RAFFICA SPECIAL: XM8: The technical look
By Dan Shea

Over the last few years, we have seen a proliferation of articles on the HK XM8 series of rifles. To date, most of them have been the result of an HK writer's meeting where the writers are given a nice demo, shown the gun, had it explained to them by HK staff, given a mag to fire and a tech package to work from. This is situation normal for the small arms writers, and the yield for the manufacturer is wide exposure with some cursory examination. A good tech writer can take this information and put together something pretty comprehensive. A gun hack will just have a couple of "Hi Mom" pictures and say the gun is superb.

That is not good enough for our readers, and we know it. The XM8, embroiled in controversial procurement snags as it has been, may well be the next weapon of the United States Army. As such, it deserves an in-depth look. It has been a long road to get where we can dig inside the XM8, and HK has been trying all along to facilitate the testing; but SAR readers and your faithful correspondent are not simple people to please - we need to put a lot of rounds downrange to see what the nature of this beast is. We have broken this into two sessions. The first session is this technical detail session, the second will be a live fire, put it through the paces test that I am doing with Sal Fanelli, the HK XM8 Project Manager. SAR would like to thank Phil de Garis, Army Program Rep for HK-Defense for all of his help during this first technical detail session, and for cranking up the jack on my rental car as I frantically changed the flat tire in time to catch my plane out of Dulles. (Had to put that in, he saved the day.) This is presented as a Raffica Special, because the readers have been sending me questions on the XM8 for several years now and I see this as the best format for answering them.

Q: Dan, isn't the XM8 just the AR18 system in a different looking shell?

A: Sigh. No, it isn't. That's like saying the 2005 Infiniti you are driving is just the '57 Chevy Belair in a different shell. I like my old AR18, and I love those old Chevys, but there are so many advances in these new designs that it is impossible to say they are the "Same." They are similar, like most of the modern descendents of the multiple locking lug weapons are. Let's look at the heart of the systems; the operating assemblies.

I have chosen four basic operating assemblies from weapon systems that use the multi lug locking system bolt, unlocked by cam action, in order to show some of the differences.

Top is the Stoner 63A series bolt carrier. Note that it has a long operating rod that extends to the front gas take off point. The recoil/return spring is a single spring on a long guide rod (not shown) that is inside the operating rod, but a separate piece.

Second down is the AR18 bolt carrier assembly and recoil/return springs. The major change here from the AR15 gas tube system which was sold to Colt in 1959, is the piston drive that is separate from the bolt carrier, used to drive it rearward. The other major change was going to a dual return spring system - this stabilizes the bolt out of any side to side motion, and keeps an even recoil and return. Less wear on the interior of the receiver and better presentation of the bolt/cartridge/chamber cycle. This allows for the sheet metal receiver, keeping internal wear down. Very smooth action.

Third down is the British SA80 bolt carrier assembly and recoil return spring. This design uses three guide rods for the bolt carrier to travel on, but only the central one has a recoil/return spring on it. Bottom is the HK G36 bolt carrier and recoil/return spring. Since the G36 system is very close to the XM8, we can use this as a comparative example. As in the SA80 and AR18, the gas drives a separate piston rearward, moving the bolt carrier back and unlocking the bolt head with the cam action. In this case, the carrier assembly also contains the charging handle and the recoil assembly is a single spring on a larger diameter guide rod, with buffer to the rear. This recoil section is more reminiscent of the G3 series than an AR18, even though the unlocking is accomplished in a similar gas piston drive to the AR18. Similar, but different from years of field testing for a better system.

Q: They are just using the old M16 bolt in a fancier package.

A: No, again. The fact that a bolt head "looks" similar doesn't mean it is the same. Typically, the multi lug bolt heads are eight position lugs, with one missing for the extractor position. This symmetry must be maintained for proper surface presentation to the barrel extension support surfaces. If you look in the picture of the two bolt heads, <3584> you will notice a couple of things right away. On the left is an original AR18 bolt head with seven lugs showing and the eighth position is the extractor. On the right is the G36 bolt head, with six lugs showing and the seventh position is the extractor. The wider width that is on the G36 bolt lugs means that there has to be slightly more rotation to lock and unlock the lugs. The designers felt that this was also a better way to get the locking surface ratio more into their comfort zone. Same amount of area, but thicker, stronger lugs, and only sacrificing a small amount of additional locking and unlocking rotation for the cam path to make up. Also note the extractor has a pyramidal shaped strengthening area, as opposed to the rectangular on standard extractors for this type of bolt.

The best way to show the differences in the bolts is probably by a comparison of the ones in the family of Stoner inspired multiple locking lug bolts. A major thing to remember in this is that the M16 bolt was designed to work in a gas system, not a piston driven system. The M16 bolt had to address very different issues from the XM8.

On the left is the granddaddy, the AR10 7.62 caliber bolt circa 1959. Note that towards the bottom is a gas pressure reservoir area. Second from left is the Stoner 63 bolt circa 1966. This bolt is short, and needs no gas pressure area since the system is piston driven. The extractor is entirely different in concept from the others as well. Third from left is an AR18 bolt from the same time period. This bolt has the form of the AR-15 series, but no gas pressure area or rings are needed as this system is piston driven. Fourth from left is a chromed bolt carrier from an M16 circa 1967. Note the gas pressure area at the bottom and the addition of three gas rings. Fifth from left is the SA80 bolt. There is no gas pressure area needed. Far right is the G36 bolt. There is no taper to the body and is more reminiscent of a Stoner 63 series bolt than an AR18.

Q: Isn't the XM8 just a G36 with some different outside looks?

A: Sort of. Well, not really. There is an awful lot of the very reliable G36 operating system in the XM8. However, the package it is put in includes a variety of lot of new features, totally new. We will cover those later in this article. That being said, I would have to agree that the core is the same. If you compare the appearance of the heart of the system, the bolt carrier group of the G36 in the comparison photo, you will see that this actual picture of the XM8 Bolt Carrier Group is virtually identical to that of the G36.

There are a multitude of interesting design innovations that have been incorporated into the XM8 system. In keeping with the initial question about this being a G36 in other clothing, I want to happily support one main similarity; the blow forward high pressure event fail-safe system. Essentially, most of the rotating multi lug locking systems leave the extractor unsupported in the barrel extension after rotating into position. This is a flexible arc of metal giving minimum support to the cartridge case it was holding in position. It has done its job and isn't needed until the bolt moves rearward and the extractor is overcome by the ejector system. In most systems, if there is a high pressure round, the extractor can blow out, venting the residue from the event violently out the rear of the barrel extension and into the receiver area. In the G36 and XM8 systems, as the extractor rotates with the bolt, it comes up against firm metal inside the barrel extension, giving it solid support. Now, if there is a high pressure round, the gases don't come back, and both the G36 and XM8 have two forward venting paths in place for the expanding gases to move forward, away from the operator. (Smattering of polite applause for the designers on this one!)

This article first appeared in Small Arms Review V8N9 (June 2005)
and was posted online on May 17, 2013


Comments have not been generated for this article.