To Reproduce
Steps to reproduce the crash:

1. Shoot a biplane with a barett 50cal.
2. Sometimes this appears to crash dedicated servers.

Screenshots
N/A

Versions (please complete the following information):

• Forge: 14.23.5.2824
• Flan's Mod: 5.6 beta 5

Additional context
Crash report: https://paste.dimdev.org/uyahudejam.mccrash

Seems to be caused by a packet being received (by a new player?) while the mod is handling server packets, whilst it iterates on the keyset of the map.

Can you reproduce this crash only with FlansMod?
If not, it would be really helpful if you pinpoint the mods needed for this crash to happen. I am not sure but this looks a lot like a compatibility issue and as such it is very hard to fix it unless you know which mod makes FlansMod trouble
(You are also running a whole bunch of core transformers, i.e. a whole bunch of potential conflicts)

I can't really reproduce it at all. It happened once, randomly, across about 100 hours of server uptime. It's just that this is a crash, in your mod, that you could write defensively around quite easily, so I thought you'd like to know.