RMSD is fine for items that fit it's size limit ie pistols,
for bigger stuff you need the large parcel delivery service which is Parcelforce (as opposed to Royal Mail which is only for letters & small parcels) and the only ParcelForce service approved for "guns for sporting use" is PF48.

NOTE
RMSD will usually fly which means being X-ray'd so declare it as being an air pistol & my advice is mark as "surface mail only".