Gents:

I am trying to duplicate 75 grain Hornady TAP ammo. The Hornady listed load with this bullet is 26 of BL-C2. I used 25.5 in CJ93 cases. Surprisingly, it shot to the same point of aim at 50 yards as 55 grain ball. But I got bright ejector marks on the case head. CCI400 Primers looked OK. The bright ejector marks would normally be accompanied by bolt lift resistance in a bolt rifle, but with no bolt and the primers looking OK, I am not sure if I have an overpressure load or not.

How do you judge pressure in the AR?

Thanks

It's really difficult to judge in an AR because of the gas system. I have a carbine that gives rim lifts and makes burred ejector marks with factory ammo. The same ammo in a rifle is fine. That's because the rim lifts and ejector marks aren't really due to pressure - they are due to the extraction timing. The same thing can be said about blown primers. Some people add more weight to the system which alleviates these problems. Or you can switch to a powder with a different burning rate. Or you can reduce the load.
As long as your velocities are in line with what a manual tells you to expect, I wouldn't worry too much about pressure. If the ejector marks annoy you, then do something about them. They aren't in and of themselves an indicator of a problem.

I'm not familiar with the CJ headstamp, I only use Lake City, so I'm not sure of the brass quality. I bet if you pull a bullet from the hornady ammo you will find Varget or RE15 under it, they are making "white box" ammo now that a lot of guys are using for practice and even matches with LC cases, the 75 gr, BTHP. I have never had problems with those 2 powders in High Power loads, can't say the same when using ball powders. If your ejector marks are bright, I'd say back off. Try 24.0 varget or RE15 but back off and try 23 and 23.5 first, once you hit 2700 to 2750, Stop. Reload a few cases and see if the primers go in too easy, also a sure sign of too much pressure.

I had a similar situation with ejector marks showing up at what appeared to be pretty low velocities...2750 more or less with 55g SP bullets, W748, and LC cases. It turned out that I'd overlubed the bolt carrier and a little lube was being deposited on the top round in the mag, effectively lubricating the case and allowing it to move back against the bolt face. After cleaning up the excess lube, problem over. Also, you might be sure the case heads are quite clean, since I also have seen "ejector marks" that weren't actually in the metal, just on the carbon.

It was puzzling, since I was getting no other signs of pressure; no growth in case length, no loose primers.

I was able to increase the load to book max, with another 200 fps velocity and no pressure signs.

I was loading 2230-S a few years ago and got 3450fps with no signs. I tried another .25 grain and ripped the extractor off the bolt. It did chrony over 3550 with a 55gr bullet. No loose primer though. I learned about predictable velocities from that. As I remember the case still had room too.