Has anyone ever used small pistol primers in the .25 ACP breechplugs.
There is a severe shortage of small rifle primers here.

Depends on what powder you are trying to set off. If it is one of the older subs like Pyrodex or even Black Mag'3 you should be OK with small pistol magnum primers. If it is BH209 I doubt that you will be able to ignite with confidence. Same goes for Tripple Seven as it is harder to ignite.

I think I may have to do some serious testing. I really don't want to go back to 209's because they foul so badly. The .25 acp plug keeps my action so clean you can't tell it's a muzzleloader.
I am using 777.

arty, I'll be real interested in your findings. I just sent a plug off to be modified for my Encore, I am even checking into having one of the new Endeavor barrels breech plugs the hand removable one modified for my rifle.

Obviously I haven't shot one so modified yet, but I keep hearing great things about improved accuracy and absolute minimal fouling as you said.....

I'm really curious as to when this primer thing is going to straigten up......