I'm assuming that your Parker is chambered for 2 1/2" shells, choked tight and tighter and that's great if it works for you. Rocky Mountain Cartridge will make you lathe turned all brass hulls, specifically for your chamber size. Below is how I helped a friend open up his very tight patterns.
A friend bought a 1917 Parker VH in 16 GA and after measuring chokes determined that the gun was a tight mod and a tight full. Patterning with modern ammunition verified that the gun's chokes were not suitable for the type of bird hunting that he did. I have had great success with all brass shells and my friend decided to buy a minimum order of 10. We tried the following using shot with little or no antimony: Osc cards cut half way through and slid into each other forming a +. This worked well. Then we tried inserting a straw in the middle of the shot and this was not effective in either of the Parkers barrels. Then we placed 3 ea. osc cards to divide the shot vertically into 4ths.
This method worked, but not quite as consistent as the + method. We only shot 10 shots per each barrel, but felt that each spreader configuration was tested to give us an idea of spread effectiveness.
My best patterning loads are those assembled the old way with card and fiber wads, so we tried the same loads that I use except in a 2 1/2" hull.
I'm happy to report that the the gun opened up to a mid range mod and a tight mod, very close to full choke. Bottom line: if your chokes are too tight for your hunting applications, you can buy commercial spreader wads or try some of the above ways to open up your patterns.
