Use of Steel Ammunition
Knowing what steel ammunition can and can’t be used with shotguns bearing both CIP proof marks and historic national proof marks, or “proof symbols”, can be very confusing. The proof symbols are explained in their simplest possible terms for what ammunition may be used with the relevant proof symbol above.
It must be understood there are two types of steel ammunition, Standard Steel & High Performance Steel.
Standard Steel ammunition can be used with any shotgun (other than Damascus/twist barrelled guns) bearing any of the below CIP, London or Birmingham symbols, provided the ammunition is of the correct length for the chamber and the barrel choking is less than half choke (some small cosmetic choke damage may still be incurred, even under these conditions). The ammunition packaging should signify that the ammunition is Standard Steel. But, if you are at all unsure whether your ammunition is Standard Steel, please contact the manufacturer. You maybe unsure if your shotgun is suitable for Standard Steel in which case please contact a knowledgeable gun dealer, gunsmith or the British Proof Authority for advice, however this may involve the submission of your shotgun for inspection.
High Performance Steel ammunition can only be used with shotguns bearing a CIP fleur de Lys 
Mark, as symbolised below in the table above. The ammunition should be the correct length for the chamber. The CIP recommends less or equal to half choke in relation to a given shot diameter for the Bore. Please see the table below for clarification.
The ammunition packaging should signify that the ammunition is High Performance Steel. But, if you are at all unsure whether your ammunition is Standard Steel, please contact the manufacturer. You maybe unsure if your shotgun is suitable for Standard Steel in which case please contact a knowledgeable gun dealer, gunsmith or the British Proof Authority for advice, however this may involve the submission of your shotgun for inspection.