I don't know anything about the throat, I do seat to max OAL as listed in several manuals and I get great groups as you know. I have to be kinda careful because of the rotary magazine, they can be finicky. I once seated a 100gr slug in the .250 out a bit much and it took a while to get things deconstipated! The 140gr Hornadies are a long slug, no lead, solid gilding metal and a rubber tip, so the boat tail really eats powder space. What is the length of that slug?

Aha! The HP spud looked a little newer than the rest of the mold so I didn't know if it had been recently modified or not.

I didn't think about twist rate. I be lively new, 8.5" barrel is a 1:8 or 1:7" twist. It's set up to stabilize heavy, subsonic bullets, like 210's at 1050fps. The old, 16" barrel did fine with a cast 220gr bullet and still shot125gr TNT's pretty well. I just assembled the upper with a new shorter barrel and have put it on a pistol lower. It's legal to hunt with here in IN as a pistol, but when I get my tax stamp and turn it into an SBR, the caliber isn't legal anymore!

As to cycling, the old set up was very versatile. I had one set of ammo that was loaded with a 190-ish cast roundnose. They didn't cycle the gun and were still supersonic, so they were about worthless as far as fun shooting! Other than that load, it's shot well with everthing Ive put in it!