Modifications you can legally make in Australia

Disclaimer. This is general information for licensed shooters. The categorisation rules in your state's Firearms Act and the National Firearms Agreement override anything here. When in doubt, ask your state firearms registry — in writing — before you fit the part.

What you can almost always do

• Replace the stock with a sporting stock of the same type.
• Fit a scope, mount and rings.
• Fit a non-suppressor muzzle device permitted under state law (most muzzle brakes — but not flash hiders in some states).
• Replace springs and triggers with factory or factory-equivalent parts that don't take pull weight below safe limits (typically 2 lb / ~0.9 kg dry).
• Refinish the metal (cerakote, bluing).
• Change the barrel for the same calibre and same overall length, in most states, when fitted by a licensed gunsmith. NSW and VIC have stricter rules around barrel changes — check first.

What changes the category

• Conversions to/from semi-auto: illegal without registry approval. Removing limiters from non-semi-auto firearms is treated as conversion.
• Magazine capacity: NSW and VIC restrict shotgun and rimfire semi-auto magazine capacity. Adding capacity bumps you into Cat C/D.
• Pistol-grip configurations: several states treat a thumb-hole or pistol-grip stock as creating a Cat D firearm if combined with semi-auto action.
• Suppressors: illegal for civilian possession in all Australian states.
• Folding/telescoping stocks: often treated as making the firearm "easily concealable" and may move it to Cat H or prohibit it.
• Barrel shortening below state minimums (varies; commonly 470 mm rifle / 660 mm shotgun OAL) is illegal.

Practical advice

• When you buy a part, keep the receipt and the manufacturer's description.
• Modifications by a licensed gunsmith give you a paper trail. Do this for any work that touches barrels, triggers or actions.
• Some states require notification of any modification that could affect category. NSW Firearms Registry has a specific form for this — file it before fitting the part.