In order to build a project such as a sentry gun, you’ll need a small gun you can strap to a pan and tilt mechanism. The problem with many sentry guns out there is that they use an unmodified gun and interact with its trigger. Ideally a modified electric gun would be better, since the electronics can interact with it directly. This is usually done using a switching device such as a mosfet or a relay. Where can we source an easy to modify airsoft gun?
Go the cheap way
If you’ve got an old gun that is still working and you don’t mind dismantling, the choice is obvious. Otherwise I recommend taking an airsoft toy gun to use its internals rather than buying a good $60 gearbox. By ‘toy gun’ I mean a clear plastic replica, with continuous shooting, working with standard AA batteries and with its own BB compartment. The kind you’d give to a 12 year old kid after a trip to Walmart. This will be cheap ($20-$30), and has built in features that will save you trouble in the long run.
You may be wondering if such a cheap gun will be up to the task. Well, it won’t shoot far. But even if it could, the rest of the sentry (pan & tilt mechanism, software, webcam…) would need lots of work and expensive components to be precise at long range.
As far as I know, toy guns are easily found locally in America, but not so much in Europe. In my case, I got one from a UK online retailer (BBGuns4Less). It costed about 18€ (currently at 14.18€), with a 15€ shipping rate. It was the cheapest I could find because sadly Asian retailers (DX, Banggood, eBay sellers…) don’t ship this kind of toy. Which is a pity, since they rarely charge shipping costs from Hong Kong.
The gun I got was the Defender of the World Mini Electric BB Gun, a replica with a futuristic look that would make you ‘that guy’ if you ever took it to a field. But don’t let the ugliness dissuade you. This gun packs a lot of features for any DIY project:
It’s got its own gravity operated BB container so you won’t have to design it yourself. According to the description, it holds 1500 BBs.
Full auto: if the motor is powered it will shoot continuously all the balls in the container (provided there is no jamming).
The clear blue plastic will let you cut the chassis precisely without breaking any union.
You can cut its handle and expose the trigger’s switch. Then, you can easily solder a mosfet or relay instead of the switch.
You can open the chassis and expose the internals. As long as you’re careful with the one or two springs that may jump away, afterwards it’s easy to assemble everything together again.
It works with 4 AA batteries providing 6 V. In series with a diode (with a voltage drop of 0.7 V) the voltage will be safe for operating the rest of the control system if you use, for example, an Arduino (bypassing the voltage regulator). Otherwise you may ditch the AA batteries and power it from your own external battery.
I don’t know the exact FPS of the gun. With the included BBs (look like 0.12g) it supposedly shoots at 115 FPS, but normally you’d use heavier, biodegradable BBs.
In conclusion, the toy gun may be bulkier than just a gearbox, but you save time with the feeding system, soldering the cables, attaching a cannon…
An example of real use
Well, ‘real use’ might be a bit of a stretch. Not exactly something you’d use in a match. But it does have many of the elements a sentry gun might have. Some time ago I found out that the guys at YouTube’s FliteTest channel have done the same modifications to this gun model and attached it to an RC plane: