I'll start off by saying that this is a difficult tank to play well. The bloom and time between shells really kills any quick opportunity to deal a lot of damage that every other autoloader gets except for the Foch 155. What it does get in return is more mobility and a higher potential clip damage than the rest of the autoloader bunch (except for the Foch, once again) and that's what you have to play for, every single time.
The crew is small and the tank is fragile meaning you'll need a good crew to stay alive. I personally recommend camo over repairs if you're playing SH/CW as a batchat player but the other way around for randoms. Spotting means squat in randoms anymore so my suggested order of training goes something like this:
Repairs + sixth (survivability in general because getting tracked means that you're dead)
Camo and respec to BiA once it's full (one could use gun skills or situational awareness but I don't see how they're more important at the moment this early)
Camo once again (you could do the last repair skill for the commander if you want to)
Situational Awareness/Snap Shot/Smooth Ride (even if the skills have surfaced as somewhat useless the gun is shitty enough for them to worth it, especially if you use it SH/CW where you'll have to run AFE instead of food)
After that you can go with whatever you wish, I went with firefighting because I run food in randoms.
You have a few options here. 30/0/0, 25/5 or 20/10. I personally run 20/10 as of now but I've been running 30/0/0 for ages and it's perfectly fine too. The point of this is simply the low ammo capacity and split clips. Running 15/15 (50/50 like I run in most of my meds that don't have 50+ shells) is simply too split when your ammo actually runs low. You'll have a clip of 4 shells and one with 1 when you needed them all. Your DPM goes trash tier from already shit tier.
The playstyle changes as well with the HEAT rounds. Lack of them means that you'll have to spend more time using the map to get shots you can actually pen instead of just engaging a E 100 frontally when he's on reload for 2K damage. 20/10 is the most flexible one in my opinion. I usually only fire 4 out of 5 shells before my shootable targets get safe and that adds up to an extra 4 shell clip simply by using experience on my end. 10 HEAT rounds means that you can shave off 4K HP off of the bulkier heavies frontally when there's situations you wouldn't otherwise be able to shoot them. (BC on Mali hill shooting early on an E 100 or any japanese heavy moving for example) so you don't have to constantly keep target selection at the back of your head wasting time thinking on what to do.
I know some of the better players run 25/5 because they fire that HEAT clip early and always fire them, then just switch to APCR for the rest of the game since they already sunk in 2K HP early in a heavier engagement before just rotating elsewhere.
Vertical Stabilizer is an obvious choice, I don't think it's necessary to go deeper into why.
The remaining contenders are vents, optics and GLD. I've seen every combo possible out of these three and none of them are actually wrong and it mostly comes down to how you play your batchat and if you do competitive play or not.
Optics are arguably useless in randoms (although I've personally been getting bang for my buck running them lately) but at least they'll keep you up top on the spotting game. I reach 488 meters of viewrange in my BC running optics, food, BiA and situational awareness which is more than enough to always stay on the edge of spotting. Camo piercing might not seem like much, but if you ever get camosniped that extra viewrange will help you out more than you'd think. It'll also let you actually spot tanks at max viewrange. Every tank has camo values which means that spotting an O-I or a Maus at 445m (max view range) won't happen at exactly 445m away because of camo. Now this is probably just a few meters but when it comes to other tanks like TDs, the extra view range helps you spot them through piercing them at large distances. The most notable map I noticed for this is Fishermans Bay. With a VR specced BC I can spot the the northeastern corner from the middle when they fire and net myself and my team early damage. Now this might sound very situational and anecdotal but it's just an example of when optics actively contribute to winning games.
Vents are vents, small boost to everything and hardly noticeable (food is twice the boost). Good for people who play the BC for 100% gun or want to boost their VR even more (hardly necessary breaking 500, ever)
GLD is what I run combined with optics. You'll actually make use of this because of the terrible aimtime and sitting still while dumping your clip quite often so the GLD will actually kick in more often than you'd think.
This gun is both good and bad. The clip potential is high and has the potential to kill some mediums in a single clip and shave 2/3 of any T10s HP pool, but that incredible strength is balanced by atrocious gun handling and long exposure to get those shells out. (It's about 10 seconds given aiming the first shell). The reload is long and renders you weak but at least it gives you decent downtime to look around and find new opportunities for the best way to approach an engagement. You'll be rotating a lot around the map simply because of the gun and running away like a little bitch whenever a flank might fail whenever you're on it.
The ammo capacity is low which means you'll have to think hard about how you load it. I recently switched from 30/0 to 20/10 because of increased SH/CW play where I'll need it but I personally didn't find the HEAT to be worthwhile in randoms. I always find myself running out of APCR before switching to HEAT now anyway in any given random battle anyway. The HEAT does give you the opportunity to engage heavies frontally whenever you'd need it (Malinovka hill is probably the only time I ever load HEAT because E5s are a thing) but it comes at the cost of split clips. Having 3 HEAT shells and 1 APCR makes for terrible late game cleanup possibilites (if you've fired that many shells you're probably having a decent game anyway, but it sucks to lose games because of this). I personally found APCR alone to be more than good enough for randoms since you'll spend more time deciding on what to attack than actually attacking so I end up with roughly the same damage anyway.
For competitive play it's a completely different story. The gun becomes a lot more dangerous here and it's a high risk tank. Missing a shell can kill you if you're playing a spotting game against another BC which is pretty goddamn easy to do considering the aimtime/bloom. But it can also shave off 2K HP of the enemy team in less than 10 seconds making it possible to enable pushes and allows it to solo scout/freeplay better than any other T10 in the game.
In actual thickness it's non-existent. The UFP angle can bounce a shell once in a while but never something to rely on. What it does have as pseudo-armour is speed and decent camo. Engaging at range (sending mixed messages here with the terrible gun, but once you let it aim in fully it can hit pretty well at longer ranges as well) allows your camo to work if you'd be down on HP, in the clan CHAI or simply think it's the best course of action. The speed helps you dodge shells and move in close quickly or simply get out quickly if things went south. Mobility=Armour in this case.
The strength of the tank lies in the gun and mobility, one of them also being its weakness. It can put in quite a bit of damage in a short amount of time which makes it possible to turn games around. Autoloaders always have the lovely advantage of mitigating damage taken when clipping since people can't really deal enough damage back at you unless they're grouped up. Finding lone tanks in the endgame is easy prey for a BC and incredibly easy to outplay. If it's anything short of another autoloader you can pretty much just yolo in, clip fully and either kill them from scratch or simply just run away afterwards. I find it very rare that anything that can actually chase your BC down after clipping is usually too low on HP for it to really be worthwhile if you clip fully. Combine it with your 5 shots in 8 seconds for their 1-2 (maybe 3 if they chase you a bit). In the case of an E 50M or a 907 that can survive your clip and probably will you'll still be able to get out anyway or just simply hang back until support arrives.
You should also try to engage backwards as much as possible. You expose less and driving forward will be quicker to get away from corridor pokes than backwards (small thing but still useful). Driving backwards will also benefit more from terrain as you can use rubble and stuff to drive onto to get better depression angles.
The newest strength that completely breaks some maps: Climbing!
Climbing sure as hell is useful. Mines gets completely destroyed by a single guy on the 0 line from either side and it also adds another layer of depth in the game. While it's mostly micromanaging it takes some time to figure the climbs out and climbing doesn't really get useful until you can do them consistently.
The weakness is pretty much the the long aim time that forces you to rush a shot or two because it simply won't aim in fast enough for the opportunities of damage to yield any fruit. Once again we have the long downtime between reloads and no armour whatsoever which means that you die if people catch you on reload. How you prevent this is by simply playing well guarded areas and making sure that you can actually run away and that anything chasing you will die trying, or simply playing passive for most of the game while seizing any overextensions done by enemy tanks for 2k damage.
You could also argue that the fragility of the tank combined with the speed is the cause for a lot of overextensions and while that is true it's not really a weakness itself, it's also easily preventable.
I simply play this is a very opportunity seizing TD/LT. Whenever maps allow for it I'll try and be somewhat active for spotting (middle on Sand River for example) but I hardly see anyone having trouble on open maps with a BC. Problem's most likely city maps anyway. I simply pay a lot of attention to the minimap and try to figure out where I can deal the most damage while taking as little as possible. It shouldn't be too hard considering the reload times while clipping anyway. You have about 40 seconds before you even get to shoot again and that's plenty of time in a mobile tank like this to rotate if your current engagement won't pay off anymore. I'm very patient and once someone goes in the open I make sure that I'm there wrecking him.
In a bit more concrete example I'll take Himmelsdorf as an example. I rarely ever play the hill anymore because it's such a hit or miss engagement. I play the 3 line. I almost always just sit around waiting for people to expose themselves on the lower side around the castle. With the engagement range at 400m only T10 meds will be able to spot me back and actually put damage into me. I also have a 40 second downtime anyway that I can just waste waiting on people to show themselves. From the north this is a bit more viable since you can further up the 3 line without exposing and get sideshots on heavies crossing in the 8 line as well. From the south it's not as good but still a lot more consistent than playing the hill. You can simply poke early to get spots on people crossing so you get a general idea of how many people are playing the railroad and what to expect on the hill. You're also free to move into the middle windows for shots at the enemy windows. You can simply move up, rotate, and go wherever you want. It's my initial play for that reason. It's a low risk opening play with a decent reward. TL;DR is probably keeping engagements at range early on so you can use your HP diving in later on. Applying this on other maps haved worked very well too. It's all about knowing what a favourable engagement is anyway.
I'd give the BC a solid 9/10. It's stupid good for a tank that really hasn't been taken into consideration of the tier 10 powercreep and it still remains one of the strongest T10s in the hands of gods while being absolutely terrible for worse ones.