I don't think there is such a thing, I searched myself and found 3 different groups, but they were all a scam, I was able to find out without having to buy. My desperation was very great.
The first one used the name of another store, I contacted them and they confirmed that it was a scam.
With the second one, I searched for group photos on Google and one of them appeared in a group selling weed on Facebook.
The third one did not respond to my messages.
But apart from all that, stop and think for a moment, does a group really exist (that openly says it sells weapons) and that is not monitored by the authorities? If they exist, why would they meet in person? They could be arrested, you could get beaten up and robbed or maybe you could get a gun but it wouldn't work (they won't let you try it I think)
I suggest caution with this, I don't know if you are in the USA, but there are probably more "classic" ways to obtain it on the black market or from a friend of a friend of a drug dealer. He only spoke from what I see in the movies.
I hope nothing happens to you.