I have experimented with different loads in my 556's and have found the sweet spot in reloading. The Hormany 68 BTHP give me tight groups using BLC2 and Varget. The 75gr loads do well but the tightest groups I get are using the 68gr Hornady bullet.
about 26 gr of bl-c2 and 25 gr varget sound about right? did you try the 69smks? Reason I ask is that I have about 1k of them and the rifle I originally bought them for is sold. 26 grn of BL-C2 is the old standard shortline service rifle load with 69 smk. It seems I was running 25 grns of varget in a recce type AR and it loved the smk. How big /small are the groups and what type of sig, DMR or carbine? trying to decide if I want to load for this rifle, only have a single stage press and would be slow going for 2 moa ammo if that is best it will do.
I'm loading for my Sig 556's and at 68gr I'm gettig 1 to 1.1 MOA with both Varget and BLC2. The 1-7 twist barrel seems to really like the 68-68gr as a sweet spot. It also performs well in my AUG and AR with a 1-8 twist. Get close to that with 4895.