Pull it back when it is soft and be well lubed, you should be able to get it in. I started doing this when I was a horny teen, and I have shot cum into my own ass many times. BTW: my cock is about 6 yo 7 inches fully erect, but I found if I start with it soft and get it back there, then when it starts to firm up, I can get it in. A slight grinding motion after it's in does the trick and it feels so good to have the feel of the orgasm pumping and feeling the cum pumping into my ass.
I've tried and been close
I have always wanted to do this. I tried a couple of time and was able to rub the tip on my hole but not get it in.
Get A Life
Get a boyfriend
I am 8" and can get about 3 of it in my ass and love to cum in myself. Do it all the time and if the wife is gone all day shopping I do it several times a day. Love fucking myself in the ass and always shoot a huge warm load up my ass. The bonus for me is That I can also suck my own cock and with my lips closed tightly around my thick shaft I cum a lot and enjoy swallowing my own cum and some times do this with my wifes small dildo in my ass.