Hello, tacotime. Let me tell you my experiance with fillers. I was experimenting with a lighter than normal cast bullet (275grs.) in a .40-70 str. (2 1/2") in a Riflesmith No. 1 long-range, 34" brl. 77' Sharps. I also was using H4227..a powder I have much experiance with, but never before used in this rifle. I was looking for a light 100yd. practice load.
Since this was a "new" powder, I started low, & filled excess space with 1.5gr. Kapok..pulled out to long thin cylinder, & lightly inserted into case. A .030" Veg. fibre wad was over Kapok & under bullet base. I didn't compress Kapok any way..the act of bullet seating pushed it to final place.
At range, I fired first start load..recoil light, but normal, bullet impact was very low on backer..normal for such a light loading. Blew thru brl..lots of smoke, but clear. Next shot also fired like first..sound & recoil the same.. bullet impact still low, near first shot. blew thru bore & still black! First thought was that smoke sure was thick in there. Took rod & pushed in breech..obstruction right at chamber end. Not wanting to push what I thought was a stuck bullet thru 34" brl., I reversed rod..and thought maybe that 2nd. hole was there on backer all along?
I couldn't believe what came out that breech...looking for all the world like an old style over-powder shotgun wad..that long Kapok filler was compressed into a firm 1/4" thick .40 wad..with the fiber wad at front!
Now my question is..how did that wad and filler end up at the breech end of a 34" long brl. AFTER the bullet exited bore?? Could a very low muzzle pressure due to the light fast burning charge of H4227 in that long brl. have allowed the wads to have been sucked back into bore when bullet base cleared muzzle? After this experiance, I have given up on fillers over smokeless.