If you can keep your questions about your build in the same thread, it will be much easier to give appropriate answers.
What receiver extension do you have? What is the interior depth? What buffer do you have currently?
Back in the 80s, I got some Morgan dollars from the grocery store I worked at that some kids used to buy ice cream. I figure they got into their grandparent's stash of silver coins.
I have a FiveseveN as well as an AR57. I think it would be a great boost to the caliber if you could carry a pistol and carbine that shared the same magazines.
There is a new Ruger pistol in 5.7 x 28. It looks pretty cool and costs less than the Five seven.
https://ruger.com/products/ruger57/models.html
It would be cool if someone with a gun manufacturing company could covert it to a Carbone using the same magazines. @Robocop1051