I just shot an IPSC match yesterday, and had a failure to seat problem with a mag that occurred with a round in the chamber.

I did an experiment this morning to reproduce the problem.  Took my Les Baer rifle, loaded a *dummy* round in the chamber, and then inserted a full 20 round magazine, which worked.  I then took a 30 round magazine containing 20 rounds, and inserted it.  But when I loaded it to 25 rounds, the magazine would not insert at all, no matter how much [reasonable] pressure I used.

Repeated this with my Bushmaster, and got the same results.

Then tried a Thermold magazine, which worked even when full (although 28 worked easier).  The first 30 rounder had Parsons marked on the floorplate, and had a new green follower.  Don't remember if there's a new (Wolff) spring in it or not.

Comments, anyone?

That is a strange problem. When I first read the title, I was assuming that the mag failure was with a Thermold - that rib in the center sometimes makes for a difficult fit. But since it was a Parsons, I really don't have any idea. From your desription, it certainly sounds like spring/follower issue. I never put 30 in a mag. In fact, I only put 9 on each stripper clip so that empties will only get 27 when I fill them up.

Curious.

Is this only one mag or all the mags?  It sounds like part of the mag wall may be slightly compressed inwards at the point where the follower would be if there were 25 rounds or more.  Take a caliper to it and see.  Or, try with a wooden stick and push the follower down.  The tension should be relatively constant all the way down, until the bottom is reached.  

Ghost

I didn't (unfortunately) mark the magazine that caused the original problem.  I just grabbed one out of my bag (out of 6) and tried it, so odds are it isn't the same one.  Also, the mag works fine when inserted with the bolt open.

I'm leaning toward the spring as being a bit overstrength.  I'm going to try to compress it out of the magazine, then repeat the experiment.

Eventually, I'll have to go through *all* of the magazines [sigh].

The only way the springs can get [i]overstrength[/i] is if someone dicked with the magazines.  Start searching for that culprit in ever increasing circles around your chair. [;)]

Mags need a sharp rap on the bottom to ensure they're seated.  Mark 'em all or at last mark the ones you have trouble with.  Save 'em for the range.  

-- Chuck

... or you could be dealing with magazines bought used.

I'm assuming that the magazine may have had a +10% extrapower spring put in it.  Just a guess, because I didn't keep a record of where each magazine came from, and I don't recall replacing springs myself, just the followers.

Oh well...

If I slap it any harder, I'll get hauled in for abuse.  [:)]

Seriously, I hit it as hard as I could.  It wouldn't seat.

Well, problem solved.

I replaced the mag spring with a new Wolff +10% spring, and then the mag worked perfectly.  Since I believe they were bought as used mags, I guess somebody put in a bad spring originally.

Oh well, time to go through all my mags...