The best thing I ever did to mine was change out springs as was suggested here.  I also learned here to cycle three times for each charge to get really uniform charges.  Tom

When I got my first B&M, the cap was missing from the plastic tube. Casting about for a lid of some sort, I discovered that the bottom of a starter pistol blank container fit the tube perfectly. Last year I picked up a brass tube B&M at a gun show. Surprisingly, it still had the original brass cap. It also has an unusual fluted clear lense on the powder chamber.

Have you ever heard of this static electricity or plastic hoppers causing a static explosion John?
I've heard this mentioned often over the 45 years I've reloaded, but honestly nobody has pointed me to a case where this actually happened. I've used several powder measures with plastic hoppers to dump BP for a long time, and no issues here. Maybe I'm just lucky?
There have been some people who deliberately tried to ignite BP with static and couldn't.

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Somehow I've ended up with several B&M but never have used one. I always had a hang up with mounting the measurer. I notice a weaker spring helps and that makes a lot of sense.  Would polishing the chamber behind the glass window help any? Is there any other tips on making the B&M run better. The soft plug is a good idea. I've thought of making a brass hopper for black powder too. Thanks

Val I have read about the static electricity injuring someone in the past. 
Not sure what the B'& M powder container was actually.made.of ? 
Most replacements that I have seen being used over the years where made out of copper tubing and so was the cap.
Not ever being a BP user I am also not also sure if that was best choice or not ?

What are the new B&M and their clones now using for BP ? 
Even though it might not ever be used for BP.
Someday somebody else might and if it were ever to go KaBoom and right in their face. 
I myself could never ever recommend what one should actually use and I absolutely would not even attempt to even go there myself.
Our God only gave us all two eyes and in no way would I ever even attempt to share what one might able to use.
Why because just one BP Kaboom could actually leave someone being completely blind for the rest of their own life.

Blackpowder, specifically the dust, absolutely can be set off with a static discharge in the right conditions.

Then again so can flour, sugar, saw dust and a long list of other seemingly harmless things. Explosions in flour mills used to be fairly common. The Washburn mill is a famous example. 

Coal dust in mines ignites much easier than the coal itself.

Some of the most  dramatic explosions in pyrotechnic shows are often dust particles blown into the air and ignited, electronically.

OSHA has some pretty detailed safety protocol for “dusty” work environments and static electricity. Lessons learned the hard way.

Possible and probable are completely different things.

It’s possible I could get ran over by a guy smoking a cigar driving a Desoto. It might be more probable I blow myself up sifting flour in the kitchen.