AD&D rules can be used to run games in the present day, or have people/items incorporated into an AD&D campaign, this is possible due to Spelljammer and Planar travel. Note that while sci-fi weapons are terribly destructive, most of them can't harm creature only hurt by magical weapons! The exceptions to this are explosive and energy weapons (laser, phaser and plasma and even then it's at the Dm's discretion), but explosives won't effect non-corporeal creatures. Other problems exist such as sounds causing cave-ins and that when someone caryring lots of ammunition is hit by a Fireball, there ammo may explode if they fail their saving throw...

  These tables have items, weapons and such divided into categories and prices. Prices are only approximate, especially since it's dealing withthe future! DMs should rule on types of weapons: exact ammo capacity and cost, the better they are, the more expensive they become.

  When using firearms, they often have two modes of fire, single shot and full automatic. Full automatic weapons have a tremendous recoil, this imposes a -4 attack penalty. And firing "from the hip", fast shooting generally used in close quarter fighting, inflicts a further -4 hit penalty, shooting is always considered "from the hip" unless the characters are at ranges greater than 20 yards, or if they state, or have time to aim properly. So someone firing full auto without taking time to aim gets a -8 penalty to hit! But the amount of bullets fired tends to guarantee a hit...

  Note that conventional armour (and thus most creature's armour class due to thick hides, except dragons, bulettes and similar with truly dense hides) is of no use against these inherently armour piercing weapons, and they are considered AC 10. However, Dexterity, cover, and magic all apply as normal.

  Reloading a weapon takes 1 round.

FUTURE WEAPONS

Range

Rate of Fire

Full Auto ROF

Magazine

$500,000 for "super" version

ARMOUR

Future body armours are designed to resist armour piercing attacks such as bullets, and to disperse the energy, reducing damage inflicted, but damage cannot be lowered beyond 1 hp. It cannot be repaired without access to a special workshop, it must be replaced after sustaining damage (i.e. if a character is injured more than once in a battle he must get new armour, or it's abilities will decrease at the DM's discretion)
Ablative armour is specifically designed to deal with laser and phaser weapons, partially reflecting or redirecting the attack, abalative armour costs double, but it increases AC versus lasers and phasers by +4.

CYBERNETICS

  These are bio-mechanical improvements to the person's body. Each improvement has a certain rating, major improvements can increase that person's Strength, Dexterity, Constitution and Intelligence by boosting the body in many ways, and adding increased mental processing. These proceedures takes 1 week to recover from per level of the improvement. Only some of the miriad enhancements are detailed. Note that Characters must have an original Strength of 10 and Constitution of 13 or better to have cybernetics fitted legally, below that there is a danger of death by over taxing the body (in other words the character must roll a Constituion check or die!)
   However, there is a serious side effect of all this: cyber-psychosis. Due to changes in the nervous system, and personality as they are now "super human", the character loses either 1 point of Wisdom or Charisma per level of the improvement! (Random chance of either). Whenever a person's Wisdom or Charisma gets to 5 or lower, they become cyber-psychotic, and generally become homicidal. Such characters then become NPCs under control of the DM. Cyber-psychosis can be overcome by spending 1 day per month in cybertherapy, this restores the character's Wisdom and Charisma to their previous levels. It is not a permanent cure, and statistics will lower by a random amount (1d4) per month not attended at the clinic, there are generally laws enforcing treatment in most cultures.