I have a Rugar #1 that is chambered for the .218 Bee. It has been rechambered for the .218 Maushburn [ spelling?] I fireform using cci 400 primers WW cases 13.0grs of IMR 4227 and a 37gr. Calhoon bullet. I get groups fireforming of around .765. Once I have my fire formed case I load using 15 gr. of IMR 4227 and a 37gr. Calhoon bullet and my groups open up to 1.750 to 2.00

Does anyone have any ideas of what might happen once the case has been fireformed to the new deminsion?

First you need to do a couple of things.
#1. chronograph your fireforming load- evidendly you barrel likes the vibrations of that velocity.
#2. chronograph your "improved" load. I have had 0 experience with the Bee, but on any other AI cartridge an increase of 5% in the powder charge over the max charge of the parent cartridge is around top end- your load is a 15% + increase.
#3 do a case runout of your fired, improved brass before and after going through you resizing die. Fired case should be less than .001, and the resized case .003 or less.
#4 Do a runout on your loaded ammo- bullet runout should be less than .005 or your groups will suffer. Good luck.

http://www.duanesguns.com

Shortrange ,
Duane is right . Make the checks he suggests .
You need to do some more experimentation with bullets and powders to find an improved load that your rifle will like . There is no guarantee that your rifle will shoot a near - maximum load accurately .
I have to drop 200 FPS off the top of my .243 AI / 55gn Nosler loads to get best accuracy .
I suggest you look for a load with minimum vertical dispersion , then play with seating depth to fine tune to a nice round 5 shot group .

my mashburn bee load is 16.2 grains of H4227 and a 40 gr nosler BT. This is in a martini cadet and is quite accurate. In a #1 you might be able to do a bit hotter.