I have a FNC, and while all AR style mags fit it, Lancers and P-Mags will jam when used, about 10 percent of the time.  

The fresh round doesn't seem to line up correctly with the bore, and the bolt crushes and mushrooms the case behind the shoulder area.

While FNC steel mags work fine.  Interesting MSAR mags for their E4 AUG clone work fine as well.

Anyone else have issues with Lancer and P-Mags?

100% with GI aluminum, HK steel, Beta and P-60 drums, and factory FNC steel.

Ever try Lancer and P-mags with yours?

Alum. GI mags. And whatever cheap Alum mags PSA sells (D&H?).
Never tried the plastic mags as I had little faith in a Belgian gun from the 80's running a modern polymer mag.

I feed mine GI and Gen2 and Gen3 PMags.

ETA: I've never had a jam in it with PMags, but only very rarely use them, since I'm flush with GI. I only own one original steel FNC mag, and have never used it - though I imagine it'd work well, too. My usual STANAG mag range loadout is one or two 50 cans of loaded GI 20s and 30s (14 GI 30, 8 GI 20), and sometimes a few bandoliers of GI 30s.

GI mags both 20 and 30

I unsure if I ever ran lancers in mine.  Magpul Gen 2 ran extensively without issues.  It does not like Gen 1 Magpuls at all.

Ran USGI, steel and many different types of drums without much issue.  Mine is full auto.

USGIs.

When I had a FNC, I typically ran USGI and FNC mags.

PMAGS ran fine, never jammed, but would not drop free in my rifle.

If you have the cash to run an FNC, run with original FN mags.  They work 100% 100% of the time.

Never tried with polymer, the guy i bought it from said it likes GI mags like the SCAR so thats all i run and havent had any issues with them.  the surefeed mags seem to lock up a little tighter so I have a bunch of those.  Also just saw aluminum FN FDE mags for $35 somewhere as well.

Also FYI, I am a bit skeptical of any "FNC" mags that dont come in the blue box.   I have been burnt with that from a major supplier.

100%  with gen 3 pmags

Hexmag worked just fine in full automatic for a couple hundred rounds.

Mine runs on USGI, pmags, and E-mags.

Mine likes pretty much any USGI mag and pmags but occasionally the mag catch won’t fully seat on the USGI mags and it will slip out causing a failure to feed.

There are at least two styles of FN magazines.

One is steel with a metal follower (first design) with the raised side of the follower on the opposite sidefrom the US military magazine.

The other I've seen have been later steel FN mags with plastic followers, with the raised side of the follower on the same side as US military magazines.

Indonesian FNC magazines that get imported, also have plastic followers with steel bodies.

The US military magazines with black or green followers work alright in semi auto, but the FN and Indonesian magazines are more reliable in full auto.

Maybe newer US magazines with stronger springs and / or different followers would be just as reliable as factory FNC magazines, but I haven't tried them yet.

I think the metal follower factory magazines have been better than the newer ones and imported ones with plastic followers.