On one of my visits to a mates house and me getting naked as per usual, as he likes to feel in control and watch me walk around doing as instructed for him, he asked me if I had ever fancied bdsm and maybe a few more guests at our meetings, I asked him to explain what he had in mind, he laughed and took my hand and led me through the hall , then into a side room that was very dark, he said he had been making his sex dungeon for a few weeks and wanted me to be the first to see it, then turned the lights on. Oh my word there was a big leather mattress like thing on the floor and all sorts of chains , whips and cuffs hanging on the walls, pride of place in the middle of the room was a device that up till then I had only ever seen on porn movies, at one end there was a gallows like thing with holes for a neck and wrists, then a padded bench with knee supports and leather restraint straps, behind that was a huge gadget with wheels , motor , piston etc with a massive dildo attached to it, he seemed very proud of it and explained that he had made it all himself but as yet not tried it out with a person , he asked if I would like to be the first test pilot so to speak, and that he also had a few mates that were interested in watching the beasts first test run as he had told them so much about it, thinking this was something for another day I agreed but not really knowing quite what I had agreed too. Great he said, whipped out his phone , sent a text and said , wonderful, the others will be here in about half an hour, lets get you in position. I was a bit shocked and realised that he had pre planned all this, but never the less decided to go along with it, and mounted the apparatus, he closed the gallows board over my wrists and neck then tightly strapped my legs into the knee supports , it was actually fairly comfortable as he had also put a big cushion under my chest, he then put a latex hood over my head with just a mouth opening. he then said that he would be back in a bit as he needed to change, on his return I asked him to take off my hood as it was a bit tight, he said sure and peeled it off . he was wearing a multitude of leather and chains a very sinister looking mask with a scrotum ring around his erect cock and balls, I was starting to get quite aroused although a bit nervous to, then his door bell sounded and he left to answer the door , after a short while he returned with his friends, and to my surprise there were three guys and one lady, none of them said a word , they just walked around having a good look at my nude body trapped in the weird contraption then took their seats , the lights dimmed and changed to red, the door closed and our host began doing things behind me, then put some very cold lube on my bum, I could feel the head of the dildo just touching my anus and hear the machines motor running it was fairly noisy and at this point some regret was entering my head, but I kept quiet as my mate gently massaged my buttocks and genitals to relax me and said are you ready? I just nodded, then the noise changed and I felt the dildo very slowly pushing into me just a bit and then back out , with every forward thrust it went in a bit further but still a very slow movement, it felt as if it was right in and then stopped , my mate or master shall we say, said , so far so good and explained to his guests that he had perfected the control to be limitlessly variable and that the led display even told him that at this point I had 8 inches of rubber dick in my ass, then the motor sound altered again and the huge dildo began to withdraw and re enter me at a slightly faster speed and was jolting me forwards as it reached its hilt 9 inches now as we were all told by the master who kept applying more lube until my bum hole was squelching with every stroke. The lady then said is that it or does it go faster? My master just replied, lets see shall we, and then started to increase the speed gradually, bloody hell the thing was soon pounding my ass so hard and fast that I shot my load on the bench, I was in heaven but also well aware of how helpless I was in a room with 5 people being fucked senseless by a huge dildo machine, then I heard my masters daunting words, That is half speed, watch this , and he turned the thing up to flat out, Jesus Christ the thing was going at me like a jack hammer, I pissed myself and screamed stop, master instantly turned it off but left it fully in me, I was sweating like pig and shuddering from empty orgasms. my master and his guests then gathered around me , touching me and looking at my bare shaved ass with 9 inches of massive black dildo rammed up it, one of the guys was running his finger around my stretched ring and said can we have the lights up as we would like to see close up that thing going in and out slowly if that was ok with me, I nodded again, my Master repositioned the dildo so that it was coming right out of my ass and then back in and they were all watching very closely and the lady was holding my cock and gently pulling on it, master then came around in front of me and shoved his cock in my mouth and began face fucking me whilst the others watched my anus being demolished, one of them had increased the speed and exclaimed , my god look he is just hanging open slack and gaping ! un beknown to me the other guys now had their cocks out and soon took turns in my mouth, while the lady carried on tugging my cock and slapping my ass cheeks, soon they all finished having ejaculated either in or on my face, master asked if I was ok and did I fancy a bigger shorter dildo experiment, I said yes through a mouth full of semen. after a bit of fiddling around he Had changed the dildo,s over, and the lady exclaimed, oh my god that is never going to fit up there, this made me very uneasy but being unable to see the size of it and unable to move I kept silent and let him continue showing of his new toy, the machine started again and I felt a very big object pressing against my by now almost numb hole, it seemed as if it would not go but then with a bit more pressure it popped into me and then out with an audible plopping noise, the faster it went the easier it became, I was by now almost delirious with a mixture of pain , shame, pleasure and yet more orgasms, I could hear the clicking of cameras and knew that they were taking close ups of my obliterated bum hole , soon master said I think that,s enough for one day and released me from the machine, and bought my clothes in for me to ashamedly dress in front of them all, I saw the big dildo still attached to the device and could not believe my eyes, it must have been 3.5 inches in diameter, like a big ball on a stick, they all thanked me, the lady gave me a big kiss tasting the cocktail of spunk on my lips, we all said our goodbyes and left, I have not been back since but feel sure I will as soon as my rectum recovers .