If you ordered the 6an fitting from NX, was the part #11702 and look like this (but yours may be balck anodized alum)
It sounds like you already had the bottle nut itself since you were using it with a 4an already? Since you say it's a Sniper bottle what you probably have is the 2 piece bottle valve nut which is some type of a metal to metal seal (no gasket needed) and does it look like the vavle on the right in these pictures?
If it looks like the pic on the right i don't think it's a medical bottle valve and you can use it to spray. The orifice in most of those type valves are pretty small though (ZEX uses this type of valve a lot because most of their kits are low HP kits). A couple of hundred HP on down you'd be ok IMO.