According to the retroblackrifle website "The 10" barrel on the Model 609 would prove to be an undesirable length due to function reliability and accuracy issues, and it would later be replaced with the Colt Model 629 / XM177E2 which was basically the same Carbine with a 11.5" barrel."

Okay.  What was the problem?  As I understand it, the modern m4 (MK 18, Mod 0) is essentially the same length as the 609.  How can we retro-heads build a 609 style carbine in SBR form with improved reliability?  Is it just a feed ramp issue?  Megaro is confused  

I could imagine that modern ammo plays a part in reliably running at 10.5"

As for us retros, the vast majority of us are only ever running Semi.  Some of those functionality issues mentioned about the 609 could have been when running full auto.

The heavier carbine buffers available today also greatly help 10 inch and 11.5 inch barreled carbines' reliablitiy by delaying the unlocking of the bolt.  

In my younger days I had a 11.5 inch barreled carbine that wouldn't eject (the old feeding the swolen, spent case back into the chamber then rip the extractor groove off the brass trying to get the casing out deal).  I tried everything under the sun to try to get that gun to work (to include altering the gas port size). I had almost given up, and someone told me to use a heavy buffer. My instinct said, I need a lighter buffer, not a heavier buffer...since the gun, in my mind, didn't have enough "gas power" to recoil enough to kick the casing out as it was. I didn't really believe it would work, but I tried it.  I went from about 40% reliablity to no trouble at all.  I was flabergasted.  Now I understand a little more about the recoil pulse/timing etc, and can see why the heavier bufffer works.

There's my 2 cents.

John Thomas

They cured the issue with some 609's by using A1 fixed stocks - the rifle buffer is almost the same weight as a current 9mm buffer. Remember, everything changes once you go FA also - things like bolt bounce become an issue. All carbines are 'overgassed'. This is why the LMT improved carrier has 3 gas holes (as did Ar15 prototypes). It took me awhile to wrap my tiny little mind around this, but it now makes sense to me. Unlocking the bolt too quickly and violently was also an issue with 601's and the early 603's - changing from the the lighter Edgewater to the now standard weight buffer made a HUGE difference in reliability. You can have too much of a good thing - like a big block in a Pinto. Okay, maybe not such a good example...

I don't have a 609 replica, but I have never had a reliability problem with my 10" AR's with the correct gas port size and an H2 buffer.  They even work well during the Winter.

I suspect a part of it was simply inexperience.  Even now, I would only trust a 10.x" barrel from a very small number of companies and even fewer to cut one down and re-size the gas port.    

The M4A1 CQBR / MK 18 is the product of almost thirty years of experience with AR15 platform weapons, and many small tweaks like buffer weights and gas port sizing.  

Building shorter M16s was a lot of trial and error at first, for all intents and purposes, the SMG variants of the M16 just cut the barrel of an M16 in half.  The 605 just chopped it at the end of the FSB.

I wonder if anyone still has access to a R609 or R607 or some other 10" XM177-type variant barrel and could measure the gasport size?  I bet it would not be the same gasport size used on today's 10" barrels.

Also, the moderator fouling caused issues, plus even today, gas port erosion is a problem on shorter M4-variants, perhaps they didn't know what was causing it and could only observe that the barrels stopped working satisfactorily too quickly.  

Remember too that employment of the XM177 series was as an SMG, not a carbine, which often tends to imply a slightly different method of use.

~Augee

I've been shooting a 609 since 2004.

This is what makes mine trouble-free

0.093" diameter gas port

McFarland gas ring

Wolff extra-strength extractor spring

Crane extractor O-ring (the D-fender ring will also work but is more expensive)

M-16 carrier for added mass

H-buffer



I really didn't have much trouble with my 609 in Central America.  Other than the barrel being too short to stabilize tracers, but that was endemic to all of the 10" carbines at the time.

UXB, I've said it before, but I've always liked that pic of your carbine. Very sweet.

With the 609's, my understanding is that the units that were using them on the SOF side leaned a lot on FULL AUTO fire during break contact drills, pre-mission test-firing, and repelling enemy assaults on their positions if they were fixed.

Nowadays, the marksmanship trends focus on accurate SEMI AUTO fire on target, and the CQBR is often used with the KAC QD suppressor, which will help with back-pressure.  Someone already mentioned heavier projectiles, which helps with back-pressure too.

Colt seems to be very guarded about sharing any of its TDP on these issues, as they don't want to release their competitive advantage to every other company out there making carbines, and I can't blame them from a business perspective.

I've seen issues with an off-brand 10" gun where it is separating case heads left and right, and the owner has tried different buffers, extractor inserts/springs, etc. but he may be looking at an erosion issue.  It's unlocking way too early.

This is one of the reasons I try to stick with no shorter than 11.5" guns for a shorty though, as I've not had issues with them in high-volume shooting schedules.

Everything I've read about the 933 and 6933 mentions their reliability. Clearly the Mk18 / CQBR's being built today are reliable, but I'm sure quality parts with the correct gas port size and the right buffer is the key. I would be sorely tempted to try an LMT 'enhanced' BCG or at least the BC in a 10" barrel - I ever get this trust set up, maybe I'll find out!

The LMT enhanced BCG only works on 14.5" systems.  Don't waste your time.  

If I were building a 609 clone, I'd run it internally exactly like a MK 18 MOD 0, same port size, same extractor parts, same buffer, ect.  Other than the barrel profile and .3" of length, it'd more or less be the same thing.  Including the lower.  

~Augee

607. its the Firebird of or retro rifles