Preface: Using a modern-esque alternate reality type steampunk world, with magic, and World War-I era guns....Backstory will explain the state of things, but the focus of this post here is on the rule set for guns; the largest task for me. Fiddlin with this alot to come up with a system where guns are accessible but not overpowering. No musket type weapons in this system, to avoid shoot and drop, and the cartridge has been invented and is in use in most firearms. Added some gun mage type feats for those taking these feats as a wizard, for example. Also, I just had to have a gunblade 
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-HEROIC TIER FEATS
* Simple Firearms Trainee: (the 1st multiclass Firearms feat) you gain proficiency with the Simple Firearms weapons Derringer, Breechloader, Revolver, and Bolt-Action Rifle; also faster reloads than non-proficient users (move actions instead of standard actions). Prerequisite: 13+ Dexterity and Simple Ranged (like all but strictly melee classes get) Proficiency. Also grants access to give up an Encounter Power to gain an At-Will or Encounter power that enables a special Simple Firearm action.
* Simple Firearms Mastery: (next level of multiclass training) opens up the final tiers of Simple Weapon attack powers, and fastest reloads (replace standard actions with minor actions). Prereq: 13+Dex, Simple Firearms Trainee.
* Military Firearms Initiate: (1st level of multiclassing into Military Firearms) you gain proficiency with the Military Firearms weapons Shotgun, and Semi-Automatic. Prerequisite: 13+ and Simple Firearms Mastery. Also grants access to give up an Encounter Power to gain an At-Will or Encounter power for a Military Firearm action, or multi-shot type attack with the semi-auto.
* Military Firearms Acolyte: (2nd level of multiclass) opens up a more advanced weapon attack power with Military Firearms. Prerequisites: 13+ Dexterity, and Military Firearms Initiate.
* Crackshot : you gain a +1 feat bonus to damage rolls with all pistol firearms. Prequisites: Military Firearms Initiate.
* Imbued Shot: you can change the damage type of a shot to have any energy type you can normally attack with while using an At-Will attack power (at-will power).
* Debilitation: you can inflict any effect (and reduce the damage dealt to a target) that you can normally cause as part of an At-will attack power.
Thus, fastest progression to gunblade: Simple Ranged Proficiency -> Simple Firearms Initiate -> Military Firearms Initiative -> Military Firearms Acolyte -> (Paragon) Military Firearms Mastery
-PARAGON TIER FEATS
* Military Firearms Mastery: opens up the final tiers of advanced weapon attack powers, and faster reloads with Military Firearms. Prerequisites: 13+ Dexterity and 13+ Intelligence, and Military Firearms Acolyte.
* Gunslinger: you gain special attacks with the Superior Firearm Semi-automatic. Prerequisite: 15+ Dexterity, Military Firearms Acolyte
* Riot Squad: you gain special attacks with the Superior Firearm Shotgun. Prerequisite: 13+ Strength, Military Firearms Acolyte
* Superior Firearms Proficiency (Gunblade): you gain proficiency with the Superior Military Firearm weapon Gunblade. Prerequisite: Military Firearms Initiate, Military Melee proficiency, 13+ Dex, 13+ Str.
* Gunbladist: you gain access to special attacks with the Superior Firearm, the Gunblade. Prerequisite: 13+ Dexterity, 13+ Str, Military Firearms Acolyte, Military Melee weapons proficiency, Superior Firearms Proficiency (Gunblade)
* Gunslinger: you gain special attacks with the Superior Firearm Semi-automatic. Prerequisite: 15+ Dexterity, Military Firearms Acolyte
* Riot Squad: you gain special attacks with the Superior Firearm Shotgun. Prerequisite: 13+ Strength, Military Firearms Acolyte
* Superior Firearms Proficiency (Gunblade): you gain proficiency with the Superior Military Firearm weapon Gunblade. Prerequisite: Military Firearms Initiate, Military Melee proficiency, 13+ Dex, 13+ Str.
* Gunbladist: you gain access to special attacks with the Superior Firearm, the Gunblade. Prerequisite: 13+ Dexterity, 13+ Str, Military Firearms Acolyte, Military Melee weapons proficiency, Superior Firearms Proficiency (Gunblade)
You could target reflex and drop the prof bonus altogether (your proficiency feat are already granting other abilities anyway). I don't really agree with the idea that all you need to use a gun is point the barrel towards the other guy (it's like saying that all you need to know to use a sword is pointy end towards enemy). However, early guns were horribly inaccurate and massed fire was really the only effective way to use them beyond point blank range. Drop high crit. And the shotgun just does way too much damage. Really, 3d8 per attack and only have to reload after 10 shots? Likewise with the gunblade. Try to keep game balance in mind with these. Guns don't "need" to do more damage than every other weapon to be interesting, useful, and believable.
The damage seems really high, especially for ranged weapons and high crit on top of that. I'd tone it down a little bit. Remember that anyone can pick up those weapons and use them minus the prof bonus so requiring extra feats to be proficient doesn't necessarily balance extra damage.
I thought it was high. But I figured with a slower reload mechanic it would balance out. Upon further contemplation however, I realized that a PC could just as easily drop it after getting the high damage attack and not reload at all, and nothing prevents the PC from doing that still, but damage is adjusted to account for it.
Also, what are you trying to model? Early firearms that didn't have that much armor penetration ability (hence the term "bulletproof")? If so the piercing mechanic works fairly well. If you want to represent later firearms that made armor obsolete, then target reflex instead, but then you have a balance issue with a high damage weapon that targets a NAD and has a proficiency bonus.
You could target reflex and drop the prof bonus altogether (your proficiency feat are already granting other abilities anyway). I don't really agree with the idea that all you need to use a gun is point the barrel towards the other guy (it's like saying that all you need to know to use a sword is pointy end towards enemy). However, early guns were horribly inaccurate and massed fire was really the only effective way to use them beyond point blank range. Drop high crit.
Agreed; dropping high crit on all but the shotgun, which is less damage now. I started to think it was a little weird to see it applied to everything, and I think the higher damages and being an attack versus Reflex Defense will handle this well enough. Thus, piercing mechanic is out for now. It was more trouble than its worth when we have Dex vs Ref.
And the shotgun just does way too much damage. Really, 3d8 per attack and only have to reload after 10 shots? Likewise with the gunblade. Try to keep game balance in mind with these. Guns don't "need" to do more damage than every other weapon to be interesting, useful, and believable.
Agreed with taking down shotgun damage, even though you mistook the 10 ammo capacity from the semi-auto pistol for that of the shotgun. I reformatted it to make it more readable (no more underlines to separate the adjacent lines..). Cuz no way does a shotgun get multiple shots before reload in my game. These are WWI- not modern-era firearms, so one MAYBE two shots (maybe I will implement a double barrel later, but still, one shot thats it, then reload)
For the damage on the gunblade, I decided to replace the gunblade's gun with a derringer to make it more balanced, thus requiring a reload after each shot. Course then I might want to make it a two attack roll deal, since I made it Dex vs Ref for the gun portion, and make the blade hit itself being the trigger for the added shot.
With my rules, the real reason why someone would want to become proficient is that some guns (especially muskets) require multiple standard actions to load, which become minor actions when proficient. I gave the blunderbuss a burst, but not the shotgun: by itself, it's just a smooth bore rifle, it's the shot ammo that gives it the spread (which I represented by a bonus to attack but penalty to damage when using that ammo). A shotgun loaded with slugs wouldn't have any spread. But for fantastic shotguns, a burst works just fine.
I took out the spread on the shottie as suggested (forgot about the ammo being the main reason for a spread effect), so I will need to add ammunition types for it (slug, buckshot). The research I did on the blunderbuss, suggested it was wider barrels to enable faster reloads, not to spread its effect. I basically took it and muzzleloaders out as weapons. If there are multiple actions to reload a more primitive firearm, then I took it out. Adding super long reloads for weapons like this would make the basic firearms more likely to be dropped after one shot, and not used very often. I want em reachable, but not automatically usable.
Redispersed some of the firearm feats, and added requirement to take firearms feats that you are proficienct with Simple Ranged Weapons already. So that someone like a fighter or even a paladin could more easily take the firearms feats, than say a wizard with only dagger and quarterstaff proficiency. Also I planned on having some reload penalties already, but didnt include them in the first post. Added that info as a short blurb to at least clear that up.
Took out Initiate training for simple firearms, youre basically an initiate at almost any point, but get very little for it, and it is largely point and shoot to use a gun (I could use one even though I have shot one like a dozen times ever, with no training). Added a penalty for having no Simple Ranged Proficiency (as is the case for Wizards).
Also changed the gaining of faster reloads to be opened up upon reaching Mastery level; there is no Rapid Reload in 4e..so I think I will make it tougher to get faster reloads.
Added requirement for the Crackshot, Imbued Shot, and Debilitation feats that you cannot use them with weapons you are not proficient with, so even though you have Simple Firearms Initiate feat, you cant perform crackshots with a revolver you arent proficient with. Added Calculated Shot as a feat, for would-be snipers.
Added a Superior Firearms Proficiency (Gunblade) feat, and took out the proficiency granted with that weapon from the other feats. I want this to be a superior weapon, accessible after becoming proficient with some form of military firearms and the bastard sword. (Its bascially a bastard sword after all)
You can see my firearms rules here.
I have done something similar in my setting, but I made guns into just another ranged weaopn. This way you don't have to worry about special feats to use them. I made reload times almost negligable, otherwise players will just fire it once and drop the weapon. Most important thing I did for my firearms was to say "damn the reality".
You can see my firearms rules here.
I rather like them, a fine "damn the reality" rule set, which as you can see I am not doing here. Still, curiosity has gotten the best of me: What's the effect of the Brutal property?
Going off the top of my head, Brutal weapons reroll their dice damage if they roll the indicated value or lower. Most mundane brutal weapons are only Brutal 1. I think the property first showed up in the Adventurer's Vault.
Ah, yes I remember reading the property (I think it was Brutal) now in one of the manuals. Makes sense too.
Just keep in mind that balancing damage/properties with reload times doesn't work well. If it takes too long to relad a gun, like a stadard action, they will become one shot fire and forget items at best, ignored for bows and crossbows at worst.
For the damage on the gunblade, I decided to replace the gunblade's gun with a derringer to make it more balanced, thus requiring a reload after each shot. Course then I might want to make it a two attack roll deal, since I made it Dex vs Ref for the gun portion, and make the blade hit itself being the trigger for the added shot.
I would make the gunblade something that can normally be used as either a gun or a sword, but not both at the same time. To do that, you need to take a power, which would be available through the multiclass feats. For that power, I would have a normal attack and damage line for the sword. If the sword hits, you then get to make a secondary attack with the gun. So kind of like you describe, but I just suggest fleshing it out in a power the character has to swap for, not just something that comes automatically with the weapon.
I would make the gunblade something that can normally be used as either a gun or a sword, but not both at the same time. To do that, you need to take a power, which would be available through the multiclass feats. For that power, I would have a normal attack and damage line for the sword. If the sword hits, you then get to make a secondary attack with the gun. So kind of like you describe, but I just suggest fleshing it out in a power the character has to swap for, not just something that comes automatically with the weapon.
Reworded that line on the Gunblade itself. That is, you need the Gunbladist feat, not just proficiency with it to use it in that way. Its already a superior weapon which takes up a feat, but the extra feat gives you the added power of a dual attack which would replace one of those on your list.
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