Normally you need to pull the trigger to allow the breech plug tool into the breech. The breech plug or the tool can get hung up on the trigger when putting them in. So always hold the trigger back when inserting anything into the MK-85.
You will learn to love that rifle. It may take one extra minute to clean it over an Encore. But I have never seen an Encore that will shoot with an MK-85. And the MK-85 will shoot whatever you decide to load in it, they are not finicky.
Which ignition system did you get. #11??
I may have an extra musket cap ignition system laying around if you or someone else might want to buy it. I changed my two MK-85's to musket cap ignition. I like it as well as my 209 ignition rifles. Probably better because it is considerably hotter than #11's, but not as hot as a 209-which IMO at times is too hot. Tom.