Odd, I just checked mine again, I pulled the bolt handle all the way back' pushed up the BHO and it goes forward to where the bolt is half way into the magazine.
No American magazines and no conversion, stock 20 guage
FWIW - The BHO on the Saiga 12 (model 109) I have holds the bolt face approx 1" aft from the rear end on the mag opening
I'll try to get a picture posted of mine tomorrow.
Mine would not strip a round from the magazine if I could pull the BHO down from where the bolt face sets.
Take a pic with the dust cover off, top view!
All this got me to thinking so I pulled the Saiga from the safe, My belief was that my BHO locked all the way open but it doesn't, what gives! It seems that it wants to catch at full open but doesn't grab enough and then holds at half open.
Apologies to hcpookie for hijacking his thread.
The stop on the BHO has to move to the rear if you want to hold the bolt fully open.