I have a pair of BPS Hunter models...28" barrels...and yes, all of the BPS models are solid guns.
I think your understanding of capactity on the 20ga Micro model ( 3" chamber" is correct at 4 +1 if you use 2 3/4" shells ...and take the plug out.
I don't know if you can change the mag tube or not on a BPS ( I've never tried to do it ). I don't have a reason to do it ...
The downside to the Micro - is that its only available in 20ga ...and the length of pull is very short ....so it won't fit a lot of shooters very well. Barrel length is also very short ...for most shooters tastes...except for defense ...but this model is intended to be a field gun for young shooters primarily in my opinion.
BPS Micro has length of pull at 13 1/4", call it a 7 lb gun...with a 22" barrel.
BPS Hunter model - 20ga 3" chamber with a 26" or 28" barrel is still around 7 lbs but has a length of pull a full inch longer.../ and BPS Hunter is available in 12ga, 20ga, 16ga, etc....
There is a BPS carbon fibre high capacity....( but I hope you reliaze its in .410 only ) ....or the BPS Wood high capacity ( its in .410 only as well )....
A tactical shotgun in .410 ...makes little or no sense to me personally....???