The Marlin Model 20-A was made up to 1922, and the modern, high speed ammunition came out after that, so you should not use modern, high speed ammunition in the Model 20-A. Standard velocity or target ammunition would be okay, but using high speed ammunition can result in a cracked breech bolt, if it is not already. To be safe, you might consider taking it to a machine shop that has a magniflux inspection machine to see if there are cracks in the breech bolt.
Due to the increasing cost of ammunition, there be no warning shot!
The growing federal deficit = generational slavery to the national debt.