ArizonaShooting.ORG

When an AR suddenly starts going bolt over base (same magazines and ammo as always), what is the most likely cause? Needs a new recoil spring? Divine retribution for a lifetime of sin?

As in the bcg not cycling back far enough to clear the rear of the top round in the mag aka short stroking?

I would start by cleaning the snot/carbon out of the bcg, look for any excess wear/damage or a loose gas key
pull the trigger group, clean, and check for anything interfering with movement like a loose primer or damage to the trigger group and springs
check the gas system for damage like a cracked or leaking gas tube or damaged / missing gas rings and if your fsb isn't pinned through the barrel or is any of the pinch/clamp type slip ons I would make sure it didn't get loose or whacked out of alignment with the gas port.
Assuming none of that clears it up, try another lower or check your buffer tube for "straightness" if they get whacked/bent the bolt carrier can bind during travel and slow down enough to cause a malfunction.

Basically, afaik it's caused by something is slowing the bcg's movement, either from interference or lack of gas.

I am guilty of (back in the day) using mere WD40 as a general lubricant and cleaner for the AR. What I didn't know then was that it is total crap for that purpose, and I induced a lot of stoppages and short cycle life between cleanings because of it.

WD40 (and other improper lubes not meant for this application) would actually assist in not only attracting carbon fouling inside the upper receiver's carrier pathway, but also caking it in a compacted layer by layer depositing of goo as the gun was fired. Certain propellants were worse than others. But in the end, the bolt carrier got sluggish as the goo accumulated and eventually would be unreliable. Short-stroking would finally require me to clean the gun before further use. Depending upon the ammo used, this occurred as soon as only 250 rounds but never made it beyond 500 rounds.

I learned the hard way that CLP is perhaps the best (and maybe only) lube you should use for a direct impingement AR. The piston systems are far more forgiving, but why tempt fate. Just use the CLP.

I have no idea if you're using WD40 for your AR. But don't.

Thank you gentlemen. All good suggestions.
I took the gun apart and inspected. Gas key tight, no loose primer, etc., etc. WD40? Aaauuugghh! Who would do such a thing? CLP is my brand.
I will take it out again tomorrow and give it another try.

OK, took it out to the range today, and it ran 100%. Until I got to a HexMag, then it choked. So, that mag goes into the dumpster.
Fingers crossed...

Any sign the bolt carrier is dragging on the HexMag? Or something in the HexMag is hanging up the follower?

Yeah, the hex is probably seating too deep in the well causing it to drag and short stroke.