I think if you are going to try checkering, you should invest a few bucks in checkering tools.
These are special files which come in several different spacings (lines per inch).
The ones I have were made by Dem-Bart and although you can buy just one handle and several different file tips, I would recommend getting a seperate handle for each tip.
If you are going to buy some of these, I would recommend a single V (#1). This cutter is used to layout the first line and to clean up small areas where the double cutters are too large to use.
You will also need to get a spacing cutter (#3 left hand, or #4 right hand) or better yet, both.
This spacing cutter has one V shaped file with another toothless V for following the last cut and forming the next line or cut.
The #2 cutter is a double V which cuts with both blades and is used for deepening the original cuts.
To achieve a close approximation of the English style just don't cut the depth down as deeply as is done for the modern diamond point style.
Also, I would suggest using tools with the wider spacing such as the 16 or 20 lines per inch.
Although there are "beading" cutters, I don't think they are needed for the old fashioned checkering found on the old guns.
From what I've found, the checkering on the Penn/Kentucky rifles was done for decoration, not to improve one's grip so it rarely had pointed diamonds.
Here's an example of my attempt to checker a Late Lancaster using Fordneys pattern:
Although it isn't necessarly PC, I've found these little tools can also be used to create backgrounds as in the center of this carving:
As for the Rigby, properly fitting the Bar Lock looks like it would be the challanging part of the job.