I have shimmed a couple of 91/30 M/N's and it really made them accurate. You can use metal but I used pieces of those plastic fake credit cards that you always get in the mail trying to get you to use their card. I placed a piece with a hole drilled in it under the front ring where the action screw screws into the action and one under the back action screw. This raises the action up enough to allow the barrel to float. I have found brass pieces under these two spots on Finn rifles. Also I took a piece of wool felt cloth 1 inch wide and long enough to wrap it around the barrel for a length of about 4 inches. I put some oil on the felt piece and wrapped it around the barrel about 2 inches back of where the barrel exits the forearm and then placed the top hand guard on. This will cushion the barrel and allow it to move during the harmonic movement of the barrel during firing but not allow it to come up hard against the wood. It also helps keep the barrel lined up squarely. This is how the Finn's did it also.
All you need to do then is slug your bore to see which bullet size it needs. Either a .311 or .312 diameter. If you slug measures .310 to .3105 between high spots left by the grooves use a .311 diameter bullet if it measures .3105 to .3115 use a .312 diameter bullet. If it measure larger than .3115 and you don't want to shoot cast bullets then get rid of it and get you another one. It will not shoot accurately with jacketed bullets.
Once you find your bullet diameter needed then get you some Accurate 4350 powder, and CCI 200 primers and load up using either .311 174 gr Sierra Match Kings or 180 gr SP COAL 3.000 or for .312 bullets use the Hornady 174 RNSP COAL 2.790 and use a Lee Factory Crimp Die on all loads. This is the most accurate load that I have found for all M/N's.
Good luck and good shooting.