What's the best way to upgrade weapons from SL11-SL20 if you dont have grails? Is it worth leveling up non grail SRs to use as fodder? If so, to what level? I always have a lot of R fodder but a limited amount of SR.

When i get to SL20+ I can switch to SSRs as fodder, but i don't know what the best way to level up after you can't use exclusively Rs.