What does “featureless” mean? I don’t know much about firearms.
Feinstein and others in 1994 passed the federal "Assault Weapons Ban" which outlawed the sale of rifles based on "features". The features included a pistol grip, a bayonet lug, a flash-suppressor, and maybe a couple more items.
The idea was that so-called "assault weapons" were dangerous rifles that were only useful in combat and one could identify such dangerous weapons by noting the features. The 1994 ban (which expired in 2004 and was not renewed) banned the sale of any semi-automatic, magazine fed rifle which had more than one such feature. Their thinking was evidently that a pistol grip when paired with a flash suppressor is ever so much more deadly than just the pistol grip alone. (Or the flash suppressor alone.)
California, always willing to take the lead in implementing progressive nonsense, decided to go even further, outlawing (with grandfathering) of rifles that had even one of a similar set of dangerous features; that is, a pistol grip, a forward pistol grip, an adjustable stock, a flash-suppressor, or maybe some other stuff.
Somewhere along the line somebody must have realized that we don't have many mass bayonetings, so the bayonet lug, I think, wasn't included.
California realized that this definition of an "assault weapon" was inadequate so they included a long list of firearms by manufacturer and model names.
All existing so-called "assault weapons" would be grandfathered in by registering them with the state but after the cutoff date no more could be bought, sold, imported, or manufactured. (With possibly some exceptions for Hollywood, of course.)
Some of us opted to keep all the dangerous "features" by modifying our guns so as to eliminate the "detachable magazine" capability. The law required that a tool be required in order to remove a magazine, so the "bullet button" was invented which required the tip of a bullet or other small tool to release the magazine. Recently a new law was passed declaring that even those rifles with a bullet-button are "assault weapons". Registration has been allowed for those after jumping through some ridiculous hoops.
"Registered Assault Weapons" (RAWs) come with very restrictive laws regarding where they can be used, how they are transported, a ban on transfers, and other junk I just didn't want to deal with.
Others, like myself, saw where this was headed, so we invested in alternatives to the pistol grip, removed the flash suppressors, and installed non-adjustable stocks. That is what is now known as a "featureless" rifle. Because it has none of the defined "features", it can operate as a center-fire, magazine-fed rifle.
If you want to be a gun owner in California you need to learn all this crap. Don't rely on my description but refer to approximately 100 pages of California gun laws for definitive guidance.
No flash hiders, no pistol grip, and nothing else that CA has decreed “evil features” such as front grips and adjustable stocks.