Problem with a new BCM lower

[Omega9]

For what it is worth I just purchased a BCM lower about three weeks ago and it accepts a 30 round mag with a closed bolt.

[Jaysop]

I recently figured out the problem with mine not seating properly was an ammo issue and not a lower issue. The ammo I had been testing it with was slightly longer than some of the other stuff I have. In the Pmags the rounds were tight and didn't want to depress well in the M3 pmags.

I'll have to get some calipers to really measure this stuff out.

[rapomstage3]

I've sanded down all my gen3 pmags. I've had issues with noveske and Kac sr 15 lowers. My problem was mags not dropping free. Needless to say I just stopped buying them. All previous magazines worked with no problems. I'm sure the Bcm rep didn't want to come out and say "stop using gen 3 pmags meathead!!" But it sounds like he said that in a way nicer way than I could have.

[stwings]

Here's some perspective:

As much as I like, advocate, and use BCM products, who should be catering to whom? Magpul, whose gen 3 pmag works with ALL receivers that conform to the TDP, or BCM, whose receivers deviate from the TDP in this particular (non critical) dimension?

This ^^^
If I'm a magazine manufacturer building magazines for military weapons in theater (Colt/FN) I'm going to build them to the TDP. If a civilian contractor decides to buy a boutique reciever that won't function with my mags then I'm sorry to hear that but can't do much for you.

[rapomstage3]

There's numerous examples of the m3 mags not working well in different receivers. And the gen 2's all work. If a lower excepts 6 different magazines and the 7th doesn't work properly then I'm not seeing the argument. Something changed in dimension on those magazines that's all there is too it. If they work for your receiver then use them if you want. If not then use something else. If it keeps you up at night then sell the lower as the op did. I use mil spec colt and lancer mags and they work in everything boutique or otherwise.
Just adding
"3. Platforms – In addition to all AR-15 rifles that the Gen M2 operate on, the Gen M3 operates on non-Colt-spec platforms such as SCAR MK 16/16S, HK 416, HK MR556A1, M27 IAR, British SA-80, and others as well."
Excerpt from an article describing some of the changes made to m3 mags. Not sure what had to change to make the mags work on all these other platforms but it could have something to do with it.

[Jellybean]

I just got a BCM Blem lower, and you could say I'm sort of invested in G3 Pmags... So needless to say this thread did not give me happy feelings...
So, I did a test today.

I loaded a number of mags with 30 rounds- 1x G3, 1x G3 with tab filed off, 1x G2, 1x Lancer, 1x OK USGI.
Lowers were a DD, Spikes build, and BCM.
ALL the magazines locked in on 30 rounds.

Now here's where it gets interesting....

The G2, OK, and Lancer have no issues with an over travel stop touching anything, duh, no surprise there.
However, ALL the magazines were the SAME in difficulty of seating on a closed bolt. It seems it you try to slap em in real fast they bounce off the bolt, as opposed to "pressing" them in til the catch *clicks*.

There was no difference in difficulty on seating between the factory tabbed G3 mag, and G3 with tab filed off.

Here's where the plot thickens even more... (See attached pictures at bottom)

On the BCM lower, on the serial # side, I have *some* space, on the opposite, it seems I have no space with the G3 insertion tab touching the lower receiver tab for the trigger guard roll pin. So there seems to be a slight offset in the machining here (well, it was a "blem" lower after all).
On the Spikes lower, the G3 tab actually presses against the polymer Magpul trigger guard when seated with zero space between it (however, around the edges of the lower, the fit is fine) so technically... the fit is worse on the Spikes build than the BCM, as the trigger guard should be hitting the tab and stopping the mag from seating....

Intrigued by this I decided to actually go out and do a shooting comparison between the two rifles- the thinking being any mag will seat if you focus on getting it in there all the way, not so much if you're reloading quickly, ie, tac/admin reload.

For the shooting test I ditched the filed G3 mag, and instead used 2 factory tabbed mags- 1 circa 2013 model with no enhanced feeding tabs to see if I'd get round crossover slapping the mag in there, and 1 new G3 mag with the enhanced feed tabs.

For the shooting test I used the OPs patrol carry requirements- ie, load on closed bolt, then charge rifle and fire 2-3 rounds. I also interspersed that with occasional tac reloads, simply firing 2 from the first mag and then dropping it and inserting new mag and firing 3 rds.

I had 0 issues with either rifle.
Granted, I didn't fire 500 rounds- I just continuously topped off the two mags to 30, and fired about 30 total.
On the "old" G3 mag, I did begin to see some slight round crossover, but only after slapping the mag against the bolt a few times til it seated.

Now, I did notice two specific things;

1) The G3 mags are a little harder to drop (even after a few rounds fired) from the BCM lower. They drop free just fine, specifically it takes a bit more pressure on the release button to punch them out.
2) The newer "enhanced" G3 mag seemed easier to seat than the original G3 mag. I slapped it in the BCM lower, and it locked right up just as easy as any other mag. I thought it hadn't seated all the way, but I tugged on the mag, and it had. I payed attention to this multiple times, and it seemed that new G3 mag was consistently easier to insert on the closed bolt, and the old one was consistently slightly harder. I pulled a second "enhanced" G3 and it was also easier to use. Go figure.

So, in short, after 25 pages here, I have no idea what to think....
Have the new G3 mags been "fixed"?
Did the blem BCM lower somehow have more "right" dimensions than a standard lower?
And if either of those are true than why did both G3 mags work just fine on the Spikes lower they shouldn't have locked into?
Do I dare buy the standard BCM lower I *was* planning on and spend $400 to find out?
As there seems to be no real guarantee *any* lower on the market will work, does it really matter what I buy as it's ALL a $400 crap shoot?

That being said, in my own personal opinion.... I think it's a little ridiculous to have a duty or defense rifle relegated to only "specific" types of magazine. Or a magazine that will only work in "most" rifles. That's like saying you can only use certain types of ammo in a given gun. Here on M4C... that would be grounds for never owning anything from that company....
File off a tab... file down a magwell.... sure, if it solves a problem permanently I'll do it and laugh as I file merrily away on hundreds of dollars of equipment. BUT I'm doing it knowing full well I payed to fix YOUR mistake on products that seem to have had a rep of being GTG no matter what.
I can't say that particularly makes me happy....

Pics:
L-R
BCM serial side space
BCM mag hitting the roll pin area
Spikes lower, mag hitting the trigger guard before lower

[MountainRaven]

I'm happy to report that my GenM3 Pmags get along just fine with two BCM lowers and a Sionics lower. And that the overinsertion tab sits about as far from the rear of the magwell on one of the BCM lowers as it does from the same dimension on a SCAR 16S. Lancer L5s fit, but require a good smack to seat in the BCM even when downloaded to 18 rounds while the GenM3 snaps in relatively easily with one good, brisk effort at seating the magazine, even when fully-loaded.

I do find the hate for Magpul voiced in this thread curious, though, given the following:

1- When LWRC introduced the M6IC, they were roundly criticized for introducing a rifle that didn't work with GenM3 Pmags.
2- When Noveske introduced the N4 Gen2 lower receiver, they were not criticized, but they rapidly altered their lowers (within the first 2,000 lowers, IIRC) to accomodate the GenM3 Pmag.

Both the N4 Gen2 and M6IC came out at about the same time as the GenM3 Pmag and were clearly designed prior to any knowledge of the GenM3's dimensions. And both lowers were eventually modified in production to work with the GenM3 (Noveske openly, LWRC quietly).

Further, the issue of criticizing Magpul for not adhering to "mil-spec" while absolving BCM is curious, given that Magpul makes magazines for military contracts (original Pmags, RevMs, Emags, GenM2/MOEs, and Gen M3s) while BCM makes no lower receivers for military contracts (as far as I am aware, the only major government contract for BCM products is for their KeyMod vertical foregrips via London Metropolitan Police Service's SCO19 for use on their MCX carbines). The Magpul product was clearly engineered to work well with actual military magazine well specs for Colt and FNM M16s and M4s, H&K HK416s, L85A2s, and FN Mk16 SCARs - which seems to cover the majority of actual military-use 5.56mm carbines that take metal M16 magazines. And clearly, the GenM3 Pmags New Zealand bought work with their new LMT carbines and the NZDF either doesn't care that their magazines may not work in some of their allies' rifles or they must be sufficiently mollified that they will work with their allies' rifles (when those allies use rifles that are capable of accepting USGI aluminum M16 magazines, of course).

[Dirknar]

If the front trigger guard length is interfering with the tab on the gen 3pmags, then why would it matter, if the mags were loaded to max and the bolt is closed? Do they seat in an open reciever?

I just loaded a gen 3 mag to capacity and tried seating it in my Colt 6920 with the bolt closed. I could NOT seat it.. smacking and smacking.. nope it wasn't going in.

I pulled and counted all rounds in the mag. 31rnds were in it... i put 30 rnds in and it seated easily. (The over insertion tab rides close)

It does seem weird that the 31st rnd fits into the mag so easily though.. if i didn't count them out, i would have figured the mag had 30rnds in it!

[Dirknar]

Did anyone catch that?

31 rnds easily fits into the gen3 mag i have.. and will not seat on a closed bolt, reguardless of the over insertion tab!

Isn't this kind of a design flaw? And a safety issue?

Can someone else with more than one gen 3 mag confirm 31 rnds in their mag too.

[stank]

It kind of has to have some room in mag for bullets to go when you load it to 30 and insert it in a closed bolt. If they made the mag where you couldn't get 31 rounds in then inserting it on a closed bolt with 30 would be more difficult. I just press on rounds in mag as I load em and know when it drops just enough to get just one more barely in its at 30. Or you can download to 28 that will let you press stack down just below back spine of mag. People have been downloading mags forever to allow easier insertion on a closed bolt IIRC.