4.2 grains of Bullseye behind a 185 grain SWC is an old standby equivalent for commercial match ammunition that uses jacketed bullets. However, old bullseye 1911 guns generally had a lighter recoil spring than standard so they would cycle properly with it. With that light bullet weight, it sounds like your SIG just needs a bigger charge to get enough momentum (the thing that is equal and opposite) to cycle full power springs. It's not uncommon. Figure that 5 grains of Bullseye behind a 230 grain bullet is an old standby WWI era load to get about 825 fps, which is what QuickLOAD predicts for it in a 5" barrel. Your 4.5 grain load it predicts would produce about 778 fps with such a barrel. To generate the same momentum as a 230 grain bullet at 778 fps, the 185 grain bullet has to be going 942 fps. If your COL is about 1.2" with it, QuickLOAD predicts that will require 5.6 grains of Bullseye. Alliant's load book gives 6.4 grains as maximum with the plated jacket Speer Gold Dot, so you should be fine at 5.6. Still, it may well be more than you need minimally, so try 5 grains first, then 5.3.
I know you don't want to increase your powder charge, but it looks like the laws of physics are thumbing their collective noses at you in this case. You can also inquire of Sig about lightening the gun's springs for target level loads.
As to keeping the bullets straight, I've tackled that problem in both my Square Deal B and a friend's 1050. Both solutions involved my lathe. I made a Lyman M profile powder drop tube for the Square Deal that puts a step in the case mouth along with the flare and that keeps the bullet straight upright as it starts into the seating die. On the 1050 I turned a seater stem in the shape of the H&G 68 200 grain SWC's we were loading with it.
Some folks have gotten a spare seating stem from Dillon, degreased it and filled it with JB Weld and pressed a waxed bullet into it to make a custom seater. The trick here is getting the waxed bullet in straight so it aligns well. An arbor press or a drill press will let you set the bullet base flat on its platform then square up the stem of the flat bottom of the press ram or chuck it in the drill press.
