Dunno. Haven't had a cock in me yet, but I have ejaculated from prostate stimulation with toys up my ass. I have NOT had an orgasm from it, however, and those experiences are precisely how I learned the difference.
At least the handful of times I accomplished this, it was highly pleasurable to feel (and see!) cum shooting out of my cock, without a doubt. But I remained in the heightened state of pre-orgasm, and hadn't yet achieved that sense of completion and release that comes with even the smallest real orgasm.
Not saying it can't happen. It may be that guys with a lot more anal experience than me may have experienced a full orgasm from anal penetration. But I do have enough experience to know that orgasm and prostate milking are not necessarily the same thing.
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I have tried different size dildo's and even though some felt really really good, I did not achieve an orgasm. I have a prostate toy which, after a lot of practice, can give me a remarkable and longer orgasm. Several have been hands free. I havent tried pegging would welcome the opportunity or maybe one day the real thing.
I came every time my fwb put his stiff cock in my ass. I tried dildos and vibrators of various sizes and shapes. But, nothing could imitate the feeling of a live, throbbing cock moving in and out of my tight hole. He didn't have to touch my cock. I was hard almost from the instant he pushed against my sphincter and exploded within a couple minutes, usually before he did.