Currently, yes we are restricted to 100g at a time and firing this off loose.
Assembling a practical device is something we're still seeking guidance on, as we don't want to do or encourage anything illegal.
I don't know about the finer details of what constitutes BP - I have a few kilos of sulphurless powder that has the same UN number as BP, but I guess that fact just muddies the waters even more.
You are correct MSER allows the manufacture of up to 100 grams, but COER required anyone acquiring BP to have an appropriate explosives certificate and making any amount of BP under MSER is considered a means of acquisition under COER.
We can't have one person with a BP certificate acquiring all the BP that gets made. However, there will be several people on site with certificates - If a certificate holder is in charge of a particular BP mixing operation then they can use their certificate to acquire what is made - however this will need to be done on a personal basis by the certificate holders.
OK, under MSER gun powder / bp is defined as an intimate mix of ammonium nitrate or sodium nitrate or potassium nitrate with charcoal with or without sulphur. The ratio's are not important as long it has any of the nitrates above with charcoal.
I guess the best option WRT BP is for someone with the appropriate certs just to bring along the BP so that more interesting things can be done with the 100 grams.