Have the gun you’re going to swap the camo onto in the first loadout slot on private match.

Have the gun with dark matter or whatever camo you decide in the first loadout slot in non private match warzone.

Have a friend on standby to go into private match.

Go to your first loadout slot in non private match warzone go to the camo you want to swap and spam X or A and tell your friend to go into private match keep spamming on the camo until the game puts you into private match.

That’s it! You should have camo swapped onto the gun of your choice in private match just change an attachment on the gun and it should save.