Well then maybe it'll work better with under size BPI sabots.

The first full bore Lee Key slugs were 490 gr,
that we had made. Good for 2300 in 3.5" plastic
and 27-2800 in 3.5" RMC brass cases in the
long barrel NEF.

For the 8ga Hubel Falling Block pictured
above; For now using 3.3" swaged kiln cases. We swage
them to fit the 8ga chamber in falling block.
The barrel is real thick so we can run 50-60,000 psi
if we want in that action. The heavy duty 8ga plastic will
take about 23000 psi. A MRC turned brass would do
35,000 psi. But the plastic does so great,for now I'm
staying with 70 cent 3.3" plastic, primed with 239 primers,
good for 4 shots or more.

And I have the REM wadcup, on left in picture, with
its own seal, that they use for the kiln slug, which works
for various other slugs and shot loads.

In the 8ga HFB - 3.3" plastic cases-
1020gr 2000 -- 900gr to 2200 ---- 770gr to 2400---
and triple ought buckshot load---
9 - 70 gr 000 buckshot, in wadcup. 630 gr total-- 2600.Ed