A Ransom Rest is designed to measure the innate accuracy of a particular weapon by removing the human element. Each gun requires a different insert, and for some guns, you may not find one.
A Ransom Rest works as designed if the frame is steel; but can't be used with a polymer-framed gun, as those frames won't necessarily return to exactly the same position with each shot.
For aimed shots, like from sand bag rest, using the sights, it doesn't matter. if the gun is steel- or polymer-framed.
Shooting from a (bag) rest is a less complex (and expensive) way of REDUCING the human variable as much as possible, and it's most useful with new guns, in getting them sighted in -- or in evaluating repairs after some big problem, a new barrel, etc.
You can pick up a mid-sized (but cheap) plastic bag of rice, leave it in the plastic store bag from the store, and take it with you to the range. Rest your hands on the bag and slowly squeeze off your shots.
Once you get the gun sighted in, you take the bag home, and you or your spouse uses it fix some future meals, and you work on improving your skills, shooting offhand in your next range trips.