Gas tube lengths for longer barrels.

[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.

[bcs4]

If you remember I'd sure appreciate it if you could give me a link to a thread. I've still got 2 months to finalize my order with Krieger.

Thanks