Grey Knights have been called boring, bland and repetitive and this is certainly true to an extent in the basic template of the army. You've got a MEQ statline armed with a Stormbolter and Nemsis Force Weapon, access to the psycannon, psilancer and incinerator special/heavy weapons, the psychic power Hammerhand and then specific defining features depending upon which Force Organisation slot they use (i.e. A2, Cleansing flame for Purifiers or four heavy weapons, Astral Aim for Purgation Squads). In the end this leaves the core firepower and options of a Grey Knight army quite similar and these are the humble storm bolter and psycannon.
I've already made an obvious assumption here: I'm choosing the psycannon over the incinerator and psilancer. This really is going to be a staple for nearly every squad of Grey Knights due to the flexibility it brings. Able to fire on the move or whilst stationary to greater effect with two to four shots which are very capable at anti-tank. Easily accounted for throughout the army brings significant firepower to your Grey Knight army and a great ability to deal with all ranges of armor values. The psilancer on the other hand doesn't really provide anything amazing unique compared to the bog standard storm bolter and whilst the incinerator is a great anti-infantry weapon being a S6 template, the sheer volume of firepower a Grey Knight list is generally capable of putting out, extra anti-infantry often isn't needed. This is a key understanding of how Grey Knights are built and played when using Storm Bolters and Psycannons. Let's look at this further.
Since most basic infantry models (obviously excluding Henchmen and Inquisitors) carry Storm Bolters the Grey Knight army operates very effectively 24" away from the opponent whilst also remaining mobile in terms of anti-infantry. Double-tapping with rapid fire weapons is a very effective way of inflicting lots of wounds of infantry models. Having storm bolters throughout the army therefore allow Grey Knights to effectively fight infantry from a distance and on the move as Storm Bolters effectively double tap at 24". This is really important when you consider how Grey Knight lists are built and how they play on the table. Importantly you rarely have to factor in dedicated anti-infantry units into your army as you've have a lot of S4/5 firepower which is effective from very early on. We'll look at how this affects the army's play style after we look at psycannons however.
The psycannon fits very well with stormbolters in that it is effective at 24"and can fire on the move. This has the great bonus of not limiting anti-infantry by taking what is effectively an anti-tank weapon (unlike in other Space Marine armies) which puts out the same (or more when stationary) shots as the storm bolter. This makes shooting at infantry units with psycannons and stormbolters have very little opportunity cost in potential firepower wasted (i.e. overkill of a missile launcher against an Ork). The real reason to take a psycannon of course is the ability it adds in the anti-tank department and this is where you see opportunity cost. Having a couple of mobile, rending, short-ranged autocannons per Grey Knight squad is amazing to have, particularly when they are stationary but when you end up shooting at infantry with the stormbolter/psycannon combo, you lose anti-tank potential and when you use the psycannons to shoot at tanks, you're losing stormbolter firepower against infantry.
This opportunity cost is mitigated when we acknowledge the 'repeition' and 'blandness' of the codex as nearly every infantry Grey Knight unit in a given army has this decision to make. By having so many units which encounter this opportunity cost, the Grey Knight army is able to overcome the potential loss of firepower and this combined with the effective range and mobility of these weapons creates a certain effective playstyle for Grey Knight armies built around Stormbolters and Psycannons. This is where the infantry units are outside of their transports. Rather than using their metal bawkses as bunkers, Grey Knights use them as extra cheap firepower (Psybacks) and quick mobility towards midfield. From there Grey Knight units operate on foot for the most part to maximise their firepower. This is due to their basic weapon (the storm bolter) being an effective anti-infantry weapon at range. This allows the Grey Knights to fight effectively on foot and at range unlike many other basic Troop units which rely upon rapid fire weapons as their main source of torrenting firepower.
Ultimately most Grey Knight armies (particularly ones focused on Grey Knight units) are going to operate around Stormbolters and Psycannons. There is nothing wrong with this though some people may feel this makes for a bland codex but does lead to Grey Knights being a very different mechanised force compared to other armies. Furthermore, each unit has some opportunity cost in terms of what potential firepower is being lost by shooting at infantry or a tank compared to the other. This is mitigated by the amount of units you can take which have these options available but target priority becomes very important for Grey Knight units when there are both infantry and tank units abound. In the end, Grey Knights have access to a lot of anti-infantry firepower built into the majority of their armies and each of their basic units has access to a weapon which adds a lot of anti-tank whilst still improving the anti-infantry ability of the squad. This allows the Grey Knight army to operate as a mobile shooting force very effectively from 24" and should be taken advantage of when you build and play with a Grey Knights army.
