[NOTE: This ruling was revoked by Revenue Ruling 61-45]
Revenue Ruling 54-455

     The attachment of a shoulder stock to a pistol designed to
     utilize such attachment, brings the weapon under the
     classification of a firearm, and therefore constitutes an
     unlawful act on the part of the owner thereof unless he has
     made declaration, in writing, of his intention to make a
     firearm, and subjects such owner to possible criminal
     prosecution and/or tax assessment.  Also, the firearm may be
     subject to seizure and forfeiture. 

     Advice has been requested as to the registration status of
pistols which have been converted to firearms by the attachment
thereto of shoulder stocks.

     Where the owner of a pistol, which was not classified as a
firearm as defined in section 5848(1) of the Internal Revenue Code
of 1954, equips such pistol with a shoulder stock attachment, the
pistol so converted is brought within the classification of a
firearm defined in said section.

     If the owner is of good repute and did not possess the firearm
with intent to violate the law, such owner will be permitted to
register and retain the firearm involved provided the weapon is
rendered inoperable as a "firearm" within the purview of section
5848(1) of the Internal Revenue Code of 1954.  To accomplish this
objective the male lug or the receiving slot, as the case may be,
on the grip of the pistol must be ground off or welded closed. 
Such alteration will permit the operation thereof as a hand gun but
will prevent its use as a shoulder weapon.  Although no longer
considered a firearm, the weapon should be registered on Form 1
(Firearms) for control purposes, indicating thereon its present
condition.   The subsequent tax exempt transfer as an unserviceable
firearm may be permitted in accordance with the provisions of
sections 5812(a)(3) and 5812(b) of the Internal Revenue Code of
1954.