Is a UBR stock legal in California.

IMO, on an otherwise featureless rifle, it should be okay, as it does not telescope.

The assault weapon laws refer to a "telescoping stock" not generic adjustable stocks.  Telescoping refers to one tube fitting inside another.  This should be confirmed by reading the DOJ definitions (which I've done but it was a while ago).

A stock is just a stock unless it creates an AW.  But until it's attached it's just a part.

Need many more details on the build to possibly determine legality.  Maybe.

We know it can be placed on the latest fixed mag AR design, the designs which replace the bullet button.

Is a Magpul extending UBR stock a telescoping stock, by CA definition?

Once this is resolved and the answer is no, then can it be place on a featureless build?

Even if it were legal to place on a featureless build, I would proceed with caution, and would not want to be the court's cuey.

UBR extending stock


Telescoping stock

"Stock, telescoping means a stock which is shortened or lengthened by allowing one
section to telescope into another portion. On AR-15 style firearms the buffer tube or
receiver extension acts as the fixed part of the stock on which the telescoping butt stock
slides or telescopes."

"Stock, folding, means a stock which is hinged in some fashion to the receiver to allow
the stock to be folded next to the receiver to reduce the overall length of the firearm. This
definition includes under folding and over folding stocks."

Here's some more that helps clarify the issue -

"Stock, fixed means a stock that does not move, fold, or telescope."

IMO, absent the function of telescoping one portion into the other or the function of folding, the stock constitutes a fixed stock with an adjustable butt plate.