AB Hammer Kit for Air Arms Airguns - by Lane. 1: Many Air Arms air rifles fitted with regulators may have had their hammers altered by the regulator installer i.e. Lathed Down, Sides Ground Away etc. Also over the years I have come across many second hand Air Arms air rifles fitted with home made hammers or running on a hammer usually found in another Series of Air Arms air rifle etc. 2: So PLEASE Before purchasing please check your existing hammer for two things: a: Does your hammer have a 4.00mm Screw Attached to the top surface YES / NO? b: What is the A O.D. Of Hammer Spring Guide & Measurement is close ±1mm to B. 1 300 Series: S300 S310 400 Series: S400, S410, S400 Super Light, S410 SuperLight, S410 Forester, S400 MPR. 2 S300 S310 FAC, Upgraded UK. 3 400 Series: TDR. 4 500 Series: S500, S510, HFT500, S510 TC, S510 Ultimate Sporter. 5 500 Series: TDR. 6 400 & 500 Series: Xtra FAC May Fit S300 310 FAC Before Purchase "PLEASE NOTE": 1: This product is primarily for Air Arms PCPs fitted with Air Pressure Regulators, such as those by Lane Regulators, BTAS, Tenchi (Tweaky), Huma, Altaros, Audrius, BTAS etc. 2: This product is for those with a very good working knowledge of the Air Arms PCP. 3: This product is for those with a very good knowledge of Machinery & Fitting and those whom can tune PCPs as this item Is NOT a simple straight fit in (Plug & Play) product but requires tuning in to work. As with regulators their selves this takes time, effort and patience.