Fine tuning JP SCS @bigjake, et al

1

10ring1

The Zohan
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Aug 9, 2012
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#1
The other thread here had me thinking and Bigjake was gracious enough to offer advice from his experiences. I was going to pm him and ask but others may have similar questions and learn along the way as well.
I am currently working on a 6.5 CM build with AP builders set and the rest is purely JP parts (with the exception of the buffer tube). The barrel has +2 gas system, Lmos bcg, JP bolt that was matched to barrel, JP adjustable gas block, and SCS system. In full disclosure, I am using the extended buffer tube from magpul. I have a long Lop and use the magpul tube with the unmodified, JP SCS and spacer cut to fit and yes I have it applying pressure to the bcg. Running a geissle national match 2 stage trigger.
I have tuned the gas block to get the brass ejecting at about 4 oclock; however, my brass still are getting ejector swipes, some extractor marks but no other pressure signs. It is definitely not my load. I am using a very conservative load of 38-39 grains of h4350 with 147 eldm. The gas block is turned down just a few clicks from being completely off. I used the method where I turned the gas off and started working back up.
With that being said, my question is do I need a heavier spring/weights for the SCS? I would like to use a faster load but anything higher, I am blowing primers in a few. I think the primers are being violently dislodged in the ejection process in the few that lose primers with slightly higher charges.
Thanks in advance and thanks bigjake and others for sharing your knowledge.
 
#2
With the pressure signs showing, and your load on the lower end i would check your headspace on the brass and your OAL. Have you tried any store bought ammo?

Also keep in mind that it's pretty normal to get some ejector/ extractor marks with Large Frame AR Semi's, and you will also have that 100-300 rd settling/ break in period with new barrels.

After you have done all the above, i would then buy the Tungsten inserts adding one at a time to find your happy place
 
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#3
i do this regularly with 6.5cm AR's.. I add 1 tungsten weight at a time and go up in spring weight till the ejector marks, burs on the case head disappear. just tuned one for a good friend last week. 1 tungsten, 2 steel and red/blue spring.

I have tested this at the range with 5 6.5cm semis now. All +2 gas, JP LMOS Or RCA BCG, SCS Heavy, SLR, Superlative gas blocks.. when I find the right combo brass looks like bolt gun brass. Here's a few pieces I shot the other day after getting the tune right.

this brass is 3xHornady all in gas gun.










 
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#4
Thanks guys. Was going to start with the tungsten weight and spring asst kit. Accuracy is definitely there and def didnt want to reduce the load anymore. Really just trying to fine tune the rifle while getting a reasonably fast load and save on the brass. That brass looks as new padom.
 
#7
padom said:
the lmos with one small tungsten weight makes for a much softer recoil impulse and less gas needed.

a full mass does the same thing with no tungsten but has more felt recoil impulse. take your pic
Click to expand...

Are you saying that taking weight from the carrier and moving it to the buffer reduces recoil? Or are you ending up with a lighter weight overall between the low mass carrier and buffer?
 
#8
I think he's saying running the lightest mass and gas setting you can will give you the lowest recoil.

Sometimes just gas adjustment isn't enough and you need to add mass back to the reciprocating assembly. Weights on the scs are the only way to do that unless you go back to a normal carrier. Lmos and a tungsten weight is still lighter than a full mass BCG I imagine.
 
#9
Correct. Its a fine tune to get it where it needs to be. With a Full Mass BCG in the 6.5cm you will prevent the early unlock and wont have brass damage but the recoil impulse due to the heavy mass carrier is very noticable. If you run a LMOS carrier and just tune the gas block it will run great but you will have case head damage due to early unlock. Thats why I add tungsten weights and heavier springs that come with the SCS Heavy AR10 kit which prevents the early unlock. With the correct combination of springs, weights and LMOS carrier you get perfect brass and a very light recoil impulse.
 
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#11
Some really good information for tuning in here. For the folks that want to skip to the end and get to the shooting I ended up using the same setup JP recommends for 6.5 Creed - Low mass carrier group (JPBC-4 BCG) and 2 tungsten 1 steel SCS (JPSCS2-10H2).

Something to keep in mind is the compensator is providing the majority of the recoil reduction. Tuning the gas with an adjustable gas block and adjusting the mass of the system with either the VMOS carrier or the SCS has a comparative smaller effect on recoil but a very large effect on function. I did some experimenting with 2 rifles using different loads testing recoil impulse and brass appearance. Blind, I could pick out the difference between the recoil of the VMOS in the heavy configuration and the LMOS carriers. However, brass was a night and day difference for the hotter loads.

I ended up tuning the 6.5's similar to Padom to bias my systems to increase brass life and generally run the rifle cleaner. I personally do not feel the slight advantages of the lowest possible recoil impulse outweigh the advantages of reliability, cleanliness and brass life. The nifty thing about the SCS and different JP carriers is you can tune your rifle however YOU want it for YOUR purposes. You get it your way :cool:
 
#12
padom said:
Correct. Its a fine tune to get it where it needs to be. With a Full Mass BCG in the 6.5cm you will prevent the early unlock and wont have brass damage but the recoil impulse due to the heavy mass carrier is very noticable. If you run a LMOS carrier and just tune the gas block it will run great but you will have case head damage due to early unlock. Thats why I add tungsten weights and heavier springs that come with the SCS Heavy AR10 kit which prevents the early unlock. With the correct combination of springs, weights and LMOS carrier you get perfect brass and a very light recoil impulse.
Click to expand...

So, you end up with a lighter BCG buffer combo and heavier recoil spring?
 
#13
padom said:
the lmos with one small tungsten weight makes for a much softer recoil impulse and less gas needed.

a full mass does the same thing with no tungsten but has more felt recoil impulse. take your pic
Click to expand...

How do you think this related to 5.56 with LMOS?

I am new to the lightweight game, but I'm putting one together right now, 18" rifle gas, & plan to set it up with a standard (white) spring & a carbine buffer for starters, obviously with an AGB. Might get it functioning & tested with a full weight carrier & then switch to the LMOS setup.

Any comments would be appreciated.

Thanks.

MM
 
#15
I would think the same principal applies but less finicky. I would guess the 223 would require less pissing with. I have an OEM bcg with a standard scs in my 223 that is smooth and easy on brass. They get dirty of course but not beat up
 
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#17
Once again , spot on....

padom said:
Correct. Its a fine tune to get it where it needs to be. With a Full Mass BCG in the 6.5cm you will prevent the early unlock and wont have brass damage but the recoil impulse due to the heavy mass carrier is very noticable. If you run a LMOS carrier and just tune the gas block it will run great but you will have case head damage due to early unlock. Thats why I add tungsten weights and heavier springs that come with the SCS Heavy AR10 kit which prevents the early unlock. With the correct combination of springs, weights and LMOS carrier you get perfect brass and a very light recoil impulse.
Click to expand...
 
#18
Just want to make sure I’m reading you right on this.

You’re using just the standard JPSCS2-10 and also got the “spring pack JPSCS-PACK10” and played around with other spring weights also? And only have one single tungsten weight, correct? Your brass looks beautiful and I want to ask it out on a date ;)

https://www.jprifles.com/1.4.7.2_os.php



padom said:
the lmos with one small tungsten weight makes for a much softer recoil impulse and less gas needed.

a full mass does the same thing with no tungsten but has more felt recoil impulse. take your pic
Click to expand...
 
#19
elfster1234 said:
Just want to make sure I’m reading you right on this.

You’re using just the standard JPSCS2-10 and also got the “spring pack JPSCS-PACK10” and played around with other spring weights also? And only have one single tungsten weight, correct? Your brass looks beautiful and I want to ask it out on a date ;)

https://www.jprifles.com/1.4.7.2_os.php
Click to expand...

That is correct. I've played with all combinations of steel and tungsten weights and all springs. I run 1 tungsten and the yellow spring with LMOS carrier and brass has no damage and super soft shooting.
 
#25
So far so good. I’m after “sexy padom brass”... lovin my MEGA. Definitely don’t shoot it enough.
 

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#28
Darqusoull13 said:
Some really good information for tuning in here. For the folks that want to skip to the end and get to the shooting I ended up using the same setup JP recommends for 6.5 Creed - Low mass carrier group (JPBC-4 BCG) and 2 tungsten 1 steel SCS (JPSCS2-10H2).

Something to keep in mind is the compensator is providing the majority of the recoil reduction. Tuning the gas with an adjustable gas block and adjusting the mass of the system with either the VMOS carrier or the SCS has a comparative smaller effect on recoil but a very large effect on function. I did some experimenting with 2 rifles using different loads testing recoil impulse and brass appearance. Blind, I could pick out the difference between the recoil of the VMOS in the heavy configuration and the LMOS carriers. However, brass was a night and day difference for the hotter loads.

I ended up tuning the 6.5's similar to Padom to bias my systems to increase brass life and generally run the rifle cleaner. I personally do not feel the slight advantages of the lowest possible recoil impulse outweigh the advantages of reliability, cleanliness and brass life. The nifty thing about the SCS and different JP carriers is you can tune your rifle however YOU want it for YOUR purposes. You get it your way :cool:
Click to expand...
So JP recommends 2 tungsten weights for the 6.5 creed? Would anyone happen to know what JP recommends for .308? I’m currently running the red/yellow spring and one tungsten weight and I’m still getting some markings on the brass.
 
#29
#31
Can’t believe I missed this until now. I’m running ful JP internals on my 24” 6.5 (+2 Gas) and it runs beautifully. But I’m not above dicking around with it more for improvements. Tagging for later.
 

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#32
My rifle has been down for a while because I needed the optic for something else. I should get everything I need to shoot it again this weekend.

My question is: what order do you start making adjustments?

Do you tune the gas block first, then go messing with SCS weights and spring?

Or do you go ahead and put the tungsten weight in before tuning the gas block?

My barrel is a 24” Bartlein with +2 gas. Using LMOS carrier and SCS heavy.
 
#33
Throw 1 Tungsten weight in and the yellow and Red spring and run it and adjust your gas block to hold open on last round. Then fire 25-30 rds with 3 rounds in the mag at a time to ensure it locks back every time on last round. Sometimes during this test it wont and you will find you need to open the gas one more click.

This should be pretty darn close as Im running the same exact setup but a 22" +2 gas. If you are running suppressed it will be a entirely different tune with probably 2 Tungsten weights and different gas settings.
 
#34
Thanks. The suppressor is mostly just there for looks ?. I’m not willing to deal with the hassle of getting it to run suppressed and unsuppressed. Especially since the TBAC is not exactly known for low backpressure.
 
#37
I run a JP FMOS system with a JP adjustable gas block and SCS with the lighest spring and no tungsten weights on the scs. The gas block is open to around 75%, I forget the number of clicks. I also have an area 419 sidewinder brake on a 24" BA barrel. I'm shooting hornady lrp brass with 42grs of h4350 pushing a hornady 140gr eldm at around 2725fps with breautiful brass and very managable recoil. I just purchased a vortex razor hd gen2 4.5-27 optic that im planning on putting on top of it. The optic is said to weigh about 3lbs, so it should help a tiny bit with recoil as well.
 
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#38
Found this thread and thought I should ask here rather than start a new thread. I got the JP Silent Spring Kit with H2 buffer and it came with three springs, 100-110.

I’m curious as to where to start here. It will be shot on Seekins Kit 308 with 16-17 inch Bartlein with SLR gas block, JP full mass bolt and be shot suppressed probably 99% of the time.

So any ideas on what spring to start with? Also it’s my understanding that the lighter the spring the softer the gun will shoot? Or are the weights just to fine tune it so it doesn’t beat itself to death.

Thanks.
 
#39
texasleftychef said:
Found this thread and thought I should ask here rather than start a new thread. I got the JP Silent Spring Kit with H2 buffer and it came with three springs, 100-110.

I’m curious as to where to start here. It will be shot on Seekins Kit 308 with 16-17 inch Bartlein with SLR gas block, JP full mass bolt and be shot suppressed probably 99% of the time.

So any ideas on what spring to start with? Also it’s my understanding that the lighter the spring the softer the gun will shoot? Or are the weights just to fine tune it so it doesn’t beat itself to death.

Thanks.
Click to expand...

With a full mass carrier and H2 buffer (tungsten weights) on that SCS you want to start with the lightest spring. It comes with 3.... If you buy the SCS Heavy without the springs and you get the spring pack seperate, it comes with more than 3 springs... So just start with the lightest spring. To tune, shoot the gun exactly how you plan to shoot it most of the time (if with a supressor make sure the suppressor is on) with the ammo you plan to shoot most of the time.... Tune the AGB till it locks back on a round. Then shoot 25-50rd but with only 3 in the mag at a time. Make sure it locks back every time... It might not. Which means you need to open the gas block up 1 or 2 more clicks.. After you have done that, inspect your brass, is it ripped to shoot, heavier extractor marks, raised burs or looks like the case rims are getting ripped up? If so, your BCG isnt staying locked long enough and its starting to move to the rear before the pressures have subsided enough for the brass to retract, so its literally ripping it out of the chamber too early.... If you have to go up to the next heaviest spring, repeat all the steps I mentioned above again then check your brass again. You will find a combo that shoots great and damages your brass the least.

Little bit of info, if you're shooting with a can, you ARE going to get SOME brass damage no matter how much you tune it. Your goal here is to find the combo that causes the least brass damage and shoots the softest and most accurate...

With an LMOS BCG, JP SCS Heavy with Springs and SLA AGB and no supressor, I am able to tune it to where I have zero damage to my brass every single time. Looks like it was fired from a bolt gun... With a can, I do get some swipe but when tuned properly its minimal.

I dont know what barrel extension you have on that Bartlein barrel. But the other part of the brass damage on these large frame gassers is caused by non-honed barrel extensions. They are sharp, and when the brass is being pulled out of the chamber, before its ejected, it scraps against the teeth of the barrel extension between 2 and 5 oclock and puts a gauge and 2 deep scratches in your necks and on the shoulder. You can fix this by pulling the barrel, and using a cratex or similar polishing tip on a dremel and hit the back side of these lugs to remove the sharp edges.

All my large frame gasser barrels I give my smith JP barrel extensions to use since they are honed and QPQ coated. No worries about brass damage from the extension. You can also hone a Lilja or similar barrel extension before its installed on the barrel which is easy as well.
 
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#40
This isn't 100% relative to this thread, but I get marks on my brass in my Mk 12 type barrel and didn't even think about sharp edges on the barrel extension. I thought it was something in the chamber. I looked through my bore scope and didn't see anything. I scrubbed the chamber to remove any carbon in the neck and still got the ding/scrape marks that are shiny. Now it makes total sense what is going on. Thanks for pointing that put Padom!
 
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#41
I have a LWRCI REPR MKII Elite in 6.5 Creedmoor that I shoot suppressed. It's an amazing 1,200-yard-plus rifle. Only one issue was making me crazy ... Gas adjustment was at the lowest setting, and ejections were at 2 o'clock (in the dirt in front of the firing line). Got the JP SCS (Heavy) for AR10 and it got me to a little past 3 o'clock (consistently hitting the guy on the next bench in the head). That heavy SCS version comes with two tungsten weights, and one steel weight. The tungsten weight is a full ounce heavier than the steel weight. Ordered an additional tungsten weight, got it in the mail, replaced the steel weight, and headed to the range yesterday. Ejection is now at a perfect 4 o'clock , with all three tungsten rings and the heaviest spring. Problem solved ... very expensively ... but hey, it's a $4,500 rifle with $2,000 glass and a $1,000 suppressor that I shoot in gas-gun competitions ... so going cheap wasn't on the To-Do-List. Solution suggestion came from the Q&A on the "Guns & Tactics" YouTube channel ... and it worked great.

It was this episode where he answered my question with this suggestion:
 
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#42
rustyinbend said:
I have a LWRCI REPR MKII Elite in 6.5 Creedmoor that I shoot suppressed. It's an amazing 1,200-yard-plus rifle. Only one issue was making me crazy ... Gas adjustment was at the lowest setting, and ejections were at 2 o'clock (in the dirt in front of the firing line). Got the JP SCS (Heavy) for AR10 and it got me to a little past 3 o'clock (consistently hitting the guy on the next bench in the head). That heavy SCS version comes with two titanium weights, and one steel weight. The titanium weight is a full ounce heavier than the steel weight. Ordered an additional titanium weight, got it in the mail, replaced the steel weight, and headed to the range yesterday. Ejection is now at a perfect 4 o'clock , with all three titanium rings and the heaviest spring. Problem solved ... very expensively ... but hey, it's a $4,500 rifle with $2,000 glass and a $1,000 suppressor that I shoot in gas-gun competitions ... so going cheap wasn't on the To-Do-List. Solution suggestion came from the Q&A on the "Guns & Tactics" YouTube channel ... and it worked great.

It was this episode where he answered my question with this suggestion:
Click to expand...


You mean Tungsten...not Titanium....Titanium is lighter than steel....
 
#46
Appreciate the help. Probably going to start out as Padom outlines but do you think I will finish up the heaviest spring and maybe need the third tungsten weight? I have three springs now. The 100, 105 and 110.

And I guess the final answer is I’ll know when I’m there when I have four o’clock ejection with some ass behind it and brass that’s not torn up? Thats when to say when? Because I could tinker with the the springs and the gas block in infinite combinations with my firearm ocd.

Thanks again.
 
#47
texasleftychef said:
Appreciate the help. Probably going to start out as Padom outlines but do you think I will finish up the heaviest spring and maybe need the third tungsten weight? I have three springs now. The 100, 105 and 110.

And I guess the final answer is I’ll know when I’m there when I have four o’clock ejection with some ass behind it and brass that’s not torn up? Thats when to say when? Because I could tinker with the the springs and the gas block in infinite combinations with my firearm ocd.

Thanks again.
Click to expand...

My final tune on the last 4 22" 6.5cm Bartlein's, +2 gas, JP LMOS was the 2 tungsten weights and yellow spring.....you'll need less, you have full mass...
 
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