12rps is about average for a stock AEG with an 8.4V NiMh battery. You'll get 15-ish from a 9.6V NiMh. IMO 3 round bursts at 15rps are sufficient for most situations. 20rps is excessive but fun. More than that is asking for trouble from your gearbox and will ultimately cost a fair chunk either in replacing knackered bits or buying upgrade parts that can hack it from the get go.
People often say that running a 7.4V LiPo increases ROF over an 8.4V NiMh, but this is not necessarily the case. Put simply more voltage makes the motor spin faster and more current produces more torque. But when people upgrade to LiPo batteries, they also often upgrade the wiring and swap to Deans connectors. Just doing that increases the ROF from NiMh batteries noticeably and, at least in the only build I have tested this, my VP Vapex 8.4V 1600mAh NiMh batteries produce a faster ROF than the Component Shop 7.4V 5000mAh 30C LiPo I bought as an upgrade. However, I then upgraded to SHS 16:1 gears and this was enough to bring the ROF with the LiPo up to about what it was with the NiMh's and the stock 18:1 gears, which I would guess at about 14-5rps but with a significantly faster trigger response for the 1st shot.
The reason it works like that is because the chemistry of LiPo's ramps up from outputting 0mA to whatever they can deliver as a maximum much faster than an NiMh, but initially the motor requires a higher spike of current (mA) to get going than it needs once it is moving and LiPo's can deliver much more current as well as doing it faster (for eg I remember reading somewhere that, given low resistance wiring.and a battery that can produce it, a high torque motor can draw an initial spike of 67A, not mA, Amps - that's half again as much as an electric cooker with everything turned on!), so while the NiMh is still struggling to output 10A, a 25C LiPo has already delivered 40A.
If you stick an 11.1V LiPo into a stock gun (and it works) you'll get 18-ish rps, which feels fast. It's not too fast to be unable to control burst fire just by how long you hold the trigger however and the same goes for the couple or three rps more you'll get from upgraded wiring, Deans connectors, and a MOSFET. Beyond about 22rps you can't get without combining high speed gears with higher voltage batteries and/or high speed motors. You need to think carefully what you want to do with your AEG though - if you don't mind stripping it down frequently then you may enjoy the really high speeds like 25-30+rps, but even if you follow somebody else's tried and tested plan, it's still to a degree a case of trial and error. To get higher than 30rps you need to short stroke the piston (which only works with short barrels) and also choose the lightest piston and piston head (people drill holes in their pistons to shave a few grammes off), but these are ridiculous speeds...
As a simple example a convenient top speed for a running person is about 10m/s over a short distance, so if you fire 20 rounds in a second across an arc which is 10m wide at the distance your target is moving, that gives 1 round every 50cm. Theoretically our running person could slip between them, but in practice neither their nor your movement is completely smooth and even, and you correct your aim if the 1st or 2nd, etc miss - it's just not credible that you could miss unless the target uses surprising evasive tactics, or you're so shit at shooting that you may as well go home.
But ridiculous rates of fire can be used to intimidate people, especially in close quarters situations, and you can also suppress people and even make them just give up with a high volume of fire ricocheting around them, even though you can't actually hit them. Is it fair? I dunno. Is it fair that someone could turn up with a 2 grand tuned AEG and no clue whatsoever and still shoot the shit out of you from cover because they've got 10m of range on you and even an idiot can't miss when you need to run 1 second straight towards them just to have a chance to shoot them...?
A word on high torque motors: there's a popular theory going around which pairs 13:1 high speed gears with a high torque motor as the way forward for a high ROF. This is because the lower the gear ratio, the harder the motor has to work to pull the spring. But high torque motors spin more slowly than most stock ones. Bear in mind that many guns are designed to use M120 or M130 springs, which are a lot harder to pull than the M100 or M105's that we generally use. Also motors such as the ICS Turbo 3000 can be powerful and fast. IMO it isn't necessary to use a high torque motor in the UK.
Another way to increase your ROF is to fit a TBB, because this will increase your FPS, so you can fit a weaker spring to still get under 350 or 368FPS or whatever limit you're aiming for - I've just done it myself - M95 spring to get an AK with a 6.03x455mm barrel under 350FPS with the hop on. I also had to put some poxy shotgun spring (or something, I dunno it came in a job lot and basically it was the shittest spring I had) into a gearbox which has a Matrix double O-ring aluminium piston head and a plastic nozzle with an O-ring - because firing through a stock 6.08x455mm TM barrel was giving me 388-92FPS! I put an 11.1V LiPo on it for the lulz and skirmished it this weekend - this is with stock wiring, including a fuse, however and a TM motor, plus an ICS solid aluminium piston, so the ROF was about 20. It was a laugh and it lasted until about 15mins from the end of the day. I haven't even opened it up yet, but my guess is the pinion gear is chewed...
