo to focus his stickies somewhere else besides the point as well. Always a plus.

I can't remember any more not so typical spots but if I do I'll edit this.

Make sure that the sentry can't be seen from the far side of the chokepoint, so that it can't be destroyed by spam from safety. An example of this is sentries at top left on badlands can be taken out quite easily from main.

http://i.imgur.com/0DCQP1B.jpg

http://i.imgur.com/OYKlNwT.jpg

These are the most popular spots (snakewater arranged by how frequent I see it being used)
The sentries indicate how useful they actually are, level 3s usually end up with 1 pick where the level 1s might not even work. Just make sure you're able to swap back instead of dying with the sentry. It's going to slow them down but you shouldn't die to keep it up.

was bored so i went through some maps and took screenshots of common/possible spots

granary

http://cloud-2.steampowered.com/ugc/578990017779921636/DC91012F1A8DD0351B628B184082BC74D31369C4/

http://cloud-2.steampowered.com/ugc/578990017779917761/8A3F737CB5F058DB651BE9C2406444970AE07006/

http://cloud-2.steampowered.com/ugc/578990017779910532/E0EEADD495F6458081AD7E92085DFB0B7E2E6D67/

http://cloud-2.steampowered.com/ugc/578990017779913341/45E10DDE49A4B21990935EAFB5FFD1D9D0DDD630/

http://cloud-2.steampowered.com/ugc/578990017779906802/325ECB5F63F60D6C809FF531919798DD3E0CF602/

http://cloud-2.steampowered.com/ugc/578990017779903273/EEF5FAC3FD6BE78D8F724662CC145299E535B2F1/

gullywash

http://cloud-2.steampowered.com/ugc/578990017779965558/DB425E275B7EFB7C5CDE9F33F9B2A5109AF48222/

http://cloud-2.steampowered.com/ugc/578990017779971252/5DFCF8A673610798FDE44A20215614C2772FDCBB/

http://cloud-2.steampowered.com/ugc/578990017779976727/39D0155BD968490237AAA82A2B91B264B3E8EC95/

http://cloud-2.steampowered.com/ugc/578990017779980165/D4EA05CA897061EC9E57E8496FCC219572CEBB39/

http://cloud-2.steampowered.com/ugc/578990017779993814/177D226B46A4B47E24E4344E2DA54D78C2B2B71A/

http://cloud-2.steampowered.com/ugc/578990017779997271/0B961A007AFAE0B47A78076AB15A2407CDD2BCB1/

http://cloud-2.steampowered.com/ugc/578990017779954375/F7F15821F28EBABB0CD580ADEF505C55BFA4FE24/

http://cloud-2.steampowered.com/ugc/578990017779960631/0E0D3DD4C816C5D708A68DADF93F8EF4927E6778/

process

http://cloud-2.steampowered.com/ugc/578990017779948714/3E97B7EAE3DD09D6794532033B2728F543D9804A/

http://cloud-2.steampowered.com/ugc/578990017779943699/61C5D0031A00505D13EEC9410F93957DB0E09403/

http://cloud-2.steampowered.com/ugc/578990017779936940/382421C0545642BE73A242B5515407A98F54944B/

http://cloud-2.steampowered.com/ugc/578990017779929330/18535B1CC8D206BB4590B270DBD2FD7056F2BA69/

http://cloud-2.steampowered.com/ugc/578990017779926032/CC3C20156D38291A0785E3DD383A4E1C5626790C/

Thanks guys :)

My approach is a bit predictable, but I think the best place to put a sentry is in a position that has full or semi cap point coverage but is still as close as possible to your spawn doors. This allows you to build the turret fast and also allows you to switch to another class quickly if it dies. It's easy and effective.

This should also a position that cannot be spammed from long distances without forcing the enemy team to enter the room. For example, I like this process spot:

http://i.imgur.com/Q8L5M0k.jpg?1

With the jag, it's quick to build, it covers 95% of the last point, it is far away from entrances and hard to spam without first entering the room, and it is easy to abandon and change classes if it is focused down.

Just think of a sentry gun as buying your team enough time to kite an uber or defend itself. The extra second or two the other team spends trying to kill your gun makes it worth it if you don't die and can switch classes quickly.