GEMTECH offers a neat option on their suppressors called the "quickmount".

Two good things: 1. You can use the same can on several guns. 2. You're not wearing out threads taking cans on and off.

One bad thing: The system is like a bolt action rifle. slip the can on, turn to lock. Big spring in can maintains pressure. Like a bolt gun with a blown primer, some amount of gas and crud escape between the can and the adapter coming back at you.

So what to do ?

First I tried a hardware store washer, 5/8" ID. It slightly canted the can causing poor grouping. There were blowback imprints on the washer, showing the leakage.

Found a company that makes precise ground washers used to shim bearing in machine tools. Ordered some in 2 thicknesses with a 5/8" ID to fit the barrel.

Now have to wait for the hysteria over the Chinese virus abates and ranges reopen. (100,000 died of flu in 1957, country was not shut down.)