There's a few things that Essentials changes if you use the rules from their version of the Adventurer's Vault. Naturally, we'll find out most of them when it comes out:
1) All items are classified as Common, Uncommon, or Rare. This supposedly will also include every existing PHB and AV item, with nearly all of them being classified as Common or Uncommon.
2) A Common item will be found 50% of the time, and is an item that they say 'you apply a stat change to your character sheet, and forget about the item until you replace it'. This includes things like a Belt of Vigor that increases your healing surge, or other items with no daily effects or other triggered things. They sell at 20% base value, and large enough cities will sell them to people.
3) An Uncommon item will be found the other 50% of the time, and is meant for the more complex items that have daily effects, or the like. A Power Jewel, which lets you recharge an Encounter power once per day, would most likely fit into this class. Uncommon items, when sold back, sell at 50% of their value. Uncommon items cannot be purchased without DM permission--and usually purchasing or finding one of your choice is something that creates an adventure, rather than being routine.
4) A Rare item will be found once per tier per character--if lucky. These are a higher class of items, with stronger effects than what current items do--the rarity restriction is meant to overcome the fact that they're noticeably more powerful than another item of their same level, and a character would likely hold onto it for several levels before begrudgingly letting it go. When sold, a Rare item gives back 100% of its value. Rare items cannot, and should not, be bought in stores, and finding one is the culmination of an adventure, or a very lucky happenstance.
5) Daily magic item uses are out the window, as now you have Uncommon items being the items that would use them, and the DM now has control of how little--or how much--he wants to give the players that ability. This does, of course, nerf certain classes (Artificer) if this is used.
Well, Pathfinder uses largely the same ruleset as d20 Modern did, which while not the best modern gun system ever is far from the worst. Here's how to get the most out of it for a firefight.
To make anything like a realistic gunfight, people have to be concerned about being shot. This means that people should have few hit points, and that guns should do a lot of damage. Keep levels low and advancement slow. Consider doing something like the E6 variant rules that cap normal level advancement. Do NOT use the piddly official Pathfinder gun rules. Use ones like the d20 Modern rules Brian linked - 2d6 should be minimum ever for a gun. If you're at a reasonably low level, 2d12 from a .357 Magnum is terrifying. A crit there can one-shot most characters of levels 6 and under. You can also try the d20 variant vitality/wound system. Oh,and remove AoOs for using a gun in melee.
Pathfinder has the same cover and concealment rules that all d20 variants have. Learn them and use them, especially partial/improved cover. Same with the terrain, vision and light, conditions, etc. rules. You need to become really familiar with most of those kinds of sections and be applying them all the time, not on an exception basis. Heck, even if you're shooting someone in the same motel room, do you see ALL of them? Or is enough of them behind a bed, tv, etc. that at least a small cover bonus is merited? You should be using Perception an awful lot. One of the distinctive elements of firearm combat is how hard it is to see all your foes and everything that's going on. Exert these rules to the utmost - and when in doubt, give defensive bonuses. If cover and concealment bonuses are so piddly compared to to hit bonuses that no one cares, they won't take cover - make sure defensive stuff helps. It's OK for most shots to miss. Strongly consider not sharing much information with the PCs, like whether they hit a foe or not (let them have a Perception check, but don't just say "ah yes you hit and do 10 points of damage" or "he is definitely down and dead") - this forces them to be careful and double-check what's going on.
Best Answer
In Arms And Equipment Guide.
Self-Propelled Vehicle: By animating the wheels, steering mechanism, and other moving parts on a vehicle, it’s possible to build a vehicle that doesn’t require dray creatures. Any land vehicle of Large size or smaller can become self-propelled, gaining a speed of 60 feet. It still moves under the direction of the driver and retains its usual maneuverability. Unlike with a normal casting of the animate objects spell, the vehicle does not become a construct and does not gain the ability to fight on its own. Caster Level: 11th; Prerequisites: Craft Wondrous Item, animate objects; Market Price: 132,000 gp; Weigh --
If you want it faster.
Speedy Wheels: This pair of wooden wheels fits most wheeled vehicles, increasing the speed of one so equipped by 10 feet. The wheels must be placed on the same axle to be effective. Regardless of how many speedy wheelsa vehicle bears, its speed increases by only 10 feet. Caster Level: 3rd; Prerequisites: Craft Wondrous Item, expeditious retreat; Market Price: 100 gp; Weight: 20 lb. each.
When limited to Player Handbook only.
Your best bet is finding a spell that propels the vehicle such as a level one halfling sorcerer who sits in a small cart and spams Unseen Servant to push him around. When you get level 6 spells you get animate object, which would be your preferred method. Then you could use craft wondrous item to create the above self-propelled vehicle.