Western recommends mag primers so that BH209 is more reliable in a larger variety of breach plugs. The new Knight bare primer plug was designed for BH209 from the start. It was also designed using a Win209 primer. One call or email to Dave at Lehigh can confirm this.
Two plugs were mainly used during the development of BH209, the T/C Omega plug and the Savage MLII plug. Consequently both are very reliable with BH209. The Omega plug is even basically flawless with a Rem STS primer.
