The Ultimates and Dark Avengers takes are willing to use them and Clint has shown a willingness to use other weapons (Ronin) and powers (Goliath). 
 
Why hasn't the regular Hawkeye, in book (not the meta theme reasons), adopted firearms more into his arsenal?  They're more reliable, compact, and just as versatile in the Marvel Universe and it is very difficult to make a credible argument that bows are superior... so why the self-handicapping?  Iron Man has updated with Extremis, Cap has- alternatively a cybernetic arm or an energy shield, etc.  Generally characters progress with technology and, if based in something archaic, there's a reason for it... so what's Clint's?

Perhaps he sees no benefit in adapting them? Hawkeye seems to be able to fire at the speed of almost a semi-automatic gun, sometimes even multiple arrows at once. He has an unlimited number of weird arrows for all kinds of situations, which he also very rarely runs out of. With bows Hawkeye never misses his target (and it is not certain that the same would be true with firearms). Occassionally, we would see Hawkeye use his arrows in 'non-traditional' ways (such as throwing them, or as melee weapon), again it is not sure if he could do them same with firearms. 
  
Of course, in 'our' universe this arguments are all rubbish (since neither bows nor arrows work that way), but for Hawkeye the bow has always done everything a firearm could do ... so why change?  

"Perhaps he sees no benefit in adapting them? Hawkeye seems to be able to fire at the speed of almost a semi-automatic gun, sometimes even multiple arrows at once. He has an unlimited number of weird arrows for all kinds of situations, which he also very rarely runs out of. With bows Hawkeye never misses his target (and it is not certain that the same would be true with firearms). Occassionally, we would see Hawkeye use his arrows in 'non-traditional' ways (such as throwing them, or as melee weapon), again it is not sure if he could do them same with firearms.   Of course, in 'our' universe this arguments are all rubbish (since neither bows nor arrows work that way), but for Hawkeye the bow has always done everything a firearm could do ... so why change?   "

I like the "he may not be as good with guns' idea. Also unlike guns arrows are silent, so if you are going to sneak into the villains base and take out his army without being detected arrows would be a good idea.