Jagaaaaaan - TG/BodySwap/Skin Suit Change?
 

http://mangaseeonline.us/read-online...4-page-16.html

I have no idea what kind of TF this is given it just happened but basically guy turns into an octopus monster, swallows a girl and then wakes up nude in her body.

Re: Jagaaaaaan - TG/BodySwap/Skin Suit Change?
 

Parasitic transformation method really.

The way Jagaaaaaan works is that there are 'broken' humans that due to an evil implanted in them turn into monsters based on a weakness of theirs. The main character generates guns over his body as needed because he wants to kill bad guys, etc. I suspect (haven't read this in over a year) this character likely suffers from either body dysmorphia or jealousy over how the person is treated. Either way, expect the character to end up mutating worse next time. It's really a weird series.