Famed for their siegecraft and resolute firepower, the Imperial Fists offer straightforward and useful Chapter Tactics. While many of the other Chapter Tactics provide mobility or advantages in specific circumstances, the Imperial Fists’ tactics just make their firepower more reliable. Frankly, that’s great in my mind.
First up, they have Bolter Drill, which allows any model firing any type of bolter to re-roll 1’s to hit. This extends to bolt pistols, stormbolters, and heavy bolters, but does not include Sternguard firing special issue ammunition. Throughout your army, this mostly benefits Tactical Marines as they are most likely to be carrying large numbers of bolters. However, it can also increase the firepower of standard Terminators, Scouts armed with bolters, and Devastators with massed heavy bolters (though we’ll get to the reason why you won’t be taking them). It’s a nice bonus, but it doesn’t do too much. Ten bolter shots with Bolter Drill will result in an average of 7.77 hits, compared to 6.67 hits without. It’ll certainly make a difference over the course of a battle but it’s no reason to take more Tactical Marines than you normally would.
The second rule is Siege Masters, which gives Imperial Fist Devastators and Devastator Centurions Tank Hunter. This is massive, allowing re-rolls for any armor penetration rolls. Combined with the reduced cost of Devastators in this codex, this makes them by far the best anti-tank unit available to Imperial Fists. Flakk missiles even become reliable at taking down most flyers (jumping from 0.22 damaging hits per shot to 0.37 damaging hits per shot against AV12). Even better, you can man a quad gun with Devastators and it gets Tank Hunter as well, averaging 2 damaging hits against AV12 every time it shoots. Devastator Centurions, though expensive, bring even more firepower. Armed with an omniscope, twin-linked lascannons, and a missile launcher, a squad will likely destroy two targets every turn. This is probably the only circumstance I can think of where grav-cannons may not be the best armament for Devastator Centurions.
Overall, I think that these rules combine to encourage specialization in your army. Tactical squads become even better at dealing with infantry, while Devastators (and Devastator Centurions) are better at taking out vehicles. Since these are built into the rules, I say run with it. I’ll be running Tactical squads with heavy bolters (for more anti-infantry with some re-rolls) and flamers (for anti-infantry and some assault defense). I used to use my Tacticals with meltas and plasma, but this encouraged them to close with and engage hard targets. This almost always resulted in the deaths of my Tactical Marines, whether or not the managed to take out their targets. They simply were biting off more than they could chew. Tactical squads aren’t elite squads, they’re Troops. They need to target and eliminate other Troops.
That means something has to take up the slack when it comes to destroying the vehicles, which Devastators are well-equipped to do. I think a combination of lascannons and missile launchers will provide the best ability to take on vehicles (they are already good at tank hunting, but become even better in the hands of Imperial Fists). Due to their limited mobility, you’ll need longer range so multi-meltas won’t work. Plasma cannons are an interesting choice as they are fairly dangerous to vehicles with Tank Hunter but double as heavy infantry killers.