Can a Reduction of a Felony to a Misdemeanor Restore My Federal Gun Rights?

The federal prohibition on firearm possession is on anyone "who has been convicted" of a felony, 18 USC 922(g).

If the prior felony was a “wobbler”—that is, a crime that may be punished by either a felony or a misdemeanor—and you received felony probation, you must seek a reduction of your felony to a misdemeanor under California Penal Code 17(b).

A  17(b) makes a prior wobbler felony a misdemeanor "for all purposes," People v. Gilbreth (2007) 156 Cal. App. 4th 53, 57-58.

A "wobbler" becomes a misdemeanor by a judgment imposing something other than prison--but when probation is granted there is no judgment.  This is discussed in United States v. Viezcas-Soto (8th Cir. 2009) 562 F.3d 903:

In granting probation, California judges may either "suspend[] the imposition of sentence or ... impos[e] sentence and suspend[] the execution thereof." People v. Alotis, 60 Cal.2d 698, 36 Cal.Rptr. 443, 388 P.2d 675, 681 n. 3 (1964) (en banc) (citing Cal.Penal Code § 1203.1). In the former instance, no actual judgment is rendered within the meaning of section 17(b)(1). See id.; see also Gomez-Hernandez, 300 F.3d at 978. This is because under California law, neither an order granting probation nor a suspension of the imposition of sentence is considered to be a judgment imposing punishment. United States v. Bridgeforth, 441 F.3d 864, 871 (9th Cir.2006) (citing Robinson, 967 F.2d at 292-93). In the latter instance, by contrast, a judgment is actually rendered. People v. Arguello, 59 Cal.2d 475, 30 Cal.Rptr. 333, 381 P.2d 5, 6 (1963) (en banc). And in such a case, if the sentence imposed on a wobbler offense is a jail term (as opposed to a term in state prison), "the defendant has, under Penal Code, section 17[(b)(1)], been convicted of a misdemeanor `for all purposes after a judgment,'" notwithstanding the fact the sentence is thereafter suspended and probation granted. Alotis, 36 Cal.Rptr. 443, 388 P.2d at 681 (quoting Cal.Penal Code § 17).

Therefore, in the event a defendant was sentenced initially to a misdemeanor or to felony probation without an imposed but suspended sentence (that is, never received a state prison sentence, whether imposed and suspended or actually served), no judgment was ever entered. Because no judgment was ever entered, the defendant has never been convicted of a felony for federal purposes.

Most of the time, however, a wobbler sentenced as a felony includes felony probation with some unimposed sentence.

However, it is imperative that the defendant seek a reduction of that felony to a misdemeanor. Under Penal Code 17(b)(3), otherwise, you still retain a felony on your record and firearms ownership would violate both California and federal law. Then a dismissal may be sought under Penal Code 1203.4 or Penal Code 1203.4a.