This.

Probably many more 604s built with this upper than... well, 605s-ever.

Colt requested permission to use 603 forgings to fulfill military contracts for the 604 when 604 forgings were in short supply. Its continuing association with the 605 is more a testament to Colt’s tendency to use whatever parts were laying around to build prototypes than a “605 upper” configuration.

~Augee

The Air Force purchased 8,500 Colt Model 02s prior to the -508 contract, which were designated simply "AR-15," what we would call a COTS designation today, but they were U.S. Property marked.

The -508 "one time purchase" contract was for the M16 and XM16E1--both the 603 and 604 were developed from the 602, the -508 Contract, therefore, was essentially for "modified 602s" that would become the XM16E1 (603) with the manual bolt closure device added per the Army's request, and the M16 (604) with the manual bolt closure device omitted per Air Force request.

The contract was written for what existed (602), but we are ultimately discussing what they became, prior to the contract, the M16 and XM16E1 were at best prototypes [having been built by modifying 602s] that had not been granted a model number, -508 "created them," and ordered those 602-based prototypes into production, which then became the 603 and 604.

It's just like this little bugger:



Built on a 602 lower, but I wouldn't call it a 602 (I've fought this fight before), but it never went in to production, so it never got any kind of model number that anyone has been able to find.

Regardless of the semantics of the model numbers--the upper in question was primarily used for M16 rifles being delivered to the Air Force.

~Augee

NICE!

I was wondering when someone would actually build a real replica of one and actually try to replicate the FSB/gas system length versus just using a standard midlength barrel. Sadly I don’t float around the Retro forum as much as I used to, so I must’ve missed yours.

I’d been thinking that someone should build one since I saw it inTBR, and I figured it for sure wouldn’t be long once the VickersGuide came out, IMHO, that book would be worth it just for the high quality photographs of the weird and wonderful prototypes in Reed Knight’s collection, even if they did make me hack down a lot of the historical write ups!

Did you actually replicate the gas system? Or just simulate it with a hidden gas block under the handguards?

The lower on the original appears to have been a fairly “standard” U.S. Property marked AR-15/602 with lots of early parts, but with the property markings milled off in a little pocket, and obviously the fourth position added, but with the “SEMI” in the traditional “AUTO” position, would be rad if you could make a custom selector with 180^o throw to “SEMI.”

~Augee

Ah, it looked at a glance as if you’d replicated the original length, and not just modified the FSB.

I have always found it interesting that that thing is sitting in Reed Knight’s collection, and then decades later, KAC comes out with the E3 intermediate-length gas system. Not trying to imply that KAC copied it, but it’s interesting to wonder if maybe Colt “got it right the first time” all those years ago. One day I’d like to compare the two gas system lengths side by side, if only to satisfy my own curiosity and see how close they are.

It’s ironic that so many midlength fans claim that carbine gas systems were chosen for bayonet compatibility as a way to discredit them, while conveniently forgetting (or maybe not realizing) that the mid-gas length was dictated by the desire to mount bayonets on 16” guns.

~Augee