Just a quick question...Im sorry in advance if this is somewhere else..BUT I have very little luck with search engines. That being said...

I carried the M16A2 and the A1 in the US Army over 4 years. I just recently purchased a Sig M400, purchased some Magpul mags to go with it. I noticed that I am unable to seat a fully loaded mag unless the bolt is locked to the rear. So am I remembering this wrong? My weapon has to be empty with bolt locked to the rear to get the mag to seat?? Is this some new fangled safety procedure or am I just getting old and I never loaded my rifle with a fresh mag with one in the chamber?

Thanks ..

Excellent TY... I was trying to figure out...in a combat situation how am I able to keep my mags "fresh"??.....so this must be a magpul issue or maybe my mag release needs some adjustment.

Sometimes it's just a stiff spring in a new magazine which is causing the "problem."

TY for the answers. I will "investigate" further and see if I can figure out whether its the new spring or I'm overstocking the mag.

Tip: If top round is on the left, (holding mag with tips of rounds away from you as it sits in rifle) odd number of rounds in the mag, 29 or 31. On the right, even, 28 or 30. If you have 31 in you will have almost no room left if you push down on the top round, at 29 you will be able to push them down almost a half inch. With 30 you can push them down like 1/4" or less. 28 down over a half inch. Basically you could just fill it up with as many rounds as you can stuff in it, if the top round is on the right and you can push it down 1/4" or so, you're good. If the top round is on the left and little/ no slack to push down, you have 31 in it. Take the top one out and good-to-go.
This is generally true with USGI or Pmags, others I don't know.

Just remembered I was using stripper clips with 10 on so I doubt I overloaded the mags. Its possibly the stiff spring issue. When I get to my rifles I'll check it out

Current USGI magazines with the tan followers are opposite. I think they did this to make the follower "different" enough to avoid trademark infringements.

You might also try to adjust (loosen) the magazine catch one half revolution.

You're used to USGI mags that are old and worn out. Put a little more force into inserting your magazines. It should clear up the more you shoot ammo through your mags.

You using an alternate floor plate with ranger pull? If so that is why. If not, remove 31st round. All PMAGS hold 31. Can't load with 30? Not on a stock PMAG, cant happen.

ok removed 2 rounds from mag. With bolt forward mag drops right out will not "lock" in. Same on both my ARs Sig M400 and DPMS panther respectively. Will try the metal factory mags later...

PS Empty pmag locks right in....

Do you insert and give it a good smack on the bottom of the mag and it still will not seat? With only 28 rounds or 30 rounds for that matter, and it will not seat with a smack on the bottom, I would send them back or use as range mags only. I test all mine with 30 rounds and don't have any that will not seat.

You need to push upward on the base of the mag for it to click in. It takes pressure with a loaded mag and bolt forward.

I had the same issue at first, but found its much more prevelant with G.I. mags than pmags. Load your pmag and push upward hard on the floorplate. And don't worry its not going to hurt anything.

^^if this doesn't work, then your lower could be slightly out of spec. I have a feeling it's fine though. You have to push fairly hard.

I used the m16a2 in the  US army infantry (so I understand that this system is one where you vigorously do everything)..I never remembered that I needed to pound the mags in....then again..I was using GI mags that were well worn and broken in (one good tap and it locked) ..never used a new mag and a new rifle...either way I solved my problem...I believed I was vigorously seating the mag  and it just wasn't locking....so I inverted the rifle gave the mag a good crack and found the locking issue consistently gone (gravity assist?)...BUT I really hope that this goes away eventually...because at the range or my backyard inverting the rifle to load a mag is ok ...elsewhere under different conditions it might be...problematic..

Thanks again for all the ideas and help...

PS OR I could just wait til plain old empty and the bolt is locked to the rear to load a fresh mag...../shrug

Have you tried other Pmags?  Other mags?