I tested with both 1.16.4 and 1.16.5 because I had both versions installed for my client and server, unable to test 1.16.3.
Reloading worked perfectly fine in singleplayer (1.16.5).

Here's the error that pops up whenever I attempt to reload on a server (its the same for both versions I tested):
[19Mar2021 07:36:33.889] [Server thread/FATAL] [net.minecraft.util.concurrent.ThreadTaskExecutor/]: Error executing task on Server java.lang.ClassCastException: class [Ljava.lang.Object; cannot be cast to class [Lcom.korallkarlsson.matchlockguns.items.ShotItem; ([Ljava.lang.Object; is in module java.base of loader 'bootstrap'; [Lcom.korallkarlsson.matchlockguns.items.ShotItem; is in unnamed module of loader cpw.mods.modlauncher.TransformingClassLoader @39c96e48) at com.korallkarlsson.matchlockguns.items.itemStruct.GunProperties.getAcceptedBullets(GunProperties.java:236) ~[matchlockguns:1.0] at com.korallkarlsson.matchlockguns.items.NewGunItem.func_77659_a(NewGunItem.java:617) ~[matchlockguns:1.0] at net.minecraft.item.ItemStack.func_77957_a(ItemStack.java:217) ~[?:?] at net.minecraft.server.management.PlayerInteractionManager.func_187250_a(PlayerInteractionManager.java:287) ~[?:?] at net.minecraft.network.play.ServerPlayNetHandler.func_147346_a(ServerPlayNetHandler.java:982) ~[?:?] at net.minecraft.network.play.client.CPlayerTryUseItemPacket.func_148833_a(SourceFile:31) ~[?:?] at net.minecraft.network.play.client.CPlayerTryUseItemPacket.func_148833_a(SourceFile:9) ~[?:?] at net.minecraft.network.PacketThreadUtil.func_225383_a(SourceFile:21) ~[?:?] at net.minecraft.util.concurrent.TickDelayedTask.run(SourceFile:18) ~[?:?] at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213166_h(SourceFile:144) ~[?:?] at net.minecraft.util.concurrent.RecursiveEventLoop.func_213166_h(SourceFile:23) ~[?:?] at net.minecraft.server.MinecraftServer.func_213166_h(MinecraftServer.java:734) ~[?:?] at net.minecraft.server.MinecraftServer.func_213166_h(MinecraftServer.java:159) ~[?:?] at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213168_p(SourceFile:118) ~[?:?] at net.minecraft.server.MinecraftServer.func_213205_aW(MinecraftServer.java:717) ~[?:?] at net.minecraft.server.MinecraftServer.func_213168_p(MinecraftServer.java:711) ~[?:?] at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213160_bf(SourceFile:103) ~[?:?] at net.minecraft.server.MinecraftServer.func_213202_o(MinecraftServer.java:696) ~[?:?] at net.minecraft.server.MinecraftServer.func_240802_v_(MinecraftServer.java:646) ~[?:?] at net.minecraft.server.MinecraftServer.func_240783_a_(MinecraftServer.java:232) ~[?:?] at java.lang.Thread.run(Thread.java:834) [?:?]

This is my first time submitting an issue, sorry if I missed something.