Geo Tech - welcome to the forum. We were all rookies once ourselves. Never let that stop you from asking questions. Or joining in. You have a rifle many are not aware of so all the information you want to share on it will be welcome.

Your rifle conversion to my understanding has a 1-28 twist so you can shoot shoot sabots just fine. Granted, they are legal to hunt with where ever you live. I would start my testing with something like a 300 grain Hornady XTP and use 80 grain of loose powder, or in your case two pellets. Perhaps some 250 grain Shockwaves might be another good bullet to test. They are normally very accurate in most rifles, and easy to find in local stores.

Pellets can go bad, like anything else. If the pellets are real old it can cause that "POP" that you described or the difference in sound. Another thing to be careful of .. Pyrodex Pellets need to be loaded a certain way. The way they come in their box, think of your hand as a crane. Put the pipe cleaner through the hole of the pellets, lift them, and swing over and deposit them down the barrel. There is a small amount of black powder on their base to aid in their ignition. Another thing that can cause pellets problems is when you swab the barrel you are getting things too wet down there. Make sure the swab you use is just damp. Swab both sides of the patch, and then I like at least two dry patches. I test the second patch to make sure it is dry... Then load as normal. When I store pellets after the season I vacuum seal them with my sealer machine. Then I know they are dry and not moisture in there. Also store them in the original container. Don't take then out unless you are going to use them.

Also you did not mention the primers. Some are cooler then others. With pellets I like the Winchester W209 primers. The Remington Kleenbore work great on loose powder but I have had a "PUFF" with them on pellets.

Hope this answered your questions and helps.