Sternguard will turn pretty much anything to swiss cheese that isn't T8 if you use them fully.
First: Sternguard are no longer the "special weapon" squad they used to be, that goes to tacticals and company veterans now. Their new Special Issue Boltguns are actually a combination of 2 shells they had prior put into one, Kraken and Vengeance. The range of Kraken with the AP of Vengeance which by all accounts makes their boltguns arguably the best boltguns in the game bar none (even above deathwatch special issue ammo).
The AP is just right for such weapons as going above AP2 on such a weapon is wasted really and the range means they can reach out and touch anything they want fairly easily, coupled with Beta-Bolters these wily vets actually carve out a unique place in the marine codex now unlike a lot of other units.
As you note, 20 AP2 shots does hurt at 30" and because of that 30" you can put them in cover. Do that and now your opponent can't slug it out with standard infantry going weapons and have to waste higher grade gear on such units. Yea, certainly taking a heavy bolter to the face does suck but lets face it, when that's all they have that can reach you, it's better than 9 boltguns AND a heavy bolter. This is the part that gives them durability, the range. Yea, the lack of wound sucks but we can compensate by using their range (in contrast to intercessors who want to use the extra range to buff their ability to get within rapid fire range when slugging it out on the move) as the best armour is the one you never need to use because you never get hit (read: Eldar tactics).
So what makes these guys able as I alluded to to being able to shred even a T7 battle tank? Their stratagem and going in on a little bit of combo power. First, they have one of the good stratagems in the codex (and I would rate it top 3 actually) which just raw gives them +1 to wounding rolls. That alone is pretty nasty as now even against T7 you are wounding on 4s. Remember, they have AP2 and while only one damage they can spit shots out like a punisher cannon! Get a marine command aura package nearby and these boys can actually do serious work even when on the anti-tank roster.
However if you want to go IN on sternguard then go Imperial Fist. First you now have ignore cover on your SIBs, now you have 3 of the special ammos in one gun (Dragonfire, Kraken and Vengeance) and that's all the time now, fairly boss if you ask me. Now we look at Imperial fist unique things and their stratagem is just the ticket: bolter drill. The key about this stratagem that GW have realised in later releases of the game is saying 6+ is a really bad idea if you give armies +1 to rolls and that is what Bolter Drill says: 6+ triggers an extra shot. Normally these stratagems kinda just flounder away because they are SERIOUSLY pathetic unless they are 6+ trigger and not "Unmodified 6" (which those can bite my power armour rear. It just feels bad is what they feels like) but thankfully marines do in fact have 2 forms of getting +1 to hit rolls now, with one being more accessible thanks to shadowspear. The Rhino Primaris and the Warlord trait "Target Priority".
Now with just one you start opening worlds of hurt. Hitting on 2s (re-rolling 1s because you have a captain nearby...you do don't you?) and triggering bolter drill on 5+ means of your 20 shots you will normally getting 6-7 extra shots without re-rolls being factored. If you go in on the second +1 buff then things get silly was now you trigger drill on 4s so your 20 shots become 30 and it keeps piling on. Pair that with masterful marksmenship you can make a tank wish it wasn't born as suddenly they are taking something like 10-13 armour saves at 5+. Even for a high would count model, for a unit you would think is only anti-infantry to turn around and begin making your tanks wound count dwindle is quite scary.
From my experience, they are extremely effective and while lacking the wound count of primaris the fact they have a special stratagem that is THAT good makes them a superb unit. Even without it they can quite easily chew up infantry left and right. And ofcourse being vets they have 2 attacks each so not slouches if they get charged.
I Chapter Master 454, Chapter Master of the Angels of Justice, Warboss of WAAAGH Gubskul, Commander of a Catachan Regiment, Phaeron of a Tomb World, Shas'O to a Cadre and Princeps of a lance of House Taranis hereby pledge that I will not take up any further models til all other prior have been fully built and painted to tabletop standards. There is no time limit for this task, there is no deadline. My oath is to solemnly complete the armies I have now, to see it that they can have their glory. Paint will be stripped from the old in need, thick may it be like ceramite I will see it removed so that plastic and metal alike may see light of new paint. Models yet to be, boxed and in darkness will be assembled with due care and attention. For this task I am permitted to still buy the supplies needed to do my task but not one model more.
http://www.bolterand...one-model-more/ the thread to my oath. My own reminder.
http://www.bolterand...rk-in-progress/ my own chapter
"The objective of playing a game is to win. The point of playing a game is to have fun. Never confuse the two"