I actually like shotguns as they stand, but they do have an overpowered->underpowered effect as you go through the game. The combat shotgun does entirely replace the normal shotgun, even more so with the tactical shotgun assembly being an easy to make weapon. Elephant guns are good, but a tac shotty is essentially a power-modded assault shotgun, making it THE thing to go for in almost any shotgun build. Between shottyman's full reload, less drop-off, tighter cone and using up a tech rather than power mod, I'll grab a tac shotty rather than an elephant every time. The single goes by the wayside, caught between the double for close range and the combat (or tactical) for distance, where you might just carry one through for Juggling purposes and still never really use it after the early game is over.
Here's my thoughts on how to make the shotgun more useful (and random thoughts on all the shotgun's play dynamics, with pointless real-life and scifi explanations in asterix text for no good reason).
#1. Slug firing alt-fire. Give it a slightly slower fire time, say 1.2 secs to account for aiming, with an 8d3 (or 12d2) shot at about +2 accuracy with standard bullet damage. Makes the shotgun a nice, versatile weapon being able to be modded in any way and usuable in any build.
* You don't know what type of shell Doomguy loaded, buckshot or slug, it's just loaded with a shell. Shells are shells. You just know what type of shell you're firing, not loading. *
* Sci-fi explanation. In the future, all shotgun shells are dual use. They are made up of a pre-shattered and re-fused depleted uranium slug with a micro-explosive detonator imbedded centrally in the shell, that can be set off in two ways. There is what we consider the standard buckshot form, where the micro-explosive is set off as the main charge of the round is fired, causing the round to break up into fragments whilst achieving muzzle velocity in the barrel. The other firing mode de-fuses the shell from the primary firing sequence and places it into impact-mode, firing the cartridge as a solid slug until it strikes a target, at which point the micro-explosive is detonated, causing massive internal injuries or shrapnel damage to the target. *
#2. More assemblies.
An assembly for combat shotgun like spread (AA), an assembly for DB spread (AB), a slug firing assembly (AT), a 3 shell magazine shotgun for mini-combat/high spread use (BB) or a plasma-blast-cutter wall-breacher(TB). It can be the non-whizkid, super-versatile weapon of modding choice. Whatever you find, you can slap it on a shotgun and make it your tool for the job.
* Over the years the simplicity of a shotgun has won over many fans. Whilst it's not great against modern armour, it's high reliability, large target circle, modifiability, excellent close-quarter abilities and tolerance to ammunition types (ranging from non-lethal/door-breaching/flares/electronic and pseudo-grenades all the way to the standard slug and buckshot rounds) has won over adherance in the military, police, criminal and civilian communties everywhere. A shotgun ain't just a shotgun. Is it a long barrel hunting gun, a sawn-off shotty, a mini-magazine non-pumper or has it just got a breaching charge loaded? It's your damn shotgun, you'd know. *
*Sci-fi explanation. The shotgun is the weapon assigned to all our Sergeants. They're the men on the ground, not only giving the orders and kicking arse, but damn well making sure those orders can be completed. The shotgun is their weapon. It makes sure those orders are not only followed, but that the mission is completed, regardless of the problems. Because of this, we have made their standard issue weapon both simple yet versatile. It can be modified almost indefinately to take a huge range of ammunition or to be altered into almost any configuration with all normal issue mod-packs. Grab your shotgun, fufill your mission. *
More to follow, my phone's almost dead :)