So i realized that the flash hider (SF3P) i was planning on using for a chop/pin/weld is 2.84" long, is there any kind of spacer that can be included to get that extra smidge of length? (without having a negative effect on lockup, concentricity, etc) that way i can use a factory 13" barrel and not have to hack up a perfectly fine 16"

I was planning on having PMM do the work because i've seen a lot of positive reviews for their Scar work

Assuming those lengths do not include the overlap of the threads, you will need an extension ~3/4" in length (threads are ~.5" overlap).  I don't know of anyone that makes an extension that long that I would trust with a suppressor if that is your ultimate goal.  If you're not running it suppressed, any decent machine should be able to make an extension for you.

Stephen

YHM makes flash hiders that are slightly longer.  I didn't math out how much you need.  I have had a barrel cut and threaded a few tenths too short and this fixed the issue.

You will need that barrel shoulder for the 13 barrel correct? Maybe use that and stack some shims?