Now is time to put everything together. How you do this is largely up to you.
Find the gun's center of gravity, and mount it at that point. This is so the servo doesn't have to waste energy trying to keep the gun level. IMPORTANT: when the servo has no power supplied to it, the gun should tilt freely, and when you move it with your hand it should not return to level - rather it should stay right where you let go. Spend a lot of time getting this balance just right, it is a major factor in the effectiveness of your finished sentry.
You want a strong, solid sentry. Vibration will cause a lot of issues with the camera later, if you don't suppress it now. I'm not going to teach you how to build well, but keep in mind that screws and bolts are better than nails, nails are better than zip ties, zip ties are better than staples, and even staples are better than duct tape.
You want it close to the gun, but the gun barrel can't ever move into the camera's view - it will see that as a target. Also, the webcam should be as solid as possible - vibration from the servo's and gun firing will mess it up.
For example, when bolting on the webcam, and when bolting on the servo motors, and when bolting on the gun.
Depending on the expected use for the sentry (airsoft? paintball? water jet? laser?), you may need to add a durable compartment to hold the electronics.
A better idea might be to mount the sentry to something solid and fixed, like a wall. With that said, surveyor's tripods are great for sentry guns - they are heavy, stable, and often adjustable too.
I have documented the entire build process of the Gladiator II sentry gun, including sketched designs and lots of pictures.
Here's a video tutorial on how to find your gun's center of gravity, for mounting purposes:
Here's a video tutorial for an older build of mine:
Another nicely documented build: http://newplans.net/RDB/Sentry%20Gun.pdf
Next step: Wiring
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