Surefire shared some hot footage of a 100 round High Capacity Magazine function test with us. They ran 5 100-round mags back-to-back turning an M4 gas tube into a light bulb in the process. Don’t try this with your own, stock AR, though. They used a heavy-barrel, select-fire milspec Colt with a beefed up gas tube to deal with the extreme heat produced by 500 rounds of continuous fire. Watch it glow. Try this with your AR and you’ll watch the gas tube melt.

As far as the ship date of the new mags, Surefire representative Ron Canfield tells us the company has delayed both models if the HCM to make minor changes to the mags to guarantee they function in the broadest number of AR platforms possible. The new ship date is June 30, 2011.

Tolerances vary widely across the spectrum of AR manufactures and while Surefire’s current design works fine in milspec Colt rifles, it runs less reliably in some commercial ARs.

The best we can figure is that the HCM, being based on the USGI service mag, works just fine with M4 and M16 rifles that adhere to Colt’s technology package. Commercial M4 Rifle manufacturers that deviate at all from the spec present a problem for the HCM because they are already using a steeper, M4 style feed ramp (M4 feed ramps are steeper than older M16 feed ramps). Some of these guns also cycle faster and harder which lead to a condition called bullet bounce. I witnessed this firsthand at SHOT Show 2011 when I shot the SF HCM in a brand new LMT rifle that was running a little faster than a milspec Colt M4. At first, I thought the gun was short-stroking, but it turns out it was a case of bullet bounce. Somewhere around round 60 on the second mag, the LMT stopped with the bolt carrier held up about an inch from it’s fully seated position. We switched over to a Colt M4 and carried on through 3 more 100 round mags without a stoppage.

Bullet bounce happens when the top bullet in the magazine is upset as it’s fed. The tip of the bullet hits the steeper M4 feed ramp and bounces back instead of sliding smoothly up and into the chamber. In some fast/hard cycling guns, this can cause bullet setback (the bullet gets pushed back into the casing, reducing OAL)  which will turn into a partial bolt override when the shortened round fails to feed. To prevent bullet bounce, Surefire is likely adjusting the lips on the HCM to feed a little more smoothly. This jives with what I was told by Mr. Canfield when he told me, “we’re adjusting the ribs at the top of the magazine to make it compatible with more commercial ARs.”

The most obvious follow-on question for Surefire is why not use this tactical pause to address compatibility with Heckler & Koch’s peculiar HK416/M27 IAR magazine well? The H&K magazine well is a little longer and a little tighter than the milspec package. The sides of the HCM flare where the quad stack is throated down to a double stack. The top of this flare is positioned low enough to allow the mag to work with a standard mag well; but the flare prevents the mag from seating in H&Ks deeper mag well. Further, H&Ks mag well is dimensioned a few thousand’s smaller internally which makes a snug fit for a service mag, and a dangerously tight, non-dropfree fit for any continuous-curve designed mag like a PMAG or the HCM.

Aftermarket designs take advantage of the larger volume of the milspec mag well to increase feed reliability. These mags are tight in the H&K, can’t engage the mag catch and will fall out. (This is why Magpul made the EMAG. It’s pared down externally and the depth stop is lowered compared to a PMAG to allow the mag to go deeper in the mag well.)

Canfield explains the scope of work and leaves the door open for a more changes. He tells me “the current SureFire HCM is designed to fit and function reliably in STANAG 4179 mag wells. Because a mag well isn’t the only part of the rifle that affects how well the mag will feed and to ensure reliability with as many platforms as possible, we’re making design changes to account for the differences in the unprecedented amount of M4/M16-style rifles in production. The HK mag well is a different spec that we don’t intend to address with this design modification.”

It’s clear from the video and my own trigger time that the HCM feeds rounds to a milspec M4 like it’s breastfeeding its own leadthirsty spawn. So, Surefire is doing whats right by the AR crowd; making sure the mag works in more than just rifles fielded by the DoD. Plenty of law enforcement agencies run rifles made by companies other than Colt (or FNH). Holding back production ’till the mag proves itself in a variety of M4 style platforms will hold things up a bit, but it will be worth it in the end.

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