want larger pattern/pardner 20 ga.

[Gallahad]

besides cutting down the barrel, or threading it for chokes, is there a way to open up the pattern on an old pardner? like ream the barrel or something?

[SHOOTALL]

I have seen test where oval chocke were installed to spread patterns left right , the same was done making the bbl. also oval. If a open choke is not enough then go tight for 3 inchs or more in the tube. , it will rub or deform the pellets causing flyers which often open the pattern. The use of unplated shot often opens patterns also. But at some point if the pattern opens to far it looks like a doughnut .

[mcwoodduck]

There are some spreader wads that aid in spreading the shot.  It is a circle with a post that sits in your wad and the pressure from the wad pushes the shot out.  Most reloading catalongs that sell assorted wads will have them.
You are talking about shooting shot shell and not Buck correct?
Why do you want to widen the shot pattern?
At some point the 7/8 oz or 1oz shot will have holes in the pattern.  The pattern shows up as hits on a page but you have to remember that the pattern is 3 dementional and that the larger you spread out that shot the wider and longer that shot string is going to be and the more holes you are going to have in the pattern that could defeat your objective.

[woodsrunner]

Reamers are available to remove some or all of your existing choke--woodsrunner

[Ol BW]

Just curious, what choke do you have?  If it's full choke, we might have a trade brewing!