I have found an easier way to make a 3.5 extension for the Pardner shotgun.
Choate makes a 7 shot extension for the Remington 870.
Get a Choate extension from Choate Machine & Tool, Inc.
Take a pipe; put it through the open end hit the pipe end the front end cap will come off.
Place tube on shotgun, mark the extension where the cut is to happen flush with the barrel.
Remove it and use a tube cutter, on the extension tube.
A little cold blue on the fresh tube cut. Take end cap and tap it back on the end cap with mag nut at the opposite end & your done! A new spring and hardware comes with the kit.
P.S. The Cost is about $45.00 for the tube, $6.00 to $10.00 for the cold blue. $50.00 to $55.00 Dollars total if you have a tube cutter.
MAX 100 seem to of dropped off the earth! If you want that extension this is the way to go.