Tell them this...
Show me a radio that can actually hear 300% modulation - and I'll tune yours to it.
He may balk at the idea - look at him squarely between the eyes and tell him to make his claim provable you need a radio that "hears" the 300% modulation - not just some of the modulation - but all 300% of it.
Because it'll be distorted in the AM receiver - period - you can get some signal from low level carrier and even an IF image but not from sheer modulation - that is Dual Sideband Suppressed Carrier endeavor - and you need a carrier power signal dead power, weakly present or modulated - to even hear it without sounding like you're on sideband. or even FM - that's been a debate for years.
Now if he says I can hear them clearly - then tell him that "modulation" requires a carrier to decode that information in another receiver - whether that receiver makes it (BFO) or not (AM Detector).
Now, what you want is power envelope to develop 300% power in modulation ENVELOPE - meaning that the radio in question requires power above and beyond that which is available naturally in the radio. Means that the carrier will have to supplant the power in envelope to offset the modulation embedded in it else all you hear again is clipping in any typical AM receiver. There is simply not enough room or power that a typical CB can provide in AM carrier without further modification and additional power applied to make the 300% modulation possible.
Tell them, the typical 4:1 ratio of peak to carrier is needed to obtain and maintain all information - so the 300% requires that "4" to becomes 64 (4 cubed the 100% treated as a level of increase 4^3)
Now, in another thread, you can obtain 150% modulation easily enough by simply getting rid of the "mid point" carrier restriction and use supply rail power on the Collectors (or across Source to Drain) and let "mid point" be the actual 1/2 supply rail and by doing so, allows you to avoid the downward clipping distortion present in any other typical AM regulated radio. But in using this method, you have to operate components (A.K.A. - Solid State) that can tolerate the input power to the base / Gate as well has the Breakdown limits established by junction or construction (PN or Gate Substrate Oxide) and the power supply requirements - so unless he owns a vehicle with a 72 volt battery he can tap off of - forget the idea...
So yes, it's possible - but it'll cost them dearly.
