A lot is going to depend on what your hauling that makes up the 40-50 pounds, along with your personal preferences in a pack. The Gunslinger II could definitely fit the bill, and with the new Intex II frame system, should do just fine in making that load as comfortable & stable as possible. The scabbard should have no issues with your weapon, so I think you'd probably be GTG with that pack.
If you're wanting other suggestions for comparative purposes, then consider the X3 LoDrag and M5 Team Elk packs from Eberlestock. While the M5 is more of a dedicated hunting pack with wide mouth scabbard (only comes in hunting camo patterns, no solid color packs), it offers a great layout and could accommodate a good amount of gear and still be comfortable. The X3 LoDrag is a VERY popular pack among 'Hiders, and works great in the roles of daypack, match/range pack, etc. It won't be quite as stable as the Gunslinger II with heavier loads (uses a plastic framesheet for stability vs. the Intex II frame of the Gunslinger II), but I have seen them loaded to 50 pounds & perform well. It does offer a weight savings over the GS II, and could work, but it is hard to say that it would be more versatile or a better performer for your applications if you're leaning toward the Gunslinger II.
Hope this helps & please ask if you need further clarification.
Ceylon
deltaoutdoorsman.com