So I'm loading up some 20 round AR mags with 300AAC rounds and one of them the bolt won't catch the cartridge's rim to chamber. I notice that the rim of the cartridge, as it sits in the mag is a few mm low, not fully pushed up, so the bolt slides right over the top of it.
Is this just in need of a new spring?

Which AR Mags (5.56 or dedicated 300 BO one's) , and what 300 Blackout loads? Sonic or Subsonic??

It's a Colt AR-15 .223 UI stamp Mag. (20 round)
I load 7 rounds into it, pull the charging handle back, let loose and bolt goes into battery leaving the round in the magazine. I can visually see that the rim of the cartridge is low as it sits in the mag.
Was not a sonic or subsonic load issue. I am setting it up for subsonic w/suppressor but it's new and I have to work down a load for it. loaded up 10.9gr of 1680, as a start, and will go from there.
220gr ELD-X coal 2.248"


Don't worry about it, I'll figure it out, just thought it would of probably happened to someone else.
I'll pull the spring outta one of the other 10 I loaded up that don't have an issue. Never wore a spring out before, but it's one I've had since the early 80's so that was my first thought.

Thanks

That or maybe it’s just a worn out spring.. 300bk being heavier than 556 could be the issue..

Or a combination of both.

I ordered some new Okay 20 round mags, and they don't seem to have the issue. All rounds are fully snug tight along the cartridge and mag lips, no visible gap at the cartridge rims and all rounds line up nicely. If all goes well with them I'll dedicate all the Okay mags to the 300AAC. The Colt SP1 I have, has never had an issue with any mag I put into it. The 450 Bushmaster doesn't have any issues ether.
A YouTube search showed why some of my other mags don't work, but the Colt mags should, thus why I still think it's the springs.
Any advice for a good site to get mag springs.

I'll post up after another range day to say how it goes.

Thanks for all the input