While the base sounds of the gun in RO2 are perfect (the actual detonation, as well as any bolting sounds), they lack any good reverberation. RO1 had the gun make one sound, combining reverberation and the actual detonation. This cheap method led to overly powerful gun sounds, while the low volume of reverberation in RO2 has led to overly weak gun sounds.
If you replace the RO2 gun sounds with RO1 however, it will end up sounding quite odd, as you'd have 2 different qualities and volumes of reverberation.
But the main thing for me is bullet cracks. TWI has upped the volume etc etc and we appreciate it. But the sounds aren't right. They need changing. Listen to this for example:
TvT Aliabad Region: Contact Front - YouTube (the sound mod used is JSRS, quite low quality but nails what I have heard in real life for many sounds)
That first bullet crack is a sound generated when the bullet reaches a plane spread out perpendicular to the players eye direction, each calibre has a different range of cracks, that change in volume due to their distance away from the player on said plane. A close bullet has a loud crack, a further away bullet has a lower echoing sound (closer to what we have in RO2 now). Each time a gun fires, you have the firing sound, environmental reverb and bullet crack. This makes firefights sound VERY epic.