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Discussion Starter · #21 · (Edited)
So the results are in and they are.....very interesting. I first tried the Sentry 7 non-adjustable with an EZ Tune 0.078, chronographed all the results, but had cycling issues with that port size, so I opened it up a bit more to get it to cycle reliably. Remember this was my "back up option" just in case I ended up with the Sentry Adjustable having issues again. I like options and options are good to have even if you don't use them initially.

DHA 120gr SST: 2416 FPS avg. (better than Factory SST, but not much, this load simply didn't want to lock back on the last round no matter what I did, except use the weakest buffer spring, needs more gas to run and that's the downside of a fixed tuned solution like the EZ Tune, but I was expecting more port pressure)
Hornady 110gr OTM Factory: 2582 FPS avg. (literally within a few FPS of last 100 times I've chronographed this load, it or VMax are my standard tuning loads)
Hornady 120gr SST Factory: 2405 FPS avg. (up 17 FPS from when I tested it with the Sentry 7 Adjustable)

Inspection up front and there was NO carbon around the gas block or tube anywhere, so no leaks. Now for what surprised me. Once I got home, I decided to try the new replacement Sentry 7 adjustable since this was the solution I originally wanted. Same process, done with care and precision.

DHA 120gr SST: 2484 FPS avg. (much closer to what I was expecting, fastest shot was 2530 FPS, slowest was 2446 FPS, overall this is reasonably good performance from a 16" 5R SPC II barrel)
Hornady 110gr OTM Factory: 2596 FPS avg. (fastest average I've gotten from this load)
Hornady 110gr Black (VMax) Factory: 2558 FPS avg. (no reasonable change from other testing which yielded about 2555 FPS avg.)
Hornady 120gr SST Factory: 2405 FPS avg. (no change).

No explanation for why the DHA SST load shot that much slower (I never had a chance to test it with the leaky Sentry 7) with the non-adjustable Sentry 7 and EZ Tune gas tube. Could have been the chrono wasn't reading correctly, but it was consistently reading the other loads....I did all my testing sat / sun, similar weather, same location, same equipment etc. I decided to open up the gas port to pos. 15 (fully open) due to the DHA 120gr SST not cycling fully when I tested it on the EZ Tune gas tube. I got 3 o'clock ejection with full gas flow on the SLR adjustable, low mass carrier, SCS set for h1 weight and it locked open on the last round 7x in a row (kept doing single round mags to test consistency).
 

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Discussion Starter · #22 ·
That's when I decided to run the 110gr OTM and VMax with full gas flow as well this time around just to see where the carrier speed ended up as I never actually ran it fully open since I started from fully closed and worked my way up until it cycled. Both loads ejected at 2 consistently which is what I want for my hybrid quasi-low mass setup (anywhere between 2 and 3 really). Changing the gas settings from 8 to 15 only moved carrier velocity a small amount with these loads as I'd get around 3 on setting 8. Below 8 and it wouldn't always lock back on the last round. However Factory 120gr SST ejected at 1, much too high of carrier speed with the port fully opened. First time I tested that load with the leaky Sentry 7, even with gas setting down to 8 it was still spitting at 2.

Factory 120gr SST runs the highest port pressure of any load I've tested between Hornady, DHA and Wilson Combat. I'd suggest it if you have issues with reliable cycling in a rifle length gas system. But the new Sentry 7 adjustable has no carbon buildup on it after 72 rounds. Just some oil from oiling the adjustment screw. I believe the issue with the leaky block was the boring tolerance, maybe an end of life bit rounding out the hole etc. What ever the case, that block was caked in carbon after the first hundred rounds, this one is much more like Sentry 7 on my 5.56 upper. The issues is indeed resolved and the block is now pinned as well. At the very least, I have some restored confidence in the design for 6.8 SPC applications, verified what works and got results I'm satisfied with.

The end goal was to find a 120gr load with similar drop to a 110gr load. So I needed something in the 2450 to 2500 FPS range, not the 2388 FPS I was getting from 120gr SST Factory on the leaky Sentry 7. All the effort and scrutiny was a success and DHA's loads have yet again performed well. So I now have 3 loads that each perform well to fulfill my requirements. 110gr OTM for training / barrier blind use. 110gr Black (VMax) for home defense. DHA 120gr SST for ranged applications (training, SHFT). I guess my last question would be, will the 120gr MKZ shoot faster now as well? With the leaky Sentry 7 I was only netting 2371 FPS from this DD 5R 1:11 SPC II barrel. I may have to just test it again.
 

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Discussion Starter · #23 ·
Just an FYI someone mentioned they preferred having a spacer behind the gas block. While I don't think that matters in the end, especially if you pin the block like I do, it doesn't hurt either. Maybe everyone know this, maybe not, but PRI makes a spacer you can buy separately: 0.750 x .037 Diameter Gas Block Spacer (precisionreflex.com)

For those wanting to have every aid in gas block alignment, it doesn't hurt! I dimple all of my barrels with a jig and as mentioned, slowly tight the rear screw down while wiggling the block a bit to get the cup screw head to center in the dimple. I've never had a misaligned block using that method, but it also doesn't hurt because then you only have to worry about rotation and not also fore / aft location since you can just push it up against the shoulder.
 

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Discussion Starter · #24 · (Edited)
I did get a chance to retest 120gr MKZ loaded by DHA yesterday and got 2433 FPS average this time from a 7 shot string. With the suspected bad Sentry 7, I was only getting 2364 FPS. So at this point, I believe my theory was correct, slower loads are more sensitive to gas block leaks than faster loads.

Heavier bullets have a lower rate of acceleration (due to mass) and have a longer dwell time (as their peak velocity is lower), so leaks in the gas block are going have more negative effect. I saw the same thing with DHA's 120gr SST, was only getting 2388 FPS before and it jumped way up to 2469 FPS after I replaced the leaking block which is a significant change.

At this point, have come to believe a Chronograph is an invaluable tool to hunt for possible leaks in the AR during the process of building a new upper and testing it. Even if it cycles fine, doesn't' mean your getting peak performance. And after several hundred more rounds of testing, the new gas block has almost no carbon on it unlike the other block.
 

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Discussion Starter · #25 ·
BTW, I also suspect my chronograph reads a bit slower than some other's on this forum because I never got the velocities from my ARP barrel using factory loads that others reported from their ARP barrels. Note all of these are 7 shot strings. Temps are approximate for that day.

ARP 16" 3R 1:11 Results (2019 build, total usage was 1,500 rounds)
Hornady AG 110gr OTM: 2596 FPS (85F sunny day)
Wilson Combat 110gr TSX: 2505 FPS average (64F overcast day)
Prvi Partizan 115gr BTHP: 2405 FPS average (64F overcast day)

20" ARP 3R 1:11 Results (2021 build, about 200 rounds through this barrel))
Prvi Partizan 115gr BTHP: 2575 FPS average (85F sunny day, 42.5 FPS per inch gain over 16" barrel)
120gr MKZ loaded by DHA: 2661 FPS average (85F sunny day)

My Current Daniels Defense 16" 5R 1:11 SPC II Results (2021 build, now at 750 rounds through this barrel)
Hornady AG 110gr OTM: 2605 FPS average (75F overcast day)
Hornady Black 110gr (VMax bullet): 2571 FPS average (75F overcast day)
DHA 120gr SST: 2561 FPS average (75F overcast day)
DHA loaded 120gr MKZ: 2433 FPS average (75F overcast day)
Prvi Partizan 115gr BTHP: 2415 FPS average (85F sunny day, pretty much same as the ARP barrel)

My factory loads netted zero benefit from the ARP barrel. Same chronograph used throughout. But some report Hornady factory loads getting 50 to 100 FPS more in their ARP barrels...so at this point, having compared three barrels on the same chronograph, I think mine just reads a bit slower than others based on the results of my former 16" ARP barrel.

I do believe however the 16" 3R ARP would have shot the 120's a little faster than my Daniels, particularly the solid coppers, but not by a huge margin. 5R vs. 3R...SPC II vs. ARP chambers....the differences are not vast. All in 1:11 twist.

The difference between the 20" ARP 3R 1:11 vs. the 16" DD 5R 1:11 is 57 FPS per inch for 120gr MKZ, which is where the 3R comes into play with solid coppers. But for lead cored 115gr BTHP it's only the normal 40 FPS per inch, so there's no benefit other than the 4" of length. Perhaps we can test DHA's 120gr SST to see how it shoots from the 20" ARP, I suspect since it's lead cored bullet, it will be 160 FPS faster than my DD 16" or the typical 40 fps per inch.
 

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Discussion Starter · #26 ·
Note that with hand loading, you can play with powders to maximize velocity for a given barrel length. But when your not changing powder or powder volume, and only changing barrel length (with no other notable differences), about 37 to 40 FPS per inch seems to be the average velocity change. I see the same thing with my 5.56 barrels.
 
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