This isn't really a problem with this plugin but I wanted to provide a fix for an issue I had on my modded server in case anyone else experienced this.

Problem:
Stack Size Controller allowed MLRS Stack to be 2000 due to my ammunition stack size being set to 2000. A player put a single stack of 24 MLRS Rockets in the MLRS and it wouldn't fire and completely froze up the entity. A restart wouldn't fix this.

The fix:
Using the Build plugin and entity kill command, I removed and replaced the entire MLRS at the monument, then changed the stack size of the MLRS Rockets to 6 in Stack Size Controller and reloaded the plugin. This fix survived a restart as well.

i spawned the mlrs and it gives you the mount option but you cant get in, i had the same issue with stack size controller and tried to fix it too, why cant i mount it what are the spawn commands? cheers

I just tested this on my modded server. I had not problems putting a stack of 12 rockets in the MLRS. I right clicked a stack of 100 from my inventory but it only put 12 and I did not attempt to put an oversized stack in by dragging a huge stack manually into it. Then went in the MRLS and put in the aiming module thne fired w/o issue. I actually had two stacks of 12 in their and it did not fire the second. Tested it again after cool down and it fired the next 12. 

Merged post

I am not using hardstacks in my Json only category stack multipliers 3x.

"RevertStackSizesToVanillaOnUnload": false,
"AllowStackingItemsWithDurability": true,
"PreventStackingDifferentSkins": false,
"HidePrefixWithPluginNameInMessages": false,
"DisableDupeFixAndLeaveWeaponMagsAlone": false,
"GlobalStackMultiplier": 1.0,
"CategoryStackMultipliers": {
"Weapon": 1.0,
"Construction": 3.0,
"Items": 3.0,
"Resources": 3.0,
"Attire": 1.0,
"Tool": 1.0,
"Medical": 3.0,
"Food": 3.0,
"Ammunition": 3.0,
"Traps": 3.0,
"Misc": 3.0,
"All": 1.0,
"Common": 1.0,
"Component": 3.0,
"Search": 1.0,
"Favourite": 1.0,
"Electrical": 3.0,
"Fun": 3.0
},
"IndividualItemStackMultipliers": {},
"CategoryStackHardLimits": {
"Weapon": 0,
"Construction": 0,
"Items": 0,
"Resources": 0,
"Attire": 0,
"Tool": 0,
"Medical": 0,
"Food": 0,
"Ammunition": 0,
"Traps": 0,
"Misc": 0,
"All": 0,
"Common": 0,
"Component": 0,
"Search": 0,
"Favourite": 0,
"Electrical": 0,
"Fun": 0
},
"IndividualItemStackHardLimits": {},
"VersionNumber": {
"Major": 3,
"Minor": 4,
"Patch": 3
}
}