I use T7 only.
In a standard ROA cylinder with the standard rammer and 3mm wad even 15grn by mass/24grn BP equivelent will leave airspace,poor ignition being the main side effect but safety should be a concern.
I have a spout marked 28grn that gives 2mm compression but the accuracy isn't good,also get some creep that doesn't happen with well compressed lead.
My best load is 15grn mass of T7 with a lubed wad and .457 RB,crunched down firmly in a press.
Fillers are a problem on a damp indoor range as they grow mould.
BP indoors is also a bit rich.
I measure everything.
Don't have much confidence in the numbers marked on spouts,they vary too much.