Gas tube lengths for longer barrels.

[bcs4]

I've been searching for definitive information regarding the effect of gas tube length in 20 to 24 inch barrels. All I've been able to find is general information.

It seems to me that gas tube lengths designed for 223 would be pretty random choices for a 6.5 Grendel, and it's further evidenced by the +2 gas tube. It implies that a longer tube is better, but seems also to imply that that tube length is best for everything 24+ inches.

Rifle or rifle +2 simply can't be the best choices for long Grendel barrels. I doubt the rifle length is even best for 20 inch barrels. The best tube length would be a balance between cycling and velocity. The pressure of Grendel is lower and volume of gas is greater than a 223. I would think volume would be more important than pressure in cycling if PSI is in the 55,000 to 65,000 range, if that's true, Grendel should be an easier cartridge to work with than the 223. I'm not sure how the greater volume would affect dwell, but my instincts tell me that dwell should be longer on the Grendel.

Then there are the questions of secondary pressure peaks, slower burning powders, bullet weight, twist, (many others I'm sure) and how they should be considered in the location of the port. Lots and lots of variables, I know.

What it all comes down to is whether there has ever been an valid study done regarding tube length on any impingement system that is based on research instead of, for lack of better word, dogma?

Thanks!

[A5BLASTER]

Thats a great idea for a study if it has not been done yet I would be intrested in reading the results of such a study.

That said I had a 21 inch barrel with rifle length gas and it worked fine, my 18 inch barrel works fine with a rifle length gas as well.

I know JP uses a extra long gas tube with there 6.5 creedmoore barrels so maybe there is something to be had by exploring a 20 and longer inch barrel with longer then rifle length gas tubes

Edit.....Rico hit it on the head I think with the adjustable gas block, forgot to state that I also had ajustable gas on both my barrels.

[RicosRevenge]

Most of us just use an adjustable gas block and call it a day.

[bcs4]

An adjustable gas block or smaller inner diameter port can certainly improve cycle and velocity on a port that's closer to the chamber than it should be, but locating a port further out on the barrel would keep the bolt locked longer, allow a longer length of barrel at full gas pressure, and reduce the likelihood of over-gassing.

[IceAxe]

BCS4, from an academic perspective, both cartridges behave quite similar with respect to the pulse width of a 20 inch barrel with a rifle length gas system. Although the Grendel has a greater charge it also has a greater cylinder diameter with a greater increase in cylinder volume as the bullet travels down the barrel which in turn will cause a more rapid decrease in pressure per inch traveled. In QuickLoad if both cartridges are used in a 20 inch barrel, with the same powder, at their max pressure load, you can see they exit the muzzle at near the same pressure. The Grendel is slightly lower in exit pressure. Additional barrel length would increase the pulse width but consider the decay in pressure due to the ever increasing cylinder volume would tend to negate the effect. Below is the PDF comparison from QuickLoad. The bottom line is....if even needed an adjustable gas block would easily do the trick.

[attachment=5472]

[bcs4]

Thx Ice. Could you possibly run these on 24 and 28 inch lengths? It looks like these are for a non-ported barrel. Is it possible to input impingement variables (you don't need to run graphs, just wondering)? If it's possible to enter port variables, it looks like I might need to invest in QuickLoad.

[JASmith]

QuickLoad is full of surprises.

It should work just fine for looking at general relationships even though extracting the pressure versus time history at the port can be a bit challengmg.

Not all powders are modeled as well as 8208 seems to be in QL. This means that QL cannot be used for choosing loads thaat are both safe and near maximum.

[bcs4]

There are indeed many variables. After looking at QuickLoads again, it doesn't seem to be able to provide anything that helps. I've ordered the barrel from Krieger and I'd like to specify a gas tube length that's as close to optimal as possible. I think it's complex enough that barrel makers don't want to offer information that could result in something that's undergassed. I can understand that. I went to the "horses mouth" (Alexander) and they wouldn't even offer anything that was longer than rifle length. Honestly though, I'm kinda surprised that there doesn't seem to be a source anywhere online.

[LRRPF52]

While the 6.5 Grendel has a lower working pressure, the propellant volume is much larger than .223 Remington, driving much heavier projectiles, so port pressures actually are really close to each other, enough so that not only are the same port locations appropriate, but gas port diameters line up well also.

The ELGS +2" is more suitable for cartridges with higher case capacity, like .260 Remington and 6.5 Creedmoor, not 6.5 Grendel.

RLGS + .0935" port works just fine with 20-24" barrels on the Grendel.

Secondary pressure events are only seen with cheap gauges when using RSI. Upgrading guages eliminates the secondary event, and you don't see it with piezoelectric metering systems to my knowledge, so non-issue.

There definitely was valid and extensive research done on the best gas systems for barrel length when Bill A. did the years of work that it took to RDT&E 6.5 Grendel. For example, in just one lot of 200 barrels with different chamber designs, none of them performed to his expectations, and a lot of money was spent learning hard lessons about what would and would not work.

He also traveled with several guns to different climates and dust conditions to work out potential problems more like what you would see with a military rifle trials regimen, not a backyard hobbyist with a new reamer and some shop tools.

When you actually look at it, I doubt there has ever been a chamber design that was tested as thoroughly as 6.5 Grendel. What other lab of engineers has tested over 120 different reamer designs for just one eventual chamber that gained SAAMI approval?

[bcs4]

This is an interesting cartridge. It would be great to see the stuff that Bill A. did. Do you know if any of it was published? He loved the 20 inch barrel.

Secondary peaks are detected by piezo, and current wisdom is that it's a late powder burn when the powder is too slow. I mentioned it as a variable, but it was just to make the complexity of port location look like climbing K2. I certainly agree that it's moot.

The muzzle energy is what's making me want to move the tube location. It would support a longer tube, and I don't think 2 inches is enough, though I'm sure it's safe. A tube that's too short is a waste of gas. I'd think the 260 and Creedmoor would move the action out of sheer terror.

Oh well, the Twins lost and the dog just farted. I'll restart the search tomorrow. Thanks for the info everyone.

[LRRPF52]

That's the first I've heard of piezoelectric seeing secondary pressure effects.

It's always been the companies that actually use piezoelectric metering who call BS on those using strain gauges that see secondary pressure, and like I said, once better quality gauges are used with strain, the anomaly goes away.

[Klem]

My experience with Blackout is that you need a minimum of 4,500lbs of pressure at the port to cycle an action. An action that uses a FMC and MILSPEC buffer/spring. That is 4.5K lbs indicated by Quickload. Add another 2K for reliability and gas leakage and anything more is surplus; to be either tolerated or tamed.

According to the simulation Grendel pressures with typical loads don't drop to below 6.5K until 26" of barrel-length. As long as you can sort out any dwell-time issues then there is plenty of gas to cycle actions using long gas tubes.

[bcs4]

That's interesting info. The lowest pressure to cycle I've found is 10K but everything I've seen is anecdotal. I think the original specs called for a rifle length at 12.7K, but again a longer dwell would allow for less pressure than the 12.7K to operate in 223/5.56 so that would even allow longer tubes on longer barrels for that cartridge.

Even though Grendel operates at lower chamber PSI, there is more pressurized gas in the barrel than 223/5.56, as evidenced by significantly higher muzzle energy.

The more I think about this, the more ways I can think of testing to determine the correct gas tube lengths for cycling. It just seems that there would be a study somewhere that would help. Efficient operation should be a major focus, and the routine acceptance of overgassing doesn't make sense considering the popularity of the platform.

[LRRPF52]

What happens to gas port pressure in extreme cold?

That's one of the most overlooked things when gassing a rifle.

Then look at fouling and debris, and the resulting friction on the action.

It's one thing to gas a clean gun on a nice day on the range, and something entirely different in winter conditions closer to the ice caps, or in high dust.

[bcs4]

Absolutely. There is the possibility that someone might want to use a 24-28 inch barrel in environments that would cause problems for a longer tube, but there are also common problems with gassing in the standard lengths. I would think anyone that wants to approach the ragged edge would more likely be shooting from a bench, willing to put serious thought into the aspects involved. A person that's less committed could choose a length based on reliability. Frankly, either choice is likely to be accurate.

It would seem that the best approach would be to study the characteristics of a specific cartridge and adapt tube lengths that meet some standard range of operation for the action. After a second cartridge is studied, it will make changes easier to determine for a third.

The first step for any useful adjustment would likely be the determining the required pressure and dwell necessary for cycling. It would probably result in a graph that went from high pressure short dwell to low pressure long dwell. Anything along that line would likely cycle.

If alternate tube lengths begin to be used, real life experiences will be discussed more often on forums. It will become another area that users might want to focus on.

[IceAxe]

Keep in mind the port diameter is also an important variable.

I am not seeing the benefit of extending the length to the gas port. I recognize by moving it outboard on a longer barrel you can shorten the dwel or pulse width. But there are some operational risks....Consider... by extending the gas port down the barrel you move more toward a region where the bullet is moving faster and pressure is dropping. That said, consider the normal changes in; atmospheric conditions, bullet weights/loadings and their effect on cycling in such a narrow operational window.

I have both a 20" and a 24" Grendel and 264LBC with RLGS and adjustable gas blocks. When adjusting the 20" gas block, I used some 120 grain PPU ammo as it was cheap. When I switched to the 123 grain Hornady SST factory loads, it was undergased and had cycling issues. Now Imagine if I had optimized this barrel by moving the gas port into a less energetic region, the only option left might be drilling the gas port.

In the interest of reliability and a larger operating window, I would rather have some surplus gas and regulate it down through an adjustable gas block. Then if needed it only a click away. Just my thoughts.

[bcs4]

Those are all valid concerns. I'm not sure I'm following the dwell and pulse width though. I'd think they'd be lengthened.

The difference in cycling is weird because looking at pressure and velocity they should be pretty close. The gas block must have been set to almost exactly the cutoff between the two. I've got pistols that are strange.

Based on what I've found today and my rudimentary math skills, pressure at the key would be reduced about 6% by moving the port 3 inches on a 24 inch barrel. 28 inches is beyond my abilities so far. I'm still looking for some sort of a pressure and pulse width numbers on cycling.

[IceAxe]

By pulse width, I am talking about the period of time gas pressure is being exposed (exposure being a function of magnitude and duration) to the gas port. So when the bullet passes the port until it leaves the barrel. Moving the port closer to the muzzle would in effect shorten the pulse width for any given length barrel. I believe the "typical" linear length for most AR's is around 7 inches.

[bcs4]

Ahhh, I follow what you're saying now. For some reason I was thinking the pulse that you meant was at the key and I was saying that it would be longer because the lower pressure would result in a slower disconnect from the gas tube. My bad.

Yes, the dwell at the port would be shorter but the pulse duration could be longer than the rifle location depending on it's placement, but even if the duration was longer, the amplitude would be less than the rifle location. So I'm back to the problem that seems to have no answer online: what is the least required pressure and duration that's needed to cycle reliably?

A couple of hours ago I also began thinking about the pressure of the gas preceding the bullet and it's affect on cycling. Yikes.

At this point I figure that I'll undergas it to the extreme, and make the carrier out of aluminum. I'll end up with something that cycles every time the shooter sneezes.

[LRRPF52]

There was someone that built an extended length gas system here several years ago I think.

I don't remember if it worked out well or not.