I know this an old post, and it seems as if old posts that are brought up are slightly frowned upon here, but I'm new, so forgive me.
I purchased what I feel is the best 209 primer conversion for the Remington Model 700 MLS ever designed. It completely does away with a nipple on the breech plug. Instead, a machined nose slip-fits on the face of the bolt where the weather shield would be installed, and it has a groove machined into it where you install a primer. That's right- the primer is installed on the front of the bolt, like a cartridge in a controlled-feed action. The new breechplug is made for the primer to fit in with minimal space to prevent blowback, and when cammed into battery, the face of the bolt seals against the breechplug. It also has a new firing pin, which is a true firing pin, and not the junk Remington sold in their 209 conversion.
Ignition of any blackpowder or substitute (including BH209) is instantaneous. It is simple to install and remove primers without a tool, and if you want to make the rifle safe, just open the bolt. The primer will remain in the boltface but the rifle cannot fire.
I personally feel that had Remington designed the 700 ML and MLS with this configuration, they would still be selling these rifles (with a better stock, though). They are about as accurate of a muzzleloading rifle as you can buy; they are just a PITA to clean due to the blowback. The locktime on them is second to none.
Here is the boltface:
The bolt and breechplug (how it fits when closed):
The breechplug with ventliner (with primer installed):
A three-shot group with 300 grain T/C Bonded Shockwaves (90 grains BH209):
My Remington Model 700 MLS (restocked in laminate):
If you want to know where I purchased it, shoot me a PM.