28" Satern Liberty Upper - Yea or Nay?

[MegaHurtz]

I also noted that Satern does not state what the twist is in the barrel. I know that you are looking at the 6.5 vs my 6, but I went with the 1:7 to ensure accuracy with heavier bullets. In my case 110 & 115's. The whole point of my build is to get to 1000 yards and beyond

It's a 1 in 8 twist of a 5R profile. See image of bbl marks.

[MegaHurtz]

I just ordered a 28" 6mm AR barrel from X-Caliber. In talking with them, they strongly stated that the rifle length gas system was a problem and that I should go with the rifle +2" along with the AGB. In that they want a happy customer, I took their advice.
I see that the Satern uses the rifle length system on their 28" - maybe they know something that X-Caliber does not?

Maybe. I perceive that Satern is a reputable Barrel OEM, so I suspect that their guidance might be legit for the products they manufacture and sell. Time will tell, I'm sure.

EDIT: Kudos on getting your bbl ordered. I'm excited at the prospect of starting my journey of 'long-range' shooting. I just wish I owner a ranch out in the SW US' High Desert regions.

The new EnABLER ELR round coming out (from the Applied Ballistics team) gives me a goal to look forward to, as i develop my skills in this domain over the next couple of years.

[A5BLASTER]

Maybe. I perceive that Satern is a reputable Barrel OEM, so I suspect that their guidance might be legit for the products they manufacture and sell. Time will tell, I'm sure.

EDIT: Kudos on getting your bbl ordered. I'm excited at the prospect of starting my journey of 'long-range' shooting. I just wish I owner a ranch out in the SW US' High Desert regions.

The new EnABLER ELR round coming out (from the Applied Ballistics team) gives me a goal to look forward to, as i develop my skills in this domain over the next couple of years.

I wouldn't trust Saturn to build a outhouse properly. Without blaming their mess up on everything and everyone else.

[LRRPF52]

AA used to make 28" Grendels, then stopped, with 24" being the longest offering.

Bwaites has a 28" AA Grendel that is extremely accurate, but is sensitive to what loads it will shoot since it has so much more plug dwell time ahead of the gas port.

You get more pressure dwelling in the gas system than you do with 20-24" RLGS set-ups, so the carrier starts moving with more inertia, causing early unlocking and all the problems associated with that.

With hand loads and some factory loads, as you approach max, the brass takes more of a beating.

[Herzo]

Using QuickLoad, I compared the pressure difference between a rifle length & a rifle +2" gas tube. (Looking at the calculated exit pressure of a 12" & a 14" barrel. I come up with approximately 15%. That + an adjustable gas block should handle the situation.

[Herzo]

Using QuickLoad, I ran the exit pressure difference between a rifle length & a rifle +2" gas tube, coming up with an approximate 15% reduction. (I looked at the exit pressure of a 12" vs a 14" barrel). That plus an adjustable gas block should take care of the situation.

[MegaHurtz]

AA used to make 28" Grendels, then stopped, with 24" being the longest offering.

Bwaites has a 28" AA Grendel that is extremely accurate, but it sensitive to what loads it will shoot since it has so much more plug dwell time ahead of the gas port.

You get more pressure dwelling in the gas system than you do with 20-24" RLGS set-ups, so the carrier starts moving with more inertia, causing early unlocking and all the problems associated with that.

With hand loads and some factory loads, as you approach max, the brass takes more of a beating.

would a bcg, with a gas port adjustment therein (such as what is for sale nowadays), help with settling that down a bit?

Also, it seems to me (pure mental gymnastics, no emperical data) that once the key clears the tube, that any residual pressure dwelling in the gas system will dissipate into the volume of the upper, not materially contributing to further rearward acceleration of the bcg.?.

And I am still an unlearned padawan (as such, not being a smart arse). Isn't it true that with any system, that as one approaches max pressure loads in a given cartrisge that the brass will take more of a beating? I've been trying to get smarter on these things, given I am going to be endeavoring to undertake reloading my own cartridges. This seems to be a common thread at the various custom load development guidance resources on the interwebs.

-MHz

[MegaHurtz]

Using QuickLoad, I ran the exit pressure difference between a rifle length & a rifle +2" gas tube, coming up with an approximate 15% reduction. (I looked at the exit pressure of a 12" vs a 14" barrel). That plus an adjustable gas block should take care of the situation.

Thanks for the continued dialog.

I find that I am a bit cornfuzed. Are you suggesting that an adjustable gas block would serve to help mitigate potentials for gas-system related drama on a 28" w/ rlgs, or that an agb coupled with a rlgs+2" is what you perceive as being appropriate?

[Herzo]

Using QuickLoad and an arbitrary charge of IMR 8028 behind a 123 hr Nosler, the pressure at 12" (simulating a RLGB) is ~13KPSI. By adding 2" the pressure at the point that you tap off the gas ts reduced to ~11KPSI. The pressure pulse decays over the next 0.56 ms in the case of the shorter gas tube (0.49ms for the longer) to ~5KPSI. This translates into a higher volume of gas at higher pressure impinging on the bolt resulting in a more violent bolt motion.
A 20" barrel using a standard rifle length system will see the higher pressure for a substantially shorter period (0,35 ms) allowing the pressure pulse to dissipate as the bolt begins it's rearward motion.
The 2" longer gas system would help by reducing the initial pressure and reducing the duration of the pulse. An AGB (think orfice) would aid in reducing the amount of gas through the gas tube, but you will be dealing with a system that is more difficult to tune over a range of bullet weights and velocities.

[Herzo]

As to the dissipation of the gas pulse into the upper as the BCG moves rearward, the BCG has only begun to move when the bullet exits the muzzle and the effect of the pressure pulse is realized by then.

[MegaHurtz]

Using QuickLoad...

Thanks for making a couple of references to this tool that you use. I just placed an order for QuickLoad. Seems to be the best internal ballistics tool out there. Anxious to get it and start getting familiar with same. Happy Thanksgiving, kind sir.

-MHz

[MegaHurtz]

As to the dissipation of the gas pulse into the upper as the BCG moves rearward, the BCG has only begun to move when the bullet exits the muzzle and the effect of the pressure pulse is realized by then.

In the interests of getting my hands on some empirical data regarding the timing of the action, where would one look to ascertain same?

...

On another note, regarding the previously discussed RLGS vs. RLGS+2" context: From this post, I came across this interesting little tidbit:

Or, if you think your barrel gas hole is too large you can remove your gas block, then drill & tap the gas port hole, then install a set screw in that tapped hole, then drill a new gas passage hole to any size you want in that set screw.

Based on that, it would seem somewhat reasonable that one could drill/tap/plug an existing port and then drill a new one at the +2" (or other) location. The deburring of the new port hole would seem to me as the hardest thing to deal with as, when the drill's bit passes into the bore, there might be a potential issue of edge material being pushed out of the way vs. being cut/removed. I guess the right drill/fixture/pressure/speed/etc. might mitigate that, when considering the material properties of the barrel. ... And maybe a few rounds down the barrel might take care of it too...

[Herzo]

While QuickLoad is a very valuable tool, I would suggest that it be used with care. A comparison QL results and published data (notably Hodgdon's) can show significant differences in pressure with the same loads. That said however, the ability to see the effect of changes in seating depth, barrel length and the pressure difference of the same bullet weight between manufacturers is quite valuable. I also find the predicted velocities to be within a reasonable range -- probably subject to the tolerances of individual rifles.

[Herzo]

I'm certain that the timing question has been researched and would be a available on the internet, however for the bolt to move 1" in 0.3 ms. it would probably have to accelerate to something like 600 fps in that time. Highly unlikely given the mass of the bolt and carrier.
I'm order to move the gas port, you would need to have the appropriate diameter at the point that you wish to move the port.
As to plugging and redrilling the gas port, I ended up doing that to a carbine that was over gassed. I merely stopped short of drilling into the bore, tapped the hole with a #8-32, installed a stainless screw and drilled the screw with an appropriately sized drill.
I guess that Satern may have taken the size of the gas port into account when selecting the size of the gas port. You may still need an AGB in order to tune for your load

[Herzo]

I should have noted that the effect of over gassing in my case was a failure to extract and tearing a segment of the rim from the cartridge. The bolt was moving before the case had time to relax.

[MegaHurtz]

While QuickLoad is a very valuable tool, I would suggest that it be used with care. A comparison QL results and published data (notably Hodgdon's) can show significant differences in pressure with the same loads. That said however, the ability to see the effect of changes in seating depth, barrel length and the pressure difference of the same bullet weight between manufacturers is quite valuable. I also find the predicted velocities to be within a reasonable range -- probably subject to the tolerances of individual rifles.

Your words of caution ring true and are appreciated. Given that I will eventually go down the path of developing custom loads, or using other's recipes, I can see the intrinsic value of QL. As with anything complex and potentially lethal, caution is warranted. Your words are wise. Thank you, sir.

-MHz

[MegaHurtz]

I'm certain that the timing question has been researched and would be a available on the internet, however for the bolt to move 1" in 0.3 ms. it would probably have to accelerate to something like 600 fps in that time. Highly unlikely given the mass of the bolt and carrier.
I'm order to move the gas port, you would need to have the appropriate diameter at the point that you wish to move the port.
As to plugging and redrilling the gas port, I ended up doing that to a carbine that was over gassed. I merely stopped short of drilling into the bore, tapped the hole with a #8-32, installed a stainless screw and drilled the screw with an appropriately sized drill.
I guess that Satern may have taken the size of the gas port into account when selecting the size of the gas port. You may still need an AGB in order to tune for your load

Re goggle'g my way to success for action timing, surprisingly, a number of searches have yet to reveal material results. The hunt continues.

Re your plugging and redrilling, that makes complete sense. I was mulling on the viability of plugging an existing port, and drilling a new one in a different location, if the extended dwell time of a RLGS on a 28" bbl proves to be a genuine issue. Given I ordered a contourless non-fluted bbl, I suspect that doing so would net higher chances of success vs. a bbl that is of varying diameter along its chord. Doing so is outside the scope of what I would consider low-risk for me and would just flip a few bones at a competent gunsmith to do (just as I would do if I end up deciding to shorten the bbl).

An AGB makes sense, as does the potential benefit of a heavier buffer/spring assembly (to delay the mass of the BCG movement a bit), possibly coupled with a heavier BCG (netting a similar effect to the bcg's impetus, I perceive...). I've also seen potential solutions wrt carriers that have adjustable gas ports thereon, as well as bolts that are hardened against related failures of the extractor and/or lugs. I am guessing that the potential for the lugs on the bbl extension being damaged is probably of lesser likelihood.?. I'll just have to get the unit in hand and get some rounds through it to see what tuning, if any, will be needed. - 'tis an interesting problem set, if it proves to be a problem at all. Time will tell. I'm hoping that this all is just an academic exercise and that the Satern upper performs just fine. We'll see...

[MegaHurtz]

I should have noted that the effect of over gassing in my case was a failure to extract and tearing a segment of the rim from the cartridge. The bolt was moving before the case had time to relax.

Understood. Yeah, my searching for action timing revealed a number of sources that explained that specific concern rather well, just without any timing/pressure curve/projectile displacement data.

I better understand how extended dwell times may impute similar issues, especially if I employ recipes that are at the edge of the pressure envelope for the given powder, projectile mass, etc. I am hoping that the buttoned 5R rifling profile may prove to be a bit more forgiving (lower friction, higher velocities, when contrasted with a 6-land cut-rifling profile). I am wondering if (molly) coated bullets would net any meaningful differences, should those sort of issues arise.

Also, the different powders seem to provide a myriad of potential tuning options, by being able to influence the pressure curve profile a bit. I found what I perceive as a good text on some propellant grain structure designs, and how deterrent coatings are used with various grain structure designs to tweak the burn/pressure profile.

This is definitely something that one can run down the rabbit hole with, quite deeply and quickly. It is stimulating from an intellectual/academic perspective.

As always, Thanks. Take care.

-MHz

[LRRPF52]

would a bcg, with a gas port adjustment therein (such as what is for sale nowadays), help with settling that down a bit?

Also, it seems to me (pure mental gymnastics, no emperical data) that once the key clears the tube, that any residual pressure dwelling in the gas system will dissipate into the volume of the upper, not materially contributing to further rearward acceleration of the bcg.?.

And I am still an unlearned padawan (as such, not being a smart arse). Isn't it true that with any system, that as one approaches max pressure loads in a given cartrisge that the brass will take more of a beating? I've been trying to get smarter on these things, given I am going to be endeavoring to undertake reloading my own cartridges. This seems to be a common thread at the various custom load development guidance resources on the interwebs.

-MHz

Adjustable gas will help a bit. The AR15 Bolt Carrier Group vents the gas way before the carrier key clears the back end of the gas tube. There are 2 vent holes that the gas rings pass as the carrier travels to the rear, at which time any excess gas is vented out the ejection port, since carrier inertia has already been reached.

I'm currently using Bootleg Adjustable Gas Carriers instead of gas blocks.

[MegaHurtz]

Adjustable gas...

Thanks for chiming in with your thoughts. That makes a lot of sense. Your assertions about using a Bootleg BCG is reassuring. Much appreciated. Hope you had a great Christmas. Happy Holidays!