To put in perspective with Loki (he labels a few cases with a slightly different Max Rating vs AMW and Kosdon but the lengths are virtually the same)
The case you have that is 30.875" and is the Kosdon/AmW/Loki 6000n case, it is a 5 grain.
That case will house a CTI or AT 5-grain reload. The CTI reload is certified to use in the AMW case with the hub-cap and nozzle holder.
I have loaded a 4-grain reload in that same case with a CTI spacer, therefore you have many choices of 4g or 5g reloads for that case.

I made a 75-4800n case (that is a Loki EX case) for 4g reloads out of an old 6000 case that had a damaged end.
It holds the AT-M1297 reload (a fine one at that with a huge white flame), you can do a 3g reload with a CTI spacer
This is a Loki 8000n (or AMW7600) case with an AT-1850 6g reload. It took a special CTI spacer that was cut 1/2" shorter.
(I prefer using thrust washers forward and aft underneath the snap ring.
75mm CTI reloads are certified in the AMW snap ring case, with hubcap & nozzle holder.
75mm CTI reloads are certified in the AT RMS hardware
75mm AT reloads are certified in CTI hdw (provided you use a single-throat nozzle.)
ANY CONFIGURATION outside of the above will be considered EX and must be flown under EX rules.