Yikes!
Mmm- 400 rpm each gun? Twins? Synchronized firing? Forget tones as I'd say PULSOUT pin,duration will be your friend,organised in a tight loop & amplified to suit. The pulse duration (relating of course to the firing) is in 10µsec units, so ( as an example) PULSOUT 20 will give a 200 µs ( = 0.2ms) pulse. Since a typical 4MHz 08M loop (being the period between rounds) is only about 1 millisec you'll need a decent pause to reload the next round, although the H.E.S reading could provide that automatically. Of course 400 rounds per minute is ~7 per sec (thus ~140 ms between shots), meaning you have ample Picaxe time to get organised.
Is this all you'll be doing with a PICAXE on board your Fokker? Rather a waste! How about at least some pulsed LEDs in parallel to relate to the firing ? The overall effect could be so realistic that you'll probably need to provide trenches,tin hats & toilet paper (!) for the unwary too. Pseudo-code to try initially -
ww1:
(HES READADC routine about here)
pulsout 2,10 '100µs = .1ms at PICAXE pin2
pause 140 ' 140 ms pause if needed
goto ww1