A simple demonstration of how to implement guns in Bukkit.

ShootingMod.java

public class ExampleMod extends JavaPlugin implements Listener {
    final int ATTACK_REACH = 100; // meters
    
    @Override
    public void onEnable() {
        getServer().getPluginManager().registerEvents(this, this);
    }
    
    @EventHandler
    public void onInteract(PlayerInteractEvent evt) {
        final Player observer = evt.getPlayer();
        
        if(evt.getAction().equals(Action.RIGHT_CLICK_AIR))  {
            if(observer.getItemInHand().getType() == Material.IRON_HOE) {
                
                Location observerPos = observer.getEyeLocation();
                Vector3D observerDir = new Vector3D(observerPos.getDirection());

                Vector3D observerStart = new Vector3D(observerPos);
                Vector3D observerEnd = observerStart.add(observerDir.multiply(ATTACK_REACH));

                Player hit = null;

                // Get nearby entities
                for (Player target : observer.getWorld().getPlayers()) {
                    // Bounding box of the given player
                    Vector3D targetPos = new Vector3D(target.getLocation());
                    Vector3D minimum = targetPos.add(-0.5, 0, -0.5);
                    Vector3D maximum = targetPos.add(0.5, 1.67, 0.5);

                    if (target != observer && hasIntersection(observerStart, observerEnd, minimum, maximum)) {
                        if (hit == null || 
                                hit.getLocation().distanceSquared(observerPos) > 
                                target.getLocation().distanceSquared(observerPos)) {
                            
                            hit = target;
                        }
                    }
                }
                
                // Hit the closest player
                if (hit != null) {
                    hit.damage(5, observer);
                }
            }
        }
    }
    
     // Source:
    // [url]http://www.gamedev.net/topic/338987-aabb---line-segment-intersection-test/[/url]
    private boolean hasIntersection(Vector3D p1, Vector3D p2, Vector3D min, Vector3D max) {
        final double epsilon = 0.0001f;
         
        Vector3D d = p2.subtract(p1).multiply(0.5);
        Vector3D e = max.subtract(min).multiply(0.5);
        Vector3D c = p1.add(d).subtract(min.add(max).multiply(0.5));
        Vector3D ad = d.abs();
         
        if (Math.abs(c.x) > e.x + ad.x)
            return false;
        if (Math.abs(c.y) > e.y + ad.y)
            return false;
        if (Math.abs(c.z) > e.z + ad.z)
            return false;
         
        if (Math.abs(d.y * c.z - d.z * c.y) > e.y * ad.z + e.z * ad.y + epsilon)
            return false;
        if (Math.abs(d.z * c.x - d.x * c.z) > e.z * ad.x + e.x * ad.z + epsilon)
            return false;
        if (Math.abs(d.x * c.y - d.y * c.x) > e.x * ad.y + e.y * ad.x + epsilon)
            return false;
         
        return true;
    }
}

Vector3D.java

package com.comphenix.example;

/*
* Attack hidden players
*
* Copyright 2012 Kristian S. Stangeland (Comphenix)
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library. If not, see <http://www.gnu.org/licenses/>.
*/

import org.bukkit.Location;
import org.bukkit.util.Vector;

public class Vector3D {
    /**
     * Represents the null (0, 0, 0) origin.
     */
    public static final Vector3D ORIGIN = new Vector3D(0, 0, 0);

    // Use protected members, like Bukkit
    public final double x;
    public final double y;
    public final double z;

    /**
     * Construct an immutable 3D vector.
     */
    public Vector3D(double x, double y, double z) {
        this.x = x;
        this.y = y;
        this.z = z;
    }
    
    /**
     * Construct an immutable floating point 3D vector from a location object.
     * @param location - the location to copy.
     */
    public Vector3D(Location location) {
        this(location.toVector());
    }
    
    /**
     * Construct an immutable floating point 3D vector from a mutable Bukkit vector.
     * @param vector - the mutable real Bukkit vector to copy.
     */
    public Vector3D(Vector vector) {
        if (vector == null)
            throw new IllegalArgumentException("Vector cannot be NULL.");
        this.x = vector.getX();
        this.y = vector.getY();
        this.z = vector.getZ();
    }
    
    /**
     * Convert this instance to an equivalent real 3D vector.
     * @return Real 3D vector.
     */
    public Vector toVector() {
        return new Vector(x, y, z);
    }
    
    /**
     * Adds the current vector and a given position vector, producing a result vector.
     * @param other - the other vector.
     * @return The new result vector.
     */
    public Vector3D add(Vector3D other) {
        if (other == null)
            throw new IllegalArgumentException("other cannot be NULL");
        return new Vector3D(x + other.x, y + other.y, z + other.z);
    }
    
    /**
     * Adds the current vector and a given vector together, producing a result vector.
     * @param other - the other vector.
     * @return The new result vector.
     */
    public Vector3D add(double x, double y, double z) {
        return new Vector3D(this.x + x, this.y + y, this.z + z);
    }
    
    /**
     * Substracts the current vector and a given vector, producing a result position.
     * @param other - the other position.
     * @return The new result position.
     */
    public Vector3D subtract(Vector3D other) {
        if (other == null)
            throw new IllegalArgumentException("other cannot be NULL");
        return new Vector3D(x - other.x, y - other.y, z - other.z);
    }
    
    /**
     * Substracts the current vector and a given vector together, producing a result vector.
     * @param other - the other vector.
     * @return The new result vector.
     */
    public Vector3D subtract(double x, double y, double z) {
        return new Vector3D(this.x - x, this.y - y, this.z - z);
    }
    
    /**
     * Multiply each dimension in the current vector by the given factor.
     * @param factor - multiplier.
     * @return The new result.
     */
    public Vector3D multiply(int factor) {
        return new Vector3D(x * factor, y * factor, z * factor);
    }
    
    /**
     * Multiply each dimension in the current vector by the given factor.
     * @param factor - multiplier.
     * @return The new result.
     */
    public Vector3D multiply(double factor) {
        return new Vector3D(x * factor, y * factor, z * factor);
    }
    
    /**
     * Divide each dimension in the current vector by the given divisor.
     * @param divisor - the divisor.
     * @return The new result.
     */
    public Vector3D divide(int divisor) {
        if (divisor == 0)
            throw new IllegalArgumentException("Cannot divide by null.");
        return new Vector3D(x / divisor, y / divisor, z / divisor);
    }
    
    /**
     * Divide each dimension in the current vector by the given divisor.
     * @param divisor - the divisor.
     * @return The new result.
     */
    public Vector3D divide(double divisor) {
        if (divisor == 0)
            throw new IllegalArgumentException("Cannot divide by null.");
        return new Vector3D(x / divisor, y / divisor, z / divisor);
    }
    
    /**
     * Retrieve the absolute value of this vector.
     * @return The new result.
     */
    public Vector3D abs() {
        return new Vector3D(Math.abs(x), Math.abs(y), Math.abs(z));
    }
    
    @Override
    public String toString() {
        return String.format("[x: %s, y: %s, z: %s]", x, y, z);
    }
}