there were some earlier back action guns and not all were heavy target guns
but i think alot of times folks either just dont know of them .
i cant remember right now where i read it , maybe it was in Ned Roberts book . but i have read then referred to as French action .
Also there is no need for a drum bolster , an improved breech works just fine . also why use a modern lock when there are so many originals around at very cheep prices ?
here are a couple earlier pieces . earlier being 1840-1860
Notice the breech application on a couple of these
AW. SpiesJacob Linn50 caliber plains rifle with JACOB LINN SAN ANTONIO TEXAS stamped on the top flat of the barrel. The bottom flat of the barrel is stamped THURSDAY NOVEMBER 6 1851. This is the only rifle presently known made by Jacob Linn
Krauskopf56 caliber plains rifle with E. KRAUSKOPF stamped on the top flat of the octagon barrel. Krauskopf was located in Fredericksburg, TX, 1846 - 1881. Krauskopf made many of the parts for his rifles including rifling the barrels,
here is a link with more of the above information and a few other back actions
http://www.texasguntrade.com/texassportingrifles.htm i would also agree with the comment of a thin wrist . however i have seen many a later SXS with back action locks that did not have broken or cracked wrists , even with 2 locks inlet into it