Berelli sent an email they are having a sale for $1599 for the complete gun.  

It seems as If I could get a 300 blackout conversion kit if I wanted to go to 300BO right?

Seems like lots of 5.56 hit the used market - probably due to lack of suppression options and incredible blast - Its a thrill ride gun, not really efficient/practical for me.  I've tried one.  Yes loud, no need for flash bangs.

That's a great price to get into the platform.

You can get different barrels later.

However if you only ever plan to use the rattler for 300blk then your paying over two grand again at that point after getting the extra barrel.

I'd rather get the Rattler Lt with Ambi controls, lighter weight, better handguard compatibility, and a 6.75" barrel if you can save up a little more.

barrels are much more expensive than AR barrels, so price that out before you jump.

I love mine in 300. It's a powerful and portable PDW and serves as HD and a travel gun for me. It was expensive and it's loud as hell without a can (still loud with one!) but does a great job in the roles I've designated for it.

The ”Virtus” 5.56 Rattler has a longer handguard than the .300BLK version as well, so along with a conversion barrel you’d need a .300BLK handguard to go with it.

In the end, it would probably end up being near the same cost to just pick up a new Rattler LT in 300BLK.

I just looked on their website.  It's not even an SBR and doesn't come with a stock.  That would be a heck of a lot of money to spend to have to still buy the back half and front half of the rifle to get it where you want it to be.