I had a chance to chronograph some rounds yesterday. I have been toying around with BLC2 in the 308. Since the Hornady black amax rounds shot well in the AR10 I was focused on duplicating those. I picked 45gr as my starting point with the amax based on other loads with the SST. It turns out I was loading just a touch hotter than Hornady with my rounds clocking in at 2592 fps while the factory load was 2540.
After considering some input here I decided not to crimp these rounds. I was pleasantly surprised to get an extreme spread of 11 and an SD of 4. The grouping was a little loose(1.5") but it was the last 5 shot group I shot through a pretty warm gun on a fairly hot day. Based on this session I think I'm going to move away from crimping my rifle rounds. In the end I had 10, 5 shot groups with every loading under 20 sd except the Hornady black factory load. 4 of 10 were under 10 sd.
In the next few weeks I might even alter a proven 223 load(by not crimping) to see if I can further shrink down some already great grouping bullets. Always chasing the dream !
If you shoot 357sig then you will want to crimp, especially the 147gr! Not much neck on those cases! Any full auto would most likely need crimped ammo? The headspacing comment holds true! I only have bolt guns in 264wm, 300wm and 338lm, but haven't reloaded any of the later 2. Been loading the 264wm for 37 years and never crimped any of them. Tube feed guns in 44mag and 45/70 (no lead bullets) I do crimp. Crimping on auto handguns (even though it's headspaced on mouth) just feeds smoother due to no edge (or as sharp/blunt edge) that works for me. Accuracy hasn't been an issue, but haven't checked velocity vs deviation. I've thought about getting a Barrett or Nemo (just rebarrel) with the quick/caliber change barrel in 300wm and have a 264wm barrel for it for an auto loader? It's more about what works for your gun!
I'll keep crimping my auto pistols but lightly at that. Mostly for the reason you mentioned regarding feeding. The 45 Colt,357 and 38 will probably still get my heaviest crimp with the Lee FCD. Otherwise I'm going to move away from crimping all my rifle rounds except maybe the 7.62x39 and 30-06 if I'm making some garand ammo. Hard to say if my 7mm WSM should get one. I might have to take a handful of uncrimped rounds to the range with a dial caliper to see if I get any setback.