Anyone here have the Sig asp20 Piston air rifle ? I’d like to know if they are quiet? And accurate and well made!.???

One of my buddies just got one in .177.  It is as quiet as any other gas piston, which is not really noticeable unless you are listening for it.   His is in the same ballpark in terms of accuracy/ precision as my TX200, and his Diana 54 (the "recoiless" version of the 48/52).  I have not shot it yet, for I was doing other kinds of shooting at the time.  I did, however check my sling swivels in the side ports on the stock.  These do accept the standard push button type swivels.

Quiet?

Most of the sound from a spring airgun, be it a steel spring or gas spring, comes from the spring and piston itself, rather than the pop of air from the muzzle.  Gas springs tend to have more of a thump sound than the "sproing" of the steel spring.  Many tuners and tune kits include various contraptions to reduce the steel spring "sproing" and try to get more of a thump instead.  The thought is that this creates fewer harmonic vibrations that travel down the barrel before the pellet leaves.

Now for the perceived noise, gas springs are pretty quiet, but not quite as quiet as a shrouded and moderated PCP.