With regards to the best breech seal arrangement, for me it has to be the conical seal a la FWB 300. I think in modern terms the BAM underlevers might use this too (please correct me if I'm wrong)

As much as it pains me to speak badly of my TX200, the double o-ring setup is woeful. Fine for the HC that I only use occasionally, but on my full length which I use 3 or 4 times a week they were constantly giving issues