Use a cap' for de-bounce AND then low-current transistor-couple it to the 555 input. The transistor will buffer any excess noise 
A Proud Member Of 'Team Spleen!' who play mainly at Gunman Airsoft, Tuddenham, Suffolk.
That could work better.
You used to be able to get low voltage transient suppressors http://www.maplin.co.uk/Module.aspx?ModuleNo=499
but I can see a buffer cct working better maybe just feed it into a gate chip of some sort is there a way an AND gate can be used so 2 inputs have to be present to get an out. Just typing as I try to think this out.
So if you have the trigger switch the 555 and the AND at the same time the pulse will only travel out the AND when both inputs are the same
state.
Or am I being illogical in my logic 
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Transient suppressors wouldn't solve the problem, he's having trouble with switch bounce. That's a mechanical problem caused by the contacts literally bouncing as the switch is closed.
( Please note, any explanations of terms I add are for the casual observer who doesn't fully understand what we're talking about 
)
Adding logic gates would be bigger devices than the microcontroller, and far less flexible.
This idea one extra chip 8 legs.
I know this shows a 16 pin NAND but a 8 must exist I think.
Also resistors are wanted here and there etc.
I am also just guessing here a bit as I don't play with AEG internals. Just going by Richard saying he needs 1 tenth of a second pulse for each pull of the trigger and he was getting switch bounce problems which gave multiple shots.
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Single gates do exist - 5-pin SOT23 packages.
However, what you've drawn wouldn't work with an N-channel MOSFET (Drain and Source the wrong way round). Plus, the switch bounce problem would still be there.
Yes that was P channel but just showing the idea I was wondering if the 2 components would counteract the bounce problem due to different switch time characteristics.
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P channel would usually be used on the high side (Gate needs to be -ve WRT Source). But n-channel is preferred for single MOSFET use because of lower RDSon and easier drive (Gate +ve WRT Source).
As I said just showing the idea not going into specifics it was just a quick paste job, I am just a bit of a jack of all trades so will bow to the experience of others on this. However some times simple ideas work in ways you don't expect them to.
In some guns space may not be a problem the good thing with electronics over mechanical is the ability to have it remote from the mechanism besides less wear and tear than a mech.
Loads of people have told me to get into PIC's over the years but I just have too many other things on the go, however you may be able to help me out with a project I have on hold for the moment
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That's kind of the thing. PICs and digital control is simpler to me than messing about with analogue stuff like 555s, discrete debouncing, et al. 
I saw a servo reverser that had been implemented using a dual monostable. Rather than messing about with that, I reproduced the basic idea in a PIC.
however you may be able to help me out with a project I have on hold for the moment
Intriguing. PM/email me if you want to discuss it.
Well that circuit would stop the triggering on the switch being released, which would be a bonus for sure.