What happened was you did not size your cases back far enough.
Each chamber is different. You need to measure the fired brass out of your chamber. Then set up your sizing die to bump the shoulder back .003-.004 thousandths.
You will notice when your die is set up to set the should back say... .003, then you fire the brass a couple more times it will only size it back maybe .002 or less. That is because as brass work hardens it "springs back"
There are many nice ways to measure fired brass shoulder length. Also base to ogive on a loaded bullet. But....
For starters i recommend the RCBS precision mic.
Later you can get digital calipers and bullet comparators and bump inserts. Basically you buy the bullet comparator body and then buy a bump insert for shoulder bump and a bullet comparator for base to ogive.
If you do not at least,...... get the precision mic, you will be oversizing or undersizing your brass. In some rifles, undersizing is very dangerous. Luckily not so much in the AR15 platform.
Oversizing means you are pushing the shoulder back way to far and wearing out your brass
Undersizing means when your bolt closes, your chamber is finishing the FL sizing job you should have done.
Don't "cam over" your press. RCBS presses can handle a bit of that. Other won't. (a bit is designed in)
There is difference between the linkage/shell holder contact being tight and flexing a bit, and cam over. All that happens when cam over is a bunch of cast iron stress.
If groups got tighter as you went higher keep going!!! Go up in 2 tenth increments. Go at least to 26!!
You want to see pressure on the brass, or groups clearly deteriorating. Then you will have learned everything about TAC at the temperature you shot it at, CCI #41's, your chamber and LC NATO brass.
Each chamber is different. You need to measure the fired brass out of your chamber. Then set up your sizing die to bump the shoulder back .003-.004 thousandths.
You will notice when your die is set up to set the should back say... .003, then you fire the brass a couple more times it will only size it back maybe .002 or less. That is because as brass work hardens it "springs back"
There are many nice ways to measure fired brass shoulder length. Also base to ogive on a loaded bullet. But....
For starters i recommend the RCBS precision mic.
RCBS PRECISION MIC CARTRIDGE HEADSPACE TOOL | Brownells
These micrometer cartridge headspace tools will enable you to set up full length sizing dies for the precise amount of shoulder bump you wish. Precision Mic gages are initially calibrated on a once-fired case, and the micrometer will read shoulder...
www.brownells.com
Later you can get digital calipers and bullet comparators and bump inserts. Basically you buy the bullet comparator body and then buy a bump insert for shoulder bump and a bullet comparator for base to ogive.
If you do not at least,...... get the precision mic, you will be oversizing or undersizing your brass. In some rifles, undersizing is very dangerous. Luckily not so much in the AR15 platform.
Oversizing means you are pushing the shoulder back way to far and wearing out your brass
Undersizing means when your bolt closes, your chamber is finishing the FL sizing job you should have done.
Don't "cam over" your press. RCBS presses can handle a bit of that. Other won't. (a bit is designed in)
There is difference between the linkage/shell holder contact being tight and flexing a bit, and cam over. All that happens when cam over is a bunch of cast iron stress.
If groups got tighter as you went higher keep going!!! Go up in 2 tenth increments. Go at least to 26!!
You want to see pressure on the brass, or groups clearly deteriorating. Then you will have learned everything about TAC at the temperature you shot it at, CCI #41's, your chamber and LC NATO brass.