Primers blowing out ties up my AR triggers-who’d athunk it?

[Amanda4461]

Tell me when I should have realized, since this was a first in 40 years of loading my own ammo. 5.56mm, new Starline brass, CCI-41 primers, Hornady 75gr BTHP Match, Vihtavouri N-135 22.7gr. Coal 2.505, no crimp. Per Vihtavouri manual, max load is 23.3gr.
Started at 21gr and loaded 50 each progressing 0.5gr for each additional 50 rounds, ending at 23gr. Took the Chrono to my range and commenced recording velocity out of my 20” Compass Lake AR-15 1x7 twist Krieger barrel. 5 round mags.
21gr had no issues, accurate and primers looked good. Using a brass catcher, so have no idea about ejection pattern. Velocity averaged 2550fps.
21.5gr also no issues. Velocity averaged 2565fps.
22.0gr the second round wouldn’t fire. Trigger was inoperable. Ejected round, no primer indent. Separated upper/lower and found a smashed spent primer stuck below the trigger. Removed it and dumped empty from catcher. Primer was missing, and ejection marks engraved lightly in rim. My chrono records and charts SD and ES, and they were close to the same as the 21.5gr loads recorded previously. Cleared trigger and fired remaining rounds without issue. First time I had ever had a primer blow out, so I assumed the new case was bad. Primers were getting flat, but not exceptional from past experiences. Velocity 2604fps.
22.5gr the 10th round tied up the trigger again. Another blown primer. Switched to my Sig SBR 7” barrel, first round tied up the action to the point that I had to tap the charging handle with a brass hammer to open the bolt. Removed the upper, and pulled the bolt. A blown primer was flattened up above the bolt, flat against the carrier. It wasn’t looking good for the home team. Took the remaining rounds home and pulled the bullets/measured the powder. I measure each load, and verify my Hornady auto measure with an old RCBS digital, so I did not expect to find any overloads. All was good. The velocity of these rounds was averaging 2817fps. A 1/2 grain increase in powder gained 200+FPS in velocity on the CLE 20” barrel.
I haven’t checked my chrono ES and SD charts yet, but I hope I will see a noticeable trend indicating an issue. Primer flattening is no different between the initial 21gr loads and the 22.5gr loads. I never felt any gas. If the weapon had not tied up with the frozen trigger, I would have not known that I was having pressure issues. The large velocity increase at 22.5gr should have woke me up, but I was sitting at the gun, sending my chrono results home to print off later. I only noticed the delta in velocity after reviewing the printouts.
Advice is needed regarding what to be more aware of in the future, to avoid getting to the point of blowing a primer. Also, should I discard my brass from the over-pressured loads? I small base resize, but if the brass needs to go, I have others. The two weapons fired a couple mags of factory loads after the blown primers stopped my testing, and they appear in normal operating condition.
All thoughts for a safer path forward are appreciated. Thanks to all.

 

[supercorndogs]

How are you getting 2.505 into an AR-15 magazine?

How far off the lands are you, 2.5 would be really close if not jammed into the lands with any of my 223 chambers?

 

[sinister]

Is the SIG SBR chamber .223 Remington or miltary 5.56mm? Jamming a bullet into the lands at 2.505 will raise your chamber pressure much higher than a free-bore jump.

Frank's Compass Lake reamers are usually 2.450 for throat longevity. Do you have a Stoney Point/Hornady gage to check and confirm when your bullets touch the rifling leade?

Going forward I'd check my cartridge overall length to leade contact; continue to small-base size; and hand-prime (for "feel") to ensure you didn't enlarge or blow out the primer pocket.

I use 23.5 of N-135 behind an 80, seated to 2.500 (for single-loading) in a 5.56 for 600 yards.

 

[Amanda4461]

Well, my iPad on my chest as I recline and worry my Dobe’s ears doesn’t help my typing skills, so my mistype of coal sucks. I load the 75gr Hornady to 2.250”. I use a Frankford hand primer, so good there. My Hornady gauge is mia after my move from Texas, so I’ll need to find it in a box in the attic, or just buy a new one. I normally just drop a dummy round into the chamber after breaking the gun down for cleaning, with the bullet seated out, and keep seating the dummy bullet deeper, until the black marker ink on the bullet doesn’t show it touching the lands. I failed to worry with this practice using the Hornady bullet, since I normally use the SMK. Working on this issue made me wonder about the accuracy of my 30 year old Starrett caliper. I borrowed a neighbors Mitutoyo today, and I think the Starrett is maybe an issue. It is off quite a bit. The mags I was using are the new CProducts Defense that are made such that you can feed a longer than 2.260” round. I use their 6.8SPC mags for my 224 Valkyrie for running heavy bullets seated out. I may have a Murphy’s law issue. Thanks for your help. I’ll do some more digging once the wife goes back to work. Considering the heat, figure a new Hornady gauge beats time in the attic.
The Sig is a FA Upper in 5.56mm.

 

[sinister]

Interesting about your Starrett.

Once upon a time several decades ago I shot a LOT of Hornady 75s since they were $2 cheaper per hundred than Sierra 77s. I bought them in plastic sleeves, five 100-round boxes per sleeve. I found a great deal of ogive variance where I'd have to verify seating depth every 500 rounds as I broke open a new sleeve. It was annoying enough that I eventually spent the extra two bucks per hundred for the Sierras to avoid the ass-pain of checking and re-setting and locking my seating die.

 

[Amanda4461]

Interesting about your Starrett.

Once upon a time several decades ago I shot a LOT of Hornady 75s since they were $2 cheaper per hundred than Sierra 77s. I bought them in plastic sleeves, five 100-round boxes per sleeve. I found a great deal of ogive variance where I'd have to verify seating depth every 500 rounds as I broke open a new sleeve. It was annoying enough that I eventually spent the extra two bucks per hundred for the Sierras to avoid the ass-pain of checking and re-setting and locking my seating die.

Thanks for the info. I thought I was holding my calipers at an angle when I noticed the same. I use the little stainless comparator that looks like the nut on a bolt, with different caliber holes drilled in the flats. I have given up on Hornady bullets, and went back to Sierra Matchkings. I am finishing up my last box. There really is No Free Lunch
Today, I took two standards and tested the analog Starrett to the digital Mitutoyo and discovered that no matter how much I tightened the Starrett dial, it kept having errors. The Mitutoyo was consistent every time. I’ll be sending old faithful in for a new dial and calibration, and ordered a cert Mitutoyo also. Murphy is alive and well, and I have gotten complacent.