http://www.issf-sports.org/
has all the answers.
9.4.2 GUNS
9.4.2.1 Types of Guns
All types of smooth – bore shotguns, including semi-automatics,
may be used provided their caliber does not exceed 12 gauge.
Guns of smaller than 12 gauge may be used.
9.4.2.2 Release Triggers
Guns with any form of "release" trigger actions are prohibited.
9.4.2.3 Slings
Slings or straps on guns are prohibited.
9.4.2.4 Magazines
Guns with magazines must have the magazine blocked so that it is
not possible to put more than one (1) cartridge in the magazine.
9.4.2.5 Changing of Guns
Changing of properly functioning guns or parts of guns including
interchangeable chokes, is not permitted in the same round.
9.4.2.6 Compensators
The addition of compensators and similar such devices fitted to
gun barrels is permitted for Skeet but prohibited in Trap and
Double Trap.
9.4.2.7 Ported Barrels and Ported Interchangeable Chokes
Ported barrels are permitted, provided they do not extend
back further than 20 cm as measured from the end of the
muzzle.
Ported interchangeable chokes are permitted, provided their
porting plus any barrel porting does not exceed 20 cm as
measured from the muzzle end of the interchangeable fitted
choke.
9.4.3.1 Cartridge Specifications
Cartridges permitted in ISSF competitions must meet the following
specifications:
case length after firing must not exceed 70 mm
shot charge must not exceed 24.5 g
pellets must be spherical in shape
pellets must be made of lead, lead alloy or of any other ISSF
approved material
pellets must not exceed 2.6 mm in diameter
pellets may be plated
black powder, tracer, incendiary, or other special type
cartridges are prohibited
no internal changes may be made which will give an extra or
special dispersion effect, such as the inverse loading of
components, crossing devices, etc.