Circles, angles, and edge splits

If your Edge Split modifier is set to 30° (by default), then you should never add circular shapes with less than 13 sides to them. Why? Because a 360-degree circle pided by 13 sides equals approximately 27.7° per side.

Any fewer sides and your angles are going to break the 30° mark and become sharp. So, 13 is the mathematical minimum. In practice, however, it's not an easy number to work with.

You can't cut it into halves or thirds, and each angle is a long decimal number (27.6923076...). This makes it hard to rotate something (such as a cut out) around a cylinder by "one side".

Instead, I suggest using a minimum of 16 sides per circle. It's easily pisible by factors of 2, and the angles work out nicely.

Next, we'll add a Shrinkwrap modifier to our circle object, and press it up against the gun. This is exactly the same process that we just used cut out the large circle.

Just like before, apply the Shrinkwrap modifier when you're done, and then join the circle object to the gun.

Using the same techniques from before, go ahead and build these small circles into the gun's body. As they're smaller than the other circle (and many areas are flat), you will be able to get away with using N-Gons, which will really speed things up.

Once this is done, we'll extrude the edges of these circles back. Unlike our larger circle from earlier, we won't take them all the way to the centre of the object. Instead, we'll pull them back just a little bit. Then, we'll select Inpidual Origins for our Pivot Point. Once this is selected, we'll scale all of the faces to 0 on the X axis so that they become perfectly flat. Choosing Inpidual Origins will make each face perfectly flat, but it won't force all of them to be at the same point on the X axis.

You will now have some good holes for screws/bolts to go in:

Starting with a circle, go ahead and build a quick bolt/screw:

You can also extrude some 2D shapes back to create it:

Then, move this into position. Duplicate the bolt/screw, and move the copies around until all of your holes are filled in:

Next, we'll make a few other cuts to our model. One technique is to start with a circle, and then rip the two edges along the center. You can then drag half of the circle forward or backward and fill it in. This gives you more of a rounded rectangle, which can look pretty good on sci-fi models:

We'll add a few other cuts as well in various shapes. Use your imagination here! One thing we want to do is leave a nice rounded rectangle cut-out to add a pipe in later:

Here's what this gun looks like with all the cuts/holes in place: