With a bit of research I'm sure you could name a rifle chambered in almost any normal handgun caliber....
As I understand, it is not a straw purchase if the recipient of the handgun can legally own it. That would make the girlfriends purchase and transfer to you legal as long as you can legally own a handgun at 19. (I'm not going to try to look up the Indiana law.) This being the case your 21 year old girlfrind can also buy a Glock 19 or anything else through a FFL and legally transfer it to you.
If it worked once, why complicate things for the next purchase?
As I understand, it is not a straw purchase if the recipient of the handgun can legally own it. That would make the girlfriends purchase and transfer to you legal as long as you can legally own a handgun at 19. (I'm not going to try to look up the Indiana law.) This being the case your 21 year old girlfrind can also buy a Glock 19 or anything else through a FFL and legally transfer it to you.
If it worked once, why complicate things for the next purchase?