Search youtube!
That helped me. They have vids of people firing every conceivable thing on there. Great sound
I'm not sure what little Sub-machine guns like that sound like, But I think they have a fairly high rate of fire, so it definitely wouldn't be the old "rat-tat-tat"... that's too slow. I think it's more like a fast roll on a snare drum so you might have to add at "rata-tata-tata-tata" sort of thing if you went that way.
Well, for a short burst you could go with Brrrap! That seems to be the one I see most associated with that sort of weapon. tik-tik-tik might be a good one for a continuing burst.
Listening to those vids of different sub machine guns, none of them make anything like a RRrrrrrp or Barrrp sound. The rate of fire isn't THAT fast. You can clearly hear the sound of the bolt slamming home each time, it doesn't blend into one sound.
You need to indicate the loudness as well as the fast, distinctive, multiple shots... So a run-on sound with lots of shap stops in it is best maybe, like the "atta-atta" thing I suggested. You could even have it go "Bitch-Attattattatta" Gives it a distinctive look
Or following that theme, anything that continues with a vowel sound and sharp cracking stops... "Reckeckecka!" "Akackacka!"
Hmm, T and K and the only sounds that can really do the cracking part...