AR barrel throat depth

[LRRPF52]

I measured my 20” with he Hornady gauge I had and got a measurement of 2.340 to the lands. I came up with the earlier measurement of 2.223 by coloring a bullet with a sharpie and clambering it and looking for marks. I was making loads of varying lengths to test and decided to seat one at 2.245 since that is the max norm for 123 SST. I chambered it and had no marks and it measured the same afterwards. I’m kinda stumped. I’m thinking when I chambered the first colored rounds that the bullet may have moved forward a smidge from the bolt slamming closed and caused the marks and extra length. I originally thought I was into the lands and the extra length was from the bullet being pulled when I ejected them. This is confusing the hell out of me. The best groups I got were with 28.5 and 29 grains of CFE at COL 2.223. I loaded some at 2.245, 2.230, 2.225, 2.218, and 2.210. Before I blow myself up or ruin my barrel is ther a minimum recommended COL? I’ve been reloading for bolt guns forever but semi autos is new to me.

If you use the Stoney Point gauge, it's very easy to not get a full seat of the dummy case, so COL numbers can be all over the map.

I personally use dummy cartridges with marker on the ogives, and let the bolt fully send them home when determining my max COL.

[LRRPF52]

Make a load ladder at the bottom of the load spectrum working up with the bullet jammed into the lands - my gun seems to like 0.025". You may see pressure sooner than without jamming into the lands, but I went up to 32 grains of CFE223 under a 123, had a wildly compressed powder charge, and jammed 0.025" and still had no pressure signs. My good load with cfe223 is currently 29.8grains IIRC, COAL is 2.245" or something analogous to that. That particular load is about 1.5 minutes lower at 625 yards than Hornady BLACK, so it is moving a tad bit slower.

(The main reason I cant remember the COAL is because I use the ogive gauge that hornady sells - as someone else mentioned, seating to a specific case head to ogive length is far more consistent than COAL.)

I do not recommend jamming the lands in a gas gun because you will have a very hard time extracting them if you don't get to fire. You are also not allowing the jump with start pressure transition into the lands, so pressures can spike on you, especially from the small volume case of the 6.5 Grendel and its efficient 30˚ shoulder.