6.5cm gas system length -- brass condition between rifle and +2" gas port?

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Kiba

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#1
For those of you who started with rifle length gas systems on their 6.5CMs and later changed to barrels with +2” gas ports, how dramatic was the difference in brass condition between the two gas port lengths, and were you able to lighten up the carrier or buffer mass with the +2” gas port to cut down on reciprocating mass and still have the brass come out in relatively good shape?

I have a pair of 6.5CM JP LRP07s, one 20”, one 22”. These were purchased about 10 years ago in the infancy of companies building 6.5 CM AR10s and at that time nobody was making barrels with +2” gas port locations. (I also had a 6.5CM GAP10 during that same timeframe, and it had issues piercing primers like crazy even with factory loads because the large firing pin GAP shipped it with didn’t play nice with 6.5CM. After 2 trips back to GAP they swapped the bolt for an Armalite bolt with a small firing pin and the piercing issue went away.)

I’ve always dealt with ejector swipes on these 2 uppers even with mild loads of H4350 or RL16 and 123gr Scenars. I’m 90% sure it’s because of the rifle length gas port not playing nice with elevated port pressures generated by the common 6.5CM powders with slower than typical 308 powder burn rates, and premature bolt unlocking and movement because of that.

Right now both are set up with LMOS carriers, adjustable gas blocks, and JP SCS buffers with 3 tungsten weights to try and delay/slow the bolt unlocking. JP initially shipped them with 1 tungsten / 2 steel weights on the buffer and the swipes were pretty bad. I then tried 2 tungsten / 1 steel which improved things a bit, and am now running 3 tungsten which improved brass condition a bit more. Gas block is set to just barely lock back on the last round. The brass comes out in pretty decent shape with 3 tungstens, but still not as good as I’d like (certainly not as good condition as brass from my old Armalite AR10 in 308.) Felt recoil from cycling isn’t bad, and the rifles don't “nosedive” excessively when chambering a round even with the H3 equivalent buffer weight slamming closed.

I run cheap Hornady 6.5CM brass that I usually buy once fired off the PX here and usually toss it after 4 trips through the ARs. Even with the 3 tungsten weights in the buffer sometimes I have to take a file to the ejector swipes before I resize the brass.

Debating if I want to swap these barrels out to new +2” gas port barrels before their end of life, or just run them as is until they’re shot out; they’re both very accurate so I’m not in a hurry to toss them. I suppose I could also sell the current uppers complete and buy 2 new complete uppers with +2” gas ports using the newest design JP semi-monolithic uppers with the integrated handguard mount… but that seems silly just to improve brass condition. The current uppers are very accurate, and I can buy a lot of Hornady 1x fired brass from the PX and toss it every 4th firing before the money saved by getting a few more reloads out of the brass would cover the cost of 2x new +2” gas barrels and 2x longer handguards or 2x new complete uppers from JP.

Appreciate any input and experience you've gathered going from rifle length to +2” gas port location.
 
#2
Any input? I know there must be a few of you out there currently running +2" gas ports that used to run rifle length gas ports on your 6.5 CMs, and I'd like to hear your opinion if it's worth it to swap barrels now or just live with getting 4 reloads out of the brass until my current ones are shot out and change them then.
 
#4
I have a JP with the +2 barrel. When I first got it shooting it with light loads the brass was black(ejecting to soon) and the ejector swipe required filing. This was Lapua brass. Like you I went with 3 tungsten weights and the red/silver spring. That cured the soot but not the the ejector swipe.
I was getting frustrated until one day while reloading I tried to fit a a bullet into a fired shell and it wouldn’t fit. I assumed the chamber was tight, so I turned the necks down .002 on 20 rounds and shot them. No ejector marks and a bullet would now slide in. The thick neck was causing over pressure. Now my brass is almost as clean as a bolt gun.
 
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#5
Kiba said:
Any input? I know there must be a few of you out there currently running +2" gas ports that used to run rifle length gas ports on your 6.5 CMs, and I'd like to hear your opinion if it's worth it to swap barrels now or just live with getting 4 reloads out of the brass until my current ones are shot out and change them then.
Click to expand...
I ordered a 22CM with a +2.4 gas system from Craddock. When I get it I’ll post results.
 
#9
Huskydriver said:
I still top out at 38.5-39 gr of h4350 in my +2 gas gun. Which is roughly 2700 fps in that barrel
Click to expand...
With the Hornady 140 eldm I was at 40.3 and 2700 FPS. I switched over to Stabal 6.5 and to get the same velocity I had to go to 41.6 gr.
I recently switched to the Hornady 123 eldm at 43.7 of Stabal 6.5 @2900 FPS. They shoot flatter all the way to 1000 yards.
 
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#10
Good info.

I gave up on 140s in both my gas guns with the rifle length gas ports as even loaded way slow they were trashing brass. I switched to 123 sceanars.

I'm running 123 sceanars with RL16 in the 22" barrel at 2840 right now. That's the highest node as I can push things without the ejector marks getting really bad. They're still there, but not too bad at 2840.

In the 20" barrel, 2770 with the 123 sceanars and RL16 was the highest node it would push them without being really hard on the brass.

At this point I think I'll shoot these barrels until they're done and go at least +2" gas port on the next barrels.
 
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#11
I'm running a 24" , +2 gas x-caliber that is very accurate, but 130 eldm's over 41.0grs of H4350 gives me nasty ejector burrs. No other grain wt bullet or other powder has done this (yet). I've not got mine figured out either. Running a KVP 5.6oz buffer and sprinco orange spring.

I just got some small primer starline brass to try (have been using new hornady)

Good luck. I'll be watching this thread for ideas myself.
 
#12
So the short answer to your question is yes, absolutely the +2 gas system makes a huge difference and it gives you a larger operational window for your buffering Mass/gas adjustments, having your brass come out clean without any Burrs from the ejector or the extractor is just one of the advantages. All of my 6.5 creedmoors are running a +2 system with the exception of an 18" 6.5 Creedmoor I'm using a rifle length system in that, and I'm still having to utilize a H3 SCS buffer system and a full Mass 2.2 lbs BCG to keep that tamed.


When running a low mass system I would recommend only doing that on 22" on up, me personally when i run the low mass systems I don't use the JP carriers anymore I've since switched over to running the full titanium carriers from RCA which usually average about a full ounce lighter than the low mass offering from JP and you get to still keep the same surface area as a full Mass carrier, and the titanium carriers from RCA are cheaper.

When I run the low mass systems I use the standard buffer weight with the SCS and go down to the next softest spring.
 
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#14
Ive built 40+ and counting...my first 10-15 builds were on Bartlein and Krieger blanks with rifle gas... if you want targets I can post them as I have a database of all testing. Accuracy was not a concern, very very accurate. But brass was getting beat to shit...raised burs and swipes on case heads and gouges out out of necks and shoulders. I can also say notcibly more felt recoil... that was with full mass and LMOS carries from JP....and adjustable gas blocks adjusted to lock back on an empty mag reliably

When I started going +2 in the early days before it was a fad with JP LMOS carries I had less felt recoil and no case head raised burs or swipes but I was still getting the gouges in my necks and shoulders. I discovered it was the sharp lugs gouging the brass on extraction so I started hand polishing my barrel extensions and giving them to my Smith for my custom.barrels..

I then switched to only using JP QPQ honed and polished barrel extensions on all my large frame gasser barrels. They are great, no hand polishing needed...

My go to these days is a custom Bartlein or Krieger....+2 gas, JP QPQ barrel extension and a RCA Adjustable gas key bolt carrier or a JP LMOS carrier and SLA adjustable gas block.

Pair that with a JP SCS heavy with spring pack and tune springs and gas and your large frame gasser will be accurate, soft shooting and brass look like it was fired in a Bolt gun...
 
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#16
padom said:
Ive built 40+ and counting...my first 10-15 builds were on Bartlein and Krieger blanks with rifle gas... if you want targets I can post them as I have a database of all testing. Accuracy was not a concern, very very accurate. But brass was getting beat to shit...raised burs and swipes on case heads and gouges out out of necks and shoulders. I can also say notcibly more felt recoil... that was with full mass and LMOS carries from JP....and adjustable gas blocks adjusted to lock back on an empty mag reliably

When I started going +2 in the early days before it was a fad with JP LMOS carries I had less felt recoil and no case head raised burs or swipes but I was still getting the gouges in my necks and shoulders. I discovered it was the sharp lugs gouging the brass on extraction so I started hand polishing my barrel extensions and giving them to my Smith for my custom.barrels..

I then switched to only using JP QPQ honed and polished barrel extensions on all my large frame gasser barrels. They are great, no hand polishing needed...

My go to these days is a custom Bartlein or Krieger....+2 gas, JP QPQ barrel extension and a RCA Adjustable gas key bolt carrier or a JP LMOS carrier and SLA adjustable gas block.

Pair that with a JP SCS heavy with spring pack and tune springs and gas and your large frame gasser will be accurate, soft shooting and brass look like it was fired in a Bolt gun...
Click to expand...

More confirmation, thanks. I was wondering how the recoil difference was and if it would be softer with the reduced port pressure of a +2" gas port.

Several years ago I tried 123s with AR comp to try and drop the port pressure since it's a faster burning powder than H4350 and RL16, and it noticably reduced recoil. Unfortunately the brass was showing actual pressure signs like noticable ejector flow (not just swipes from a bolt carrier unlocking too fast) at about 100fps slower than moderate H4350 and RL16 loads; AR comp was just too fast for even the 123s to get decent velocity before running into pressure.

Both my barrels are very accurate, but as mentioned, a bit harder on brass than I'd like.

I'm considering 2 options...

1. Shoot them as is until they're done using Hornady 1x fired brass from the PX and toss it every 4th reload or so. Once fired Hornady brass is cheap, and I really don't shoot my large semi autos all that much so it will be quite a while before I shoot the barrels out. Of course, once the barrels are shot out they're junk.

2. Sell both barrels right now and buy new ones with +2" gas ports. This might actually be a cheaper option as I can sell the current barrels which still have plenty of life in them and use the proceeds towards the new +2" barrels and 2" longer hand guards, but not sure how much of a market there is for 6.5cm barrels with rifle length gas ports since the word is now out that +2" is the way to go on 6.5cm.
 
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#17
Kiba said:
More confirmation, thanks. I was wondering how the recoil difference was and if it would be softer with the reduced port pressure of a +2" gas port.

Several years ago I tried 123s with AR comp to try and drop the port pressure since it's a faster burning powder than H4350 and RL16, and it noticably reduced recoil. Unfortunately the brass was showing actual pressure signs like noticable ejector flow (not just swipes from a bolt carrier unlocking too fast) at about 100fps slower than moderate H4350 and RL16 loads; AR comp was just too fast for even the 123s to get decent velocity before running into pressure.

Both my barrels are very accurate, but as mentioned, a bit harder on brass than I'd like.

I'm considering 2 options...

1. Shoot them as is until they're done using Hornady 1x fired brass from the PX and toss it every 4th reload or so. Once fired Hornady brass is cheap, and I really don't shoot my large semi autos all that much so it will be quite a while before I shoot the barrels out. Of course, once the barrels are shot out they're junk.

2. Sell both barrels right now and buy new ones with +2" gas ports. This might actually be a cheaper option as I can sell the current barrels which still have plenty of life in them and use the proceeds towards the new +2" barrels and 2" longer hand guards, but not sure how much of a market there is for 6.5cm barrels with rifle length gas ports since the word is now out that +2" is the way to go on 6.5cm.
Click to expand...
Not sure which COA you selected. I had brass getting beat like Ike beat Tina with the standard rifle length gas system on a Compass Lake 6.5 upper. A member here, forgot which one, suggested that I get one of the Superlative Arms AGB. I have done some testing and so far it’s improved things with the Lapua brass I use. Could possibly provide you with an interim solution before you get your new uppers.
 
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#21
IMG_4351.jpeg


Much better than without the AGB
 
#22
bigjake83 said:
So the short answer to your question is yes, absolutely the +2 gas system makes a huge difference and it gives you a larger operational window for your buffering Mass/gas adjustments, having your brass come out clean without any Burrs from the ejector or the extractor is just one of the advantages. All of my 6.5 creedmoors are running a +2 system with the exception of an 18" 6.5 Creedmoor I'm using a rifle length system in that, and I'm still having to utilize a H3 SCS buffer system and a full Mass 2.2 lbs BCG to keep that tamed.


When running a low mass system I would recommend only doing that on 22" on up, me personally when i run the low mass systems I don't use the JP carriers anymore I've since switched over to running the full titanium carriers from RCA which usually average about a full ounce lighter than the low mass offering from JP and you get to still keep the same surface area as a full Mass carrier, and the titanium carriers from RCA are cheaper.

When I run the low mass systems I use the standard buffer weight with the SCS and go down to the next softest spring.
Click to expand...
A full mass 2.2 lbs BCG?…
Does it have a lead slug in the back of the BCG?
Thanks
 
#24
Many have the so-called cure for the 6.5creed in by having the + gas systems? Well, not so fast on that cure as it doesn't help that much! With having 8 AR10 6.5creed barrels from 5 different makers only 1 works 100%.............Aero Precision with standard rifle length gas (12.7"). It was the 1st barrel I bought in the cartridge and did not use an adjustable gb, but used standard weight buffer and spring. Their answer was having the gas port size correct! Brass ejected properly and without heavy ejector and extractor marks and no popped primers. I've found gas port size to be more beneficial than location on this cartridge in AR's. AP list port size on their site when selecting "specifications" after barrel is selected. All 5 barrel makers were tested with the same ammo of the same lot # of Hornady 123sst and a-max match ammo. Usually if not noted on the box for AR's it has been tested for bolt guns which usually has overpressure symptoms in gas guns.
 
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#25
Top O' Texas said:
Many have the so-called cure for the 6.5creed in by having the + gas systems? Well, not so fast on that cure as it doesn't help that much! With having 8 AR10 6.5creed barrels from 5 different makers only 1 works 100%.............Aero Precision with standard rifle length gas (12.7"). It was the 1st barrel I bought in the cartridge and did not use an adjustable gb, but used standard weight buffer and spring. Their answer was having the gas port size correct! Brass ejected properly and without heavy ejector and extractor marks and no popped primers. I've found gas port size to be more beneficial than location on this cartridge in AR's. AP list port size on their site when selecting "specifications" after barrel is selected. All 5 barrel makers were tested with the same ammo of the same lot # of Hornady 123sst and a-max match ammo. Usually if not noted on the box for AR's it has been tested for bolt guns which usually has overpressure symptoms in gas guns.
Click to expand...
How long were the barrels?
 
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#28
padom said:
Ive built 40+ and counting...my first 10-15 builds were on Bartlein and Krieger blanks with rifle gas... if you want targets I can post them as I have a database of all testing. Accuracy was not a concern, very very accurate. But brass was getting beat to shit...raised burs and swipes on case heads and gouges out out of necks and shoulders. I can also say notcibly more felt recoil... that was with full mass and LMOS carries from JP....and adjustable gas blocks adjusted to lock back on an empty mag reliably

When I started going +2 in the early days before it was a fad with JP LMOS carries I had less felt recoil and no case head raised burs or swipes but I was still getting the gouges in my necks and shoulders. I discovered it was the sharp lugs gouging the brass on extraction so I started hand polishing my barrel extensions and giving them to my Smith for my custom.barrels..

I then switched to only using JP QPQ honed and polished barrel extensions on all my large frame gasser barrels. They are great, no hand polishing needed...

My go to these days is a custom Bartlein or Krieger....+2 gas, JP QPQ barrel extension and a RCA Adjustable gas key bolt carrier or a JP LMOS carrier and SLA adjustable gas block.

Pair that with a JP SCS heavy with spring pack and tune springs and gas and your large frame gasser will be accurate, soft shooting and brass look like it was fired in a Bolt gun...
Click to expand...
I found this post super interesting.

Curious, where exactly are you polishing on the barrel extension? I get those marks on my brass too and would love to mitigate it.

Also, if you run a heavy JP SCS to slow down the action, Would you want to run a a JP VMOS to slow the bolt movement even more?

Curious to know why you'd want a heavy buffer and a light BCG.
 
#29
Torchy said:
I found this post super interesting.

Curious, where exactly are you polishing on the barrel extension? I get those marks on my brass too and would love to mitigate it.

Also, if you run a heavy JP SCS to slow down the action, Would you want to run a a JP VMOS to slow the bolt movement even more?

Curious to know why you'd want a heavy buffer and a light BCG.
Click to expand...
The lighter bcg keeps the mass lower which keeps the recoil less. In doing so you need to increase 1 or both (buffer weight and / or a heavier spring ). That's for starters.
 
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#30
Top O' Texas said:
The lighter bcg keeps the mass lower which keeps the recoil less. In doing so you need to increase 1 or both (buffer weight and / or a heavier spring ). That's for starters.
Click to expand...
OK that makes sense. I'm building a AR-10 in 6.5CM right now and thinking about switching up some components based on this thread.

My GAP-10 in 6.5cm beats up brass and leaves those marks on the cases. I'd like to mitigate as much of that as possible on this new build.

If anyone is wondering, only shoot the gap-10 suppressed and put a SLR agb on it and tuned it. Like that rifle a lot. Trying to build something similar but make it even better.
 
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#31
Top O' Texas said:
The lighter bcg keeps the mass lower which keeps the recoil less. In doing so you need to increase 1 or both (buffer weight and / or a heavier spring ). That's for starters.
Click to expand...
Exactly. Less mass slamming to the rear which in turn means less felt recoil. I've built over 50 now both with LMOS and full mass BCG's. The configuration is different for both.
 
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#33
Torchy said:
I found this post super interesting.

Curious, where exactly are you polishing on the barrel extension? I get those marks on my brass too and would love to mitigate it.
Click to expand...

You are polishing the sharp backside of the lugs between 12-3 o'clock... when the brass is exiting the chamber, the case is under spring tension of the ejector which is pushing the casew against the backside of those lugs as it extracts from the chamber. Polish up the sharp edges and you won't get those 2 deep lines on your case...

Another reason I use JP QPQ honed and polished barrel extensions for all my custom Bartlein, Krieger, Rock large frame gassers...

Much easier to do with the barrel extension in hand....but you can do it installed on the barrel if you pull the barrel from the upper and use a cratex bit on a dremel...
 
#35
Curious if any of you have tried RL15.5 with 130 & under in a AR.
Gives great speed, accuracy, & SD’s in my 25” bolt gun.
The reason I’m asking is per GRT burn out happens fast and port pressure should be reduced without giving up speed.

On my 308 I shortened the ejector spring just a smidge and it help with the neck scoring. Proof extension.
 
#37
padom said:
Exactly. Less mass slamming to the rear which in turn means less felt recoil. I've built over 50 now both with LMOS and full mass BCG's. The configuration is different for both.
Click to expand...
On my 308's using LMOS I can tune down via gb and use an extra power spring. On 6.5creed with same bcg it takes an extra power spring and a heavier buffer. All this regardless of rifle weight.
 
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