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Discussion Starter · #1 ·
I've recently noticed something after another outing this weekend testing loads. Since I don't currently reload and my focused use is as a fighting gun, I've spend a considerable amount of time testing accuracy, terminal ballistics and reliability of various loads for different applications. However, given the current cost and availability of 6.8 SPC ammunition, I also built a 5.56 clone (or as close as one can get) for training purposes. I'll get to the gas blocks in a moment.

Both uppers use SLR Sentry 7 Adjustable gas blocks. Both uppers use Daniels Defense 16" barrels, both mid-length gassed. Both use nitride gas tubes. The 6.8 upper is a 2020 production barrel from Daniels with SPC II chamber, 5R rifling, 1:11 twist in their heavy S2W profile, stellar accuracy with factory OTM, VMax and even SST. The 5.56 upper is NATO chamber, 1:7 twist, 6R. Both uppers have Sentry 7 Adjustable gas blocks.

I just this past weekend noticed that the gas block on the 6.8 SPC upper is absolutely caked in carbon around the metering screw and the leaf spring / detent. But the same gas block on my 5.56 upper that I shoot full snot M855 and 77gr Razor Core (both NATO spec loads), has zero carbon anywhere. I've run the 5.56 upper harder than the 6.8 in terms of training due to much cheaper ammunition as well. I would expect 5.56 to produce higher port pressures shooting NATO spec loads than 6.8 SPC shooting SAAMI spec loads.

The gas block on the 6.8 is NOT leaking out from underneath. Both barrels are dimpled, both blocks have the screws torqued to 25-in-lbs using a CDI calibrated torque dial. Both blocks are pinned also. Both blocks were sealed around the gas port using Rocksett and their fit onto the shoulder was very precise.

Has anyone else experienced SLR's gas blocks fouling like that? I have been getting some odd results from my factory load testing. All of the 110gr stuff shoots at or above it's spec. I get 2589 FPS from 110gr OTM and 2555 FPS from Black (aka VMax). But I only get 2371 FPS from 120 MKZ loaded by Druid Hill and 2388 FPS from 120gr SST Factory. Both of the 120's are well under their expected range for a 16" 5R SPC II with 1:11 twist, but both 110's are on the money.

At first I thought possibly the MKZ was slow because it's a solid copper bullet and just didn't' like the 5R and shorter 16" barrel. But now having tested 120gr SST factory loaded by Hornady, my results are notably under the spec velocity of 2460 FPS for a 16" barrel on that load as well. I have some 120gr SST from Druid Hill coming to test, but at this point, the heavy carbon buildup and low velocities of two very different 120gr loads, one NOT being a SCHP, have given me to some pause to investigate a potential issue.

I also looked up some testing others have done on the Daniels Defense Ambush line with their 18" SPC II barrel, same twist and rifling, they are getting 2550 FPS from 120gr SST factory ammunition. Typically velocity in the same barrel goes up between 30 to 40 FPS per inch, so their results seem in line with an 18"....2550 FPS - 2388 FPS = 162 FPS for just 2" of barrel length. Something is amiss.

This now leads me to believe the SLR gas block is bleeding pressure, as evidenced by the considerable carbon caking that I don't see on my 5.56 upper using the same gas block. It may be possible my 110gr loads would be faster as well if that's the case, but I would expect them to be less sensitive to leaks given their shorter dwell time.

This is my first time using SLR's gas blocks, as my previous 6.8 upper was based on the Adams Arms piston system paired with an ARP 16" barrel. Anyone else with an SLR experience a lot of carbon up around the metering screw / leaf spring on a 6.8?
 

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The first thing I would do is clean the AGBs and swap them to see if the same results occurs.

My SLR and Syrac gas blocks show carbon residue on my 6.8s. A 6.8mm has more powder thus more gas than a 5.56. I've switched to SLR because I can take them apart and clean them. CLP is your friend when it comes to AGBs and helps prevent (not eliminate) stuck adjustment screws. I lube them and run them full close and back out every couple of range sessions. I had to send my Syrac back to the company for repairs after many rounds through my 6.8. This was a couple years back (they are now Out of Business). They called me and said they had never seen such erosion on the adjustment screw and were baffelled why the screw had cracked. A 6.8 can be produce a lot of gas and pressure for those that handload to the max. If your swap shows the same results, I don't think you have a problem.

One related question. You said they "fit onto the shoulder was very precise". Just checking and may be a benefit to others, Gas blocks typically don't fit up against the shoulder ahead of the gas port with a freefloat handguard as a small gap is needed to fit the handguard cap. The gap is there if the dimple is centered (wiggled) into the dimple.
 

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Have no idea about velocity loss, sorry.

SLR are my go-to's for adjustable gas blocks. I run quite a few of them, on multiple cartridges. I have noticed some have a very small amount of carbon on the outside, it's nothing I worry about. I can usually just wipe it off with a qtip. I also do what Xman suggested with lubing them, except I use Aero-kroil. SLR also suggests doing it, too. If you're having a bad leak, I would also suggest pulling it apart and cleaning it. See if that fixes it. If not, contact SLR.
 

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One related question. You said they "fit onto the shoulder was very precise". Just checking and may be a benefit to others, Gas blocks typically don't fit up against the shoulder ahead of the gas port with a freefloat handguard as a small gap is needed to fit the handguard cap. The gap is there if the dimple is centered (wiggled) into the dimple.
This stood out to me, too. I've gotten to the point where I now run a .030" shim on all my gas blocks on all builds that get a free-float handguard. I figure I'd prefer to have the gas block up against something when I torque it, rather than hoping the set screw will keep it "floating."
 

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Discussion Starter · #5 · (Edited)
Maybe there's an official term for it or maybe not, but the portion of the barrel that's straight that the block slides onto. Technically I do think the back stop is the "shoulder", but the whole area is typically 0.750" diameter for AR's. Both of my SLR's were a very nice tight fit, I didn't have to tap them on, but they did require some wiggle / pressure by hand.

Below are some pictures showing how much carbon leakage I'm getting up front. If you look carefully, you can see that it's even peeled over sprayed paint back from the inside of the handguard.

Automotive tire Tread Bumper Tire Automotive design


Automotive tire Camera lens Camera accessory Carbon Cameras & optics


Now comparing the above 6.8 SPC to the below 5.56 upper, both are using Sentry 7's:

Tire Automotive tire Wheel Wood Tread


Automotive tire Grey Wood Automotive exterior Gas


Just for reference, I followed the same process I always do. Dimpled, bedded around the port with Rocksett, torqued to spec using a CDI torque dial (most accurate way to torque screws) and then pinned.

The 5.56 block is vastly cleaner, with only a little bit of carbon buildup compared to the 6.8 SPC gas block. I guess I'll find out because Todd at SLR said he would send me a replacement block to try.

I'll check to see if there's any velocity difference in the 120gr MKZ and 120gr SST Hornady. Also got some 120gr SST from Druid Hill to test. All of my 110gr loads shoot well (velocity wise), but I've been very disappointed with 120's.

110gr OTM Factory - 2589 FPS
110gr Black (aka VMax) - 2555 FPS
110gr Nossler loaded by Druid Hill - 2650 FPS (Smokin')

And the Sad Faced 120gr Loads
120gr MKZ Loaded by Druid Hill - 2371 FPS (pretty bad)
120gr SST Hornady Factory - 2388 FPS (not much better than MKZ)

Before I finally give up on 120's (other than maybe trying to hand load at some point), I want to be sure there aren't any other issues that have cropped up.
 

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Discussion Starter · #6 · (Edited)
It's only a theory at this point that the block is leaking excessively, hence my investigation. It may just be all this barrel can manage with 120's of any type, BUT, I found a review of the Daniels Defense Ambush using the same barrel from guns America just a few years ago.

That 18" barrel was pushing 120gr SST factory ammo at 2550 FPS with their chronograph. Yah yah, chrono's vary, temps etc....Chrono's are pretty consistent and accurate these days, 0.5% is only a handful of FPS, so it's a reasonable proxy.

Factory spec is 2460 FPS for 120gr SST from 16". So (2460 fps + (2in * 40 fps / in)) = 2540 fps expected for a n 18" barrel, right what Guns America got with their 18" barreled DD Ambush...but I'm getting a woeful 2388 FPS from a DD 16" barrel with the same rifling, chamber and twist rate.

There's no way I should be loosing 162 FPS from just 2" shorter barrel on the same load, same chamber, same rifling count, same twist and all my other Hornady factory ammo has been spot on velocity wise. Something just seems off with both 120's and that's when I gave things a good hard look for any signs of leaks and noticed how much buildup was on the gas block of the 6.8 vs. the 5.56 using the same block.
 

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COBrien,

I learned hard way from my first 6.8 build that one doesn't push the gas block tight against the shoulder. Even with the dimple, it will lock-down not centered in the dimple. With the gas block against the shoulder, the gas port is partially blocked and could make your rifle undergassed.

If the barrel has a dimple, I start the associated set screw into the dimple fist while wiggling the gas block left-right, fore-aft. As the screw tightens, the movement disappears as the set-screw hits bottom and is torqued. Once set, I've not ever had one change position. If it is a clamp gas block, the spacer is likely the best solution.
 

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Discussion Starter · #9 · (Edited)
COBrien,

I learned hard way from my first 6.8 build that one doesn't push the gas block tight against the shoulder. Even with the dimple, it will lock-down not centered in the dimple. With the gas block against the shoulder, the gas port is partially blocked and could make your rifle undergassed.

If the barrel has a dimple, I start the associated set screw into the dimple fist while wiggling the gas block left-right, fore-aft. As the screw tightens, the movement disappears as the set-screw hits bottom and is torqued. Once set, I've not ever had one change position. If it is a clamp gas block, the spacer is likely the best solution.
I do the exact same thing, although my first builds were with Adams Arms piston uppers, the gas blocks are either set screw with a pilot for pinning or factory pinned using solid roll pins (which are darn near impossible to get out). Works very well to ensure alignment. Also most gas ports are in the 0.125ish range for the gas block itself, so there is some accounting for very minor alignment error.

I think at this point, I may buy just a regular non-adjustable gas block and a few EZ Tune gas tubes as a backup method. Gives me options in case I keep having issues with the SLR on the 68. It has worked beautifully on my 5.56 upper, but that's quite a bit of carbon on the 68 block. My primary reason for using adjustable gas blocks was to allow for initial tuning, once I set it, I generally don't change it.
 

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My .223/5.56's have not required adjustable gas blocks. None of my barrels, 5.56, 224V, 6.8, or 300 BO came with 0.125" gas ports. My 6.8s have all been in the 0.09+ range. I did have to open one of my 300 BOs to 0.125 which is the maximum size and as wide as the gas blocks I have go. The eight 6.8s I have had all ran fine without AGB until I started shooting 130gr bullets with CFE & LVR. Now I need an AGB all the time on my 6.8s for any round because I always shoot suppressed.

Why have you planned for a back-up with EZ gas tubes?

Did you swap AGBs between your two rifles and seen the same results, e.g., no change?

Have you already tested the new AGB SLR has sent you and still had the issue?

Is it possible your DD barrel's gas port is too large, e.g, greater than 0.100", and flowing too much gas into the SLR gas block exacerbating this issue?

Have you tested your 120 grain bullets with the AGB closed to verify there is a significant velocity increase?

Below is a picture of my SLR gas block on my 12.5 6.8 with less than 10 rounds since I cleaned it (all suppressed). Yes it already has carbon build-up on it but my AGBs have always had some carbon on them. My entire rifle including ammo in the mag have carbon on them when you run a suppressor. Your going to have to clean your AGB frequently or it can lock up. Only run an AGB if you need one or you are causing yourself unnecessary maintenance.

Automotive tire Tire Bumper Bicycle part Rectangle
 

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Discussion Starter · #11 · (Edited)
To be clear, the gas port on the barrel isn't 0.125. I said the gas port hole in the gas blocks are typically 0.125 to allow for some minor misalignment with the barrels gas port.

I'm not sure what port size Daniels Defense uses, but I'll try to remember to mic it when I pull the SLR off the 68.

The replacement Sentry 7 is in the mail as of today. However I have limited trust in this block design given these issues this early on with the 68.

The tuning gas tubes and non adjustable blocks (which are non-adj Sentry 7s) give me a more robust option should this occur issue again. One I won't have to wait on. I like options especially in the current state of affairs.

I run adjustable blocks because I find a tuned action produces faster split times and followup shots which is advantageous for a fighting gun. It has as much impact as the brake. Having both is a boon.

To get that I need to use and adjustable or a tuned gas tube with a standard block. Now I will be able to test a new Sentry 7 and have another option in case the issue repeats. I'll see what the results are in a week or so.
 

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Mission dictates everything , that said if the mission is fighting with a gun I personally don't want a gun that has to be fine tuned to run properly with an assortment of the most likely available ammo .

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Discussion Starter · #13 · (Edited)
Agreed. If I can't use my own pile of 68 or there is no 68 in the area, that's where my 5.56 comes into play. Tuned on M855 and 77gr Razor Core. 6.8 is a rarity in the us compared to 556, 762 and other calibers, so your only likely to use a 68 as long as you have available ammo and have access to that ammo after returning from the field. You fight with what you have not with what you wish you had.

But while I'm able to use what I have, I can run the 68 for overmatch against 556, 5.45 of God for it Chinese 5.8. Both uppers are set up with nearly identical configs, just two pins and a sling and I'm back to running the 5.56.

Regardless, the weakest factory load I've shot in 68 is Prvi, generally if it cycled 110gr OTM it also cycle Prvi with only a slight shift in carrier speed (from 3 to 3:30). Only load I had real cycling issues with was DHA 110gr Nossler (due to the fast powder) at 2650 fps.

So if I tune it to 110grn OTM and 110gr VMax, thus far other loads like Wilson Combat 95gr ttsx or 110gr TSX etc. have all been reliable with the same BCG / buffer / gassing.

The biggest issue is figuring out this leaky block which I'll report back on in a week or so. Also interested in the tunable gas tubes, I will probably test both.
 

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I've recently noticed something after another outing this weekend testing loads. Since I don't currently reload and my focused use is as a fighting gun, I've spend a considerable amount of time testing accuracy, terminal ballistics and reliability of various loads for different applications. However, given the current cost and availability of 6.8 SPC ammunition, I also built a 5.56 clone (or as close as one can get) for training purposes. I'll get to the gas blocks in a moment.

Both uppers use SLR Sentry 7 Adjustable gas blocks. Both uppers use Daniels Defense 16" barrels, both mid-length gassed. Both use nitride gas tubes. The 6.8 upper is a 2020 production barrel from Daniels with SPC II chamber, 5R rifling, 1:11 twist in their heavy S2W profile, stellar accuracy with factory OTM, VMax and even SST. The 5.56 upper is NATO chamber, 1:7 twist, 6R. Both uppers have Sentry 7 Adjustable gas blocks.

I just this past weekend noticed that the gas block on the 6.8 SPC upper is absolutely caked in carbon around the metering screw and the leaf spring / detent. But the same gas block on my 5.56 upper that I shoot full snot M855 and 77gr Razor Core (both NATO spec loads), has zero carbon anywhere. I've run the 5.56 upper harder than the 6.8 in terms of training due to much cheaper ammunition as well. I would expect 5.56 to produce higher port pressures shooting NATO spec loads than 6.8 SPC shooting SAAMI spec loads.

The gas block on the 6.8 is NOT leaking out from underneath. Both barrels are dimpled, both blocks have the screws torqued to 25-in-lbs using a CDI calibrated torque dial. Both blocks are pinned also. Both blocks were sealed around the gas port using Rocksett and their fit onto the shoulder was very precise.

Has anyone else experienced SLR's gas blocks fouling like that? I have been getting some odd results from my factory load testing. All of the 110gr stuff shoots at or above it's spec. I get 2589 FPS from 110gr OTM and 2555 FPS from Black (aka VMax). But I only get 2371 FPS from 120 MKZ loaded by Druid Hill and 2388 FPS from 120gr SST Factory. Both of the 120's are well under their expected range for a 16" 5R SPC II with 1:11 twist, but both 110's are on the money.

At first I thought possibly the MKZ was slow because it's a solid copper bullet and just didn't' like the 5R and shorter 16" barrel. But now having tested 120gr SST factory loaded by Hornady, my results are notably under the spec velocity of 2460 FPS for a 16" barrel on that load as well. I have some 120gr SST from Druid Hill coming to test, but at this point, the heavy carbon buildup and low velocities of two very different 120gr loads, one NOT being a SCHP, have given me to some pause to investigate a potential issue.

I also looked up some testing others have done on the Daniels Defense Ambush line with their 18" SPC II barrel, same twist and rifling, they are getting 2550 FPS from 120gr SST factory ammunition. Typically velocity in the same barrel goes up between 30 to 40 FPS per inch, so their results seem in line with an 18"....2550 FPS - 2388 FPS = 162 FPS for just 2" of barrel length. Something is amiss.

This now leads me to believe the SLR gas block is bleeding pressure, as evidenced by the considerable carbon caking that I don't see on my 5.56 upper using the same gas block. It may be possible my 110gr loads would be faster as well if that's the case, but I would expect them to be less sensitive to leaks given their shorter dwell time.

This is my first time using SLR's gas blocks, as my previous 6.8 upper was based on the Adams Arms piston system paired with an ARP 16" barrel. Anyone else with an SLR experience a lot of carbon up around the metering screw / leaf spring on a 6.8?
6R? more like 6 groove Enfield rifling.
 

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What buffers are you using?
 

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Discussion Starter · #17 ·
6R? more like 6 groove Enfield rifling.
6 grove is what I meant. I had R in my head since I had previously mentioned the DD 5R 1:11 barrel. But yah, it's old school. That's what the US Military still uses for NATO spec barrels. Even the new URGI from Geissele uses 6 grove. Since the 556 is primarily a training upper, I focused on ruggedness and durability with high volumes of fire and high round count expectations. However it is very accurate with 77gr OTM from IMI. I get clover leaf at 50 yards using a 1-6x LVPO.
 

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I don’t believe a leaking gas port is contributing to the velocity loss. Joe Carlos, a retired AMU amorer tested this idea and debunked it.
 

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Discussion Starter · #20 ·
I guess we'll find out, but . I have options, so that's the good thing. It doesn't hurt to have different avenues of approach. If it's not actually leaking to where it is affecting velocity that's a plus, but I find it very strange that my 5.56 upper with the same Sentry 7 is so clean. And I've seen some Sentry 7's fail due to issues with the spring temper, maybe this one was part of a bad batch? I did measure the gas port on the DD barrel and it's 82 thousandths for reference. Daniels gasses their barrels for H1 and full mass BCG with around a 3 o'clock ejection.

The first thing I'll test is the EZ Tune gas tube just to see if they picked the right size gas port (0.078 is what they sent me) for my low mass BCG setup. I just paired it with a new Sentry 7 non-adjustable gas block as they are cheap and seem to be well made, plus light weight and I know they fit the handguard already. At the end of the day, nothing proves something does or does not work better than just testing it.
 
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