I would argue there's an abundance of reasons to push the port out.

Not my gun, though.

Personally I'm not very technically astute but I appreciate some of the posts so far.  I have owned an AR-10 for over 10 years and have seen some things that are perhaps lost over time so a couple of comments FWIW.

When Armalite developed the AR-10 Ultra Mag they pushed the gas port out two inches beyond regular rifle length.

John Holliinger owner of WOA was a fan of longer gas systems on the AR-10 and he built some AR-10s with extended length gas systems.  The ADCO team also did some work with extended length gas ports for the AR-10 and at one time they offered an AR-10 24" barrel with an extended length gas system.

I think one of the reasons the extended length gas system did not get more accepted is that shorter barrels have become more popular.  When I first got into AR-10s most barrels were in the 20-24 inch range.  Today it seems like everyone wants something in the 16-18 inch range.  You can't put a longer gas system on a shorter barrel.

The pigtail gas tube on the 300 RSAUM I am building measures just over 28" in length before I coiled it.
It's a nice little trip that the gas is taking. And better behaved at arrival.

G.

I'm reading that thinking to myself , " sounds like a good idea until it's time to clean it . " Short of removal and submersion in solvent there will be no quick or efficient way to do this .

I've never cleaned a gas tube on an AR-15 and don't intend to do so on my 12.5" AR-10 either.

Have gas tubes on AR-10s actually become obstructed to the point of failure before?

Thats why Armalite lost the contract to the M14 back in the 50s, a sleeved barrel.

But anyways, I was thinking about the pigtail or expanded tubes as well. It makes sense, for pressure reasons, but as with everything else with the AR10, it will probably need a redesigned gas ports in the bcg itself. Ive been wanting to check out the Lantac bcgs and their "forward porting", but havent had the chance. I keep missing the sales, so keep putting it off.

Interesting point about the Armalite sleeved barrels; I didn't know about that.

The reason I'm doing it is to make a stiffer "bull" barrel (1 inch diameter), that carries off heat better than composite plastics/carbon fiber does.
It's a fun project, difficult to predict how useful it will turn out, with lots of opportunities to practice fabrication skills.

I'm sleeving it by having the aluminum rod bored out to the exact diameter of the turned down steel barrel. The aluminum sleeve is then heated to 500 degrees F (expansion), and the steel barrel placed into liquid nitrogen (contraction). The two are mated, and at room temperature are permanently bonded together (and stay that way).
I've done that trick before with other projects, but this is the first time with long range caliber.

G.

Great thread.

I have little to add except a story on the AR-10 barrel in testing.  There apparantly was a catastrophic failure to a sleeved barrel that torpedoed the AR-10.  Up until that point, it was dominating the trials and the failure gave the ordnance corps the excuse they needed to kill it.

Yeah, I pushed my .22-250 port out 5" and am quite happy with it.  Bone stock Armalite BCG and buffer/spring with no issues, very easy on brass.

Just read the history of that AR10 barrel kaboom (during US military testing).

It happened during full auto testing, and Eugene Stoner had been adamant that it was wrong (wrong!!) barrel choice. He was right of course.

My design is different in several ways: better steel, much larger sleeve, and never, ever, full auto fire. It's meant to allow longer strings of fire (at distance) for a precision rifle. Definitely not an infantry rifle.
Thanks to all of you for pointing out that bit of history to me. That is why I love this forum. I am always learning something.

G.

That was the general consensus among a lot of people who are fans of the Stoner design, but I really think the fact that the AR10 entered the testing late, combined with the fact that the T44 was Ordnance Corps' home team design, determined the fate of not only the AR10, but the FAL as well for a US service rifle.

There was a retired Army officer who was assigned to Ordnance Corps at the time of the trials, and in his recorded personal oral history, he said he remembered that they literally left the T48 (FAL) in the corner, never really subjected it to any testing that he can recall.

If you actually read all of the reports, most are available on line, they tell a different story.

The program to replace the M1 Garand was well behind schedule by the time Armalite submitted the AR-10 for testing.  The requirement to replace the M1 was initiated immediately after WW2, by 1956, the program was eleven years old and there was immense political pressure to finish testing and find a replacement.  When the Army tested it they found it a promising design, the failure of the experimental bi-metal barrel notwithstanding.  The barrel failure was not the cause of the dismissal of the AR-10.  In 1960, the AR-10 was tested again by APG and found quite reliable, the only real reoccurring problem was the failure to fully lock on the first round from the magazine, something that a forward assist would have made a non-issue.

The AR-10 design was fairly new and did have some shortcomings, just look at all the changes between the 'Hollywood" and 'AI' production models.  Springfield figured the AR-10 was in the same state of development as the T44 and T48 were in 1952 and would require another three to five years development before it was refined enough for production (They weren't that far off, AI did not start volume production until 1960.)   So, the decision was made to shelve the AR-10 in favor of the two more advanced designs, the T44 and T48.

The T48 was hardly "just put in a corner and not really tested".  Between 1952 and 1956 the T48 and T44 were subjected to more testing than any previous rifle design.  Not only that, three different manufactures made T48s for testing, FN, Belgium, H&R, and RSAF all submitted samples for testing.  Further, over two dozen changes were recommended and adopted to FN to improve the T48 design.

Before the arctic test of 1953-54, the T48 was the odds on favorite.  However, the original FAL's small gas port provided insufficient power due to the reduced port pressure because the the cold weather.  Enlarging the gas port solved the problem, but original three three position gas regulator did not have the adjustability to compensate for both warm and cold weather operation, so FN was given recommendations and another round of arctic tests were scheduled the following winter (1955-56), which further delayed the adoption of an M1 replacement.  

The results of the 1956 tests were that the T44 and T48 were both suitable for adoption.  The T44 was standardized for several reasons, it was lighter by 3/4 of a pound, had fewer parts, the gas system was simpler for the operator (self-regulating), and training requirement changes were fewer.  But probably the one thing that was key to the failure of the FAL to win the competition was the failure of H&R to produce a rifle as reliable as the FN or RSAF examples.  In side-by-side testing of the three, H&R continually ranked last.  There was no doubt concern over whether US contractors could make rifles as reliable as FN did with H&R's converted drawings.

In any case, the T44 was standardized in 1957, however, it would not be until the end of the next year before a production contract would be approved for volume production.  Then there were all the quality issues with the first few production runs.  One of the reasons for the retesting of the AR-10 . . .

I do seem to recall similar mentions in TBR about the FAL/T48.  It appears the officer wasn't as involved with the program.

The Dutch really had an aggressive development program with the AR10.  They experimented with charge handle evolution, carbines, scoped sniper variants, belt-feds, bipods, and their production guns have excellent attention to detail on things like the bolt, ejector face, extractor, barrel quality, and overall execution of the rifle design and quality control with production.

Even with iron sights, on my friend's sample at 150m, I was able to print unbelievable groups with SA 7.62 NATO surplus.

I'm about 3/4 way through this excellent new  book
( written by an ARFCOMMER )

the AR10 was an excellent rifle, but a victim of bad timing



Attached File

Excellent thread, thanks for the info.

One thing that continues to come back with this to me is that the AR10 would have been awesome in an intermediate cartridge, with shorter OAL, resulting in a smaller rifle, smaller magazines, with a lower working pressure.

Something in 6.35, 6.5, or 7mm with a smaller case, maybe like a Carcano or BR.

That would have made an excellent rifle, without all the problems introduced by the size and pressure issues created by 7.62 NATO, while allowing performance advantages over 7.62 NATO without all the recoil and blast.

You can see the market moving in this direction, where lighter weight, less recoil, rifle balance, and efficient projectile selection versus high pressures and inefficient .30 bores trying to make up for bad projectile shape and weight with more pressure and mv.

the Dutch made some in 762x39mm

They didn't size down the receivers though, and 7.62x39 is probably the worst example of intermediate cartridges out there looking at performance.

yep

.

Gene Stoner didn't like intermediate cartridges

he was drinking 308 Kool Aide

.280/30?


.280/30 British, flanked by .30-06 on the left and 7.62mm NATO on the right.

Blindeye-
FYI The factory 300 RSAUM AR-10's had the gas port located 3 inches further out than a standard rifle length. I have a factory gun and wish the cartridge would have been promoted better- an M-110 in 300 Rsaum would have it's uses....

I have about 2k new Remington cases and was relieved to see Nosler brass at my local Sportsmans warehouse. Good luck with your project and please keep us updated!

Ronnie

I am not an engineer so I see things in simple terms. You get what you pay for. I've owned several Armalite AR10s and they have always been reliable.  Problem is when you label every AR308 an AR10 you are lumping all the crap ARs into the same category.
Soon the AR10 has a reputation  of being unreliable.

Ronnie B,

Thanks for your input about the gas port location.

I think my coiled/pigtail gas tube will net the same results (or better).
I didn't come up with the idea on my own, but some of the shooters at Snipershide forum had done the same for their WSM AR-10s.

What I am trying to do is delay unlocking of the bolt while chamber pressures fall off. 28+ inches of travel distance for the gas causes it to lose energy during the trip, and makes the cycling of the action smoother (which is just a by-product of what I am seeking).

There is almost twice as much volume in the lumen of the gas tube I made compared to a (standard) rifle gas tube, and that lowers pressures also.
I didn't mean to take credit for the idea; it's well tested in AR-10's shooting magnum cartridges.

G.

What does the pigtail do, that cant be achieved with an adjustable gas block?

Longer length for the gas to travel cuts back on pressure. Am adjustable gas block keeps that pressure in the barrel unless it's bled off somehow. Not all adjustables have an excess vent.

With the pigtail, that pressure is still there, but going through as longer tube means less pressure at the gas key.

That's where my head is wrapping around it, i could be wrong though. Can kinda give the same results i guess. Definitely something that needs more scientific studying.

The longer/pigtail/expansion chamber tubes do react differently than simply cutting back the gas. Why exactly that is, i have no clue yet.

Something like that would have been nice.

Time.  You're not only adding volume, thereby reducing pressure, you're adding time before the gas hits the gas key.  All a gas key can do is limit the gas, it can't add time.

The most important thing a pigtail gas tube does is lengthen the time it takes for the gas to hit the key.

It does other things for sure, but the longer travel time is something you can't replicate with an adjustable gas block.

G.

If we had an electronic gas block, we could store  & then release the gas shot @ a pre-determined time.