I think it was essentially a parallel design that quickly became redundant, and then when the M4 went flattop, the M16A2 carbine became not just redundant but obsolete.

Not the barrel, but the guide rails. I don't know what exactly the issue was, but given the update has a bevel, I imagine there was some interaction on the front when the barrel was pushed forward.

Man, the mental image of a grenade launcher barrel ‘crumbling’ makes my balls retreat into my abdomen faster than Ed Sheeran’s openers ditched him

Ah ok that makes sense. I never heard of the M16A2 carbine, so I figured it fit somewhere into the M4's lineage. The question was where though lol. Thanks for the info!

The XM177 was introduced in 1960s, long predating the M16A2 carbine which was developed in the 1980s.

The M4 development started only a tiny bit later than the M16A2 carbine development, as a requested improvement on the XM177. It was originally known as the XM177E2, then renamed the XM4, and then finally the M4.

The XM4s are basically identical to M16A2 carbines. The later standardized on M4s finally moved to a flat top. I believe a very small number of carbines with receivers marked "M4" (rather than "XM4") but retaining the fixed carry handle were pushed out early to the US Army Rangers, but I can't find the source I read that on.

Would this be what became the XM177/M4?

That's a very interesting story and a cool historical object!

Counterstrike 1.6 made me love the Steyr AUG and just bullpups in general. Oh not to forget the P90.

Strictly in media though.

Trying to engage the safety would probably eject the magazine

True. I also loved using the LMG variant in one of the modern warfare games. I think it was modern warfare 2.

You saw it in Die Hard.

I have always wanted this rifle for some reason

Trigger on the mag is hilarious. had to be uncomfy asf

Here is a clear frankenrifle example as you may have seen. XM16E1 upper with the triple prong, and M16A1 lower identifiable by fencing.

Webpage.

I love me a good bullpup, but a drum back þere looks powerful awkward.

Guncycles

Neat, never seen one of those before either, we've come such a long way since then lol.

Colt makes a little collar that fits on the barrel under the FSB. Attach it to one side or the other, and remove the existing swivel, no drilling required.

Man I wish cameras back then had more pixels (or maybe the shot is just out of focus) I can't see that side attachment point at all, curious where they drilled the rivet hole for the sling swivel lol. Also never seen one of those grenadier sights before, that's pretty cool too.

Sling swivels and slings can fit on a grenade launcher equipped rifle, either in standard position or reoriented to the side by an armorer but I'm sure FSB mounting looks more attractive to most grenadiers.

With carbines the swivels pretty much have to be shifted to the side, but again just removing them is a lot more attractive I think.

The Aimpoint is on a Weaver rail interfaced with the carry handle. The best I can tell, that tape is just there as extra assurance, similar to seeing zipties and 550 on other optics and PEQs as a last line of dummy defense.

Here's a repro builder's clear photos:

It also looks tough to mount a sling to the swivel when you delete it and throw a 40mm tube there haha. Also did second from the bottom tape an optic to his carry handle? Surprised if that holds zero lol. Awesome pics!

As far as I'm concerned a repro that's using backwards compatible parts is perfectly accurate. Sometimes repro builders get a little too hung up on what something is by the book rather than what it became in the field.

For the sling, yes mounting the front on the FSB is something you can see a lot in Vietnam. M16s were designed with all the sling points facing down, which reflects the existing doctrine of the time and the earlier doctrines that considered slings something used for marching and transport, but that rifles should be entirely unslung in combat. The M16s and Commando carbines (with the later carbine having their buttstock slingpoint facing up but the forward one down) were light enough that people started carrying them slung in combat, and figuring out that mounting slings on FSBs worked better.

At some point, which I unfortunately can't pinpoint, the doctrine flipped to follow what people in the field had figured out and the modern U.S. military teaches that rifles absolutely should be slung in combat. You can see in early Iraq soldiers and marines still attaching slings to FSBs, as well as a ton of using up or side facing Velcro mounting points to achieve a similar outcome.

Edit: Here's 80s and 90s Delta doing the same thing, giving a clearer view of the refined version of the technique.

Huh neat, I've definitely seen A1s with three prongs in pics before, I guess sometimes parts probably just got swapped around as time went on (well they for sure did later, but I guess maybe earlier than I thought). Makes me feel better about the inaccurate frankenstein clone build I'm working on haha, technically I need an a1 upper but it's getting an a2 becsuse I like them more and have a spare.

Cool Nam pic too, interesting he has his sling tied to the FSB instead of the sling mount, looks like a strip of ripped cloth or something.

Yeah, oop that's on me for posting too fast. There's one good source that calls it a submachinegun but everyone else, including RIA and the Army don't (and their opinion matters most) so that's a desk pop for me.

Semiautomatic != submachinegun