Picked up a Sig Rattler 5.5" in 5.56 in a trade this weekend and I need some help.
I've read so much about it and what I'm trying to do , that I'm now somewhat confused and unsure of exactly what I need.

I want to convert it to a 300blk out.
If I use the 5.5" 300blk out barrel , I will then need to find the 300blk out handguard as well , correct?

If I want to use a longer barrel, I would be better off sourcing a complete upper?
Will a complete upper from a Virtus work with the Rattler? Or am I stuck with utilizing the Rattler 5.5" uppers?

Will the new Rattler LT upper work with this lower? Or will I again run into OP rod replacement issues?

Ideally , I'd like to utilize a complete upper so I can use my 5.56 upper [original] for cheaper training with 5.56 . If possible and practical, otherwise I can skip that idea entirely.

If you want to run a suppressor, you would need a shorter handguard to go with a 5.5" 300 BLK barrel.  You can use a 6.75" Virtus barrel, but you need the longer op-rod.  You might have to buy a complete Virtus recoil spring assembly to get that.  I run a 6.75" barrel on my Rattler with the 5.5" 5.56 handguard.

The new Rattler LT upper or a Virtus upper will work with the older Rattler lower.

Thank you, I just confirmed this with Sig as well [the LT and Virtus 9" uppers]
Looks like I have an expensive decision to make.