It's a bit tough to do a complete tutorial on how to make a gun in a forum post
, but here's a rough outline of how I'd go about making one:
1) Decide on your polygon count. Will this be a low-poly model (i.e. for a game) or a high-poly model (i.e. for film or still renders)? How many polygons you want in the final model makes a difference in how you might go about modeling it.
2) Find as many reference images as you can and study the shapes. To make the modeling easier, you might also want to find or sketch some images you can use as a template. Even if you don't use it as a template, sketching is a good exercise because it helps you analyze the shape and decide what details you feel are important enough to model. You will want to find photographs or draw images of your subject from the side, top, and front if possible.
3) Set your sketches as a background image in AC3D by selecting "Set Background Image" from the "Orth" menu of the desired view.
4) The barrel of the mk.23 is pretty square. I'd probably start with a cube for the barrel and chamfer it by slicing it horizontally and pulling the vertices to form an angle.
5) The handle is more complicated. The easiest thing to do there is to trace over your template in the side view with the polygon tool. If you are making a high poly model, you can click Surface > Spline on the surface when you are done tracing to smooth the shape. If you're creating a low-poly model, then just trace it without smoothing.
6) After you've traced the basic shape of the handle, use the extrude tool to give the handle width. For a low-poly model, extrude just once. For a high-poly model, extrude by a small amount several times, slighting scaling down the extruded surface on each pass, to keep the roundness of the handle.
7) The muzzle and hammer can both be made from cylinders. On the hammer, pull on the vertices on the edge toward the gun to make the hammer more oblong.
8) The finger guard and the trigger can be made in a similar way as the handle. Trace over them in the side view, and extrude. Remember, they aren't as thick as the handle, so don't extrude them as far.
9) Ditto for the gun sights.
10) If you're making a low-poly model, you probably don't want to add more detail than this. Fine details like the accessory mounting groove and magazine release should be done in a normal or bump map, not in the geometry.
11) For a high-poly model, use the boolean tool (Object > Boolean) to carve additional detail into the model. You can use things like cylinders to make the accessory groove and other slots, and subtract them from the main model.
12) For other details like the safety lever, again, trace them from the side and extrude. Most of the objects on a gun are pretty flat, so while I wouldn't recommend this much extruding for most models, for the mk.25 you should get pretty good results.