And the purpose of the plastic jacket is ????
If blowback is increased, wouldn't that indicate a worse seal?
FPJ - Full Plastic Jacket and in your case it is a red plastic jacket. This is the red jacket primed with a 209 shot gun primer.
If the breech plug looks like this it is a FPJ system
If it looks like this.... it is a NFPJ system - Non Full Plastic Jacket
You also will need the correct bolt to go with the BP that is installed in your rifle
Hope this helps a bit..
Truthfully when I got my ultralight I picked up the bare primer kit, I really like the FPJ system and haven't even tried the bare setup. Save your red jackets and wash them, they can be reused a few times before the primer pockets loosen up.
I get the blow by theory, I've only used BH209 and blow by just doesn't seem to be an issue for me.
The FPJ provides a completely water tight breech. you could stand the breech in water for days take it pull the trigger - boom
But, if you get the compression right on a NFPJ you could do the same thing also.
The blow back comes from the stretching of the FPJ around the nipple when the blow back pushes back.