Well, if you were left to die there, you'd bleed out and your stomach acid would leak out into your body. You'd probably feel pain in your stomach and if you left it as an open wound, then you'd most likely die in 15 - 30 minutes. Altogether, you'd feel pain, but the stomach acid probably wouldn't change the pain level. I mean getting a bullet in your stomach hurts anyways.
Dunno about the 15 - 30 minutes And after a while trust me, it would increase the pain level