Resurrected Weapons: XM-8

And now time to examine another futuristic weapon, the XM-8. This was an offshoot of the failed XM-29 project, where some in the US Army tried to get a more direct replacement for the M-16.

The XM-8 was a carbine firing 5.56 mm rounds. No fancy caseless ammo, no airburst grenades, just bullets. The same bullets that cranky guy up the street shot in Vietnam, even. What was different here?

The XM-8 was designed to be lighter and more reliable than the M-16. Reliability would be improved in a number of ways. HK built the XM-8 around it’s highly successful short-stroke gas piston system that had been used in their G-36. The body of the weapon was entirely polymer, with easily swappable components, and design by the Udelhoven Design Studio.1 Plus, while there were a number of ergonomic and internal design improvements over the stock G-36, they used the G-36 magazine.

Let’s talk feed devices. Recall that the original AR-15/M-16 magazine was a 20 round box magazine with no curvature. The magwell was designed to accommodate this, and is also not curved. Army desires for a 30 round magazine required some amount of curvature to accommodate the taper of the 5.56mm cartridge. But the magazine had to be compatible with all M-16s, so the top had to be kept straight. So there’s a kink in the 30 round magazine where the curved section meets the non-curved section, and this can cause problems. The G-36 magazine has a continuous taper, and is made of translucent polymer, so you can see how many rounds are left.

The G-36 is pretty “European” with a paddle magazine release, and most bolt work being done with the charging handle, which is atop the gun under the raised sight rail. It can fold to either side for ambidextrous use, and can be locked to either side for use as a forward assist. The XM-8 made some improvements here too. A shoe, sort of like what’s on an HK pistol, was added to the mag release so you could press a lever on either side of the trigger guard with your trigger finger to release the mag. Bolt release was in the front of the trigger guard. The selector was the usual ambidextrous affair, with safe/semi/full auto on the trigger group.

The XM-8 also tried to improve accessory attachment methods. Picatinny rails are expensive to machine, and add weight to the weapon and height to the accessory mount. For the same reasons that we would see the development of Keymod and Mlok in the civilian world in 2014, HK and Picatinny came up with PCAP. Just like Keymod and Mlok, PCAP uses a bunch of negative attachment points2, but it was designed to totally replace Picatinny rails. Specifically, it was designed to be a superior sight attachment system. PCAP naturally mounts things in the same place each time, so sights naturally will hold zero when mounted and dismounted.

Further, the XM-8 had a new sight. The XM-8 had to be lighter than an M-16, so a sighting unit was designed to integrate a red dot sight and an infrared laser sight. This gives day/night capability in a single unit, with one battery, that’s lighter than the two separate units with two batteries and two sets of mounting hardware. Plus, the sight was synchronized so that sight adjustments to the red dot also adjusted the infrared laser sight. The military version was called the ISM-IR. If you’d like one with a visible laser sight, the civilian version is so equipped, and is the ISM-V.

Of course, then politics intervened. No army requirement was actually listed, so Congress started asking questions. HK’s competitors started asking why they hadn’t been given a shot to deliver something new if the army wanted that. Plus, some people had spent money earmarked for OICW on the XM-8, and that brought up more questions. In the end, the XM-8 was cancelled for being too expensive for what it was. At the end of the day, it was more reliable, but still fired 5.56 rounds downrange.

So what do we think? If you’re looking for a new carbine system, because you’re finally joining the 5.56 train or your previous issue carbine is old and busted and you want some new hotness, the XM-8 makes a good choice. Though, it does require new accessories because of the new attachment system.

Honestly, that might be one of the better parts of the XM-8. We really like PCAP, and we especially like the ISM-IR. That can be had in Picatinny railed form too if you want. Plus, at some point, you have to accept some development costs to get something better in the system. We’re seeing some forces go to Mlok3, and PCAP is better because it gives you a sight solution too. On the other hand, we’d really like to compare it to some of the more refined AR-15s and similar that have come out since the XM-8 program was cancelled.

Verdict: Funding Approved for program testing by the Borgundy War Department Ordnance Procurement Board

1.) They also do design work for Audi.
2.) Also known as holes. Precisely cut and aligned holes.
3.) Including Canada.

3 thoughts on “Resurrected Weapons: XM-8

  1. Arathar

    The thing is, in tests the mailfunctions in % was greatly reduced to the M16 (not saying at all the A15 overall is unreliable). But 2 things: 1 i dont like that the barrel is not hold together with the scope rail per steel… I like Polymer but that is one of the verry few absolutly WRONG places to use it. 2: when the XM8 was finished it was already COMPLETLY outdated compared to the Steyr ACR and Stoner/Ares ACR , just imagine a Steyr ACR with light polymer cased 5 to 6mm SCHV rounds with at Stoner ACR 80round drum!
    The XM8 was an incredible senceless programm, the money could be used much much better. We are lacking decades behind time. Even the CTSAS is a piece of shit and unbelivable waist of potential.

    1. parvusimperator Post author

      That test (the 2007 sand test) was incredibly dodgy. Anyway, I’m with you on not trusting polymer fully in the receiver components. Look at all the G-36 problems. Plus, it was done to keep the weight down, but it was still heavier than an M4 when neither had sights.

      And yeah. I seem to recall there were a few officers in the Army who where highly involved in the solicitation. But that was pretty dodgy too. Anyway, even if it really is significantly more reliable, it’s certainly not worth the changeover monies.

      Since I forgot to mention this in the G-11 bit, I’ll say it here. Polymer cases gives you most of the weight savings of caseless with a lot less hassle. Though there are still all kinds of problems with poly cases. They still don’t act as little disposable heatsinks like brass, and they’re not ductile like brass, so sealing the chamber behind the bullet is hard. Also, the LSAT weapons have incredibly goofy looking push-through extraction cycles, because those rounds lack an extractor groove for conventional reciprocating-style actions.

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