If you plan on shooting in a SASS Sanctioned match, you need a shotgun with a barrel length of at least 18".
See the sentence in red below. This is from page 33 of the shooters handbook.
Barrels
- Barrels or chambers may be re-bored, lined, or sleeved to any of the approved
calibers or gauges for that particular firearm provided industry safe limits are
maintained.
- Original barrels may be replaced with new barrels of correct styles (e.g., round,
octagon, half-round) for that type of firearm.
- Barrels may be shortened and/or crowned.
- Rifle barrels must maintain a length of at least 16 inches.
- Rifle barrels may be altered to accept mounting hardware for an alternative tube
magazine configuration and capacity appropriate to the model of firearm being
altered.
- Barrels may be altered to accept period style sights as described within the Sights
Section.
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Shotgun barrels must maintain a length of at least 18 inches.
- Shotgun barrels may have internal choke tubes installed provided they do not
extend beyond the muzzle.
- Ventilated ribs and mid-barrel beads are allowed on all types of shotguns.
- No heavy competition or “Bull Barrel” is allowed on revolver caliber firearms.
- Barrels must be made of steel or iron only.
- Visible counter-weighting devices are not allowed.
- Compensating ports are not allowed.