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Mega Arms Monolithic 308 JP Low Mass Problems

jwsnydes

jwsnydes

Private
Full Member
Minuteman
Aug 22, 2010
0
1
43
South Dakota
www.snipershide.com
Hello,

I just finished putting together my first precision rifle and everything seemed to be going great until I got to the range. I need a little help from someone on diagnosing what is going wrong.

First off my specs:
Mega Arms Monolithic upper and lower 308
JP Low Mass Complete bolt group including Enhanced bolt.
Rifle length JP adjustable gas block.
Rifle length spring tuned JP.
18 inch Rock Creek Barrel done by Predator Customs
Wilson Combat TTU 3 Gun Trigger
Vortex Viper FFP MRAD/MRAD 4-16
Magpul PRS stock
LR20 Magpul magazine
Standard AR-15 Lower Parts Kit
Ambi Safety Selector
American Eagle 7.62x 51mm 168gr OTM

What is happening:
As best as I can get it, it will eject but won't cycle in a new round or lock back on an empty magazine. It will when I do it by hand without an issue.

With the gas turned all the way closed, it won't eject or cycle, then turned out about 2 turns it will create a stove pipe, 3 turns out starts to eject reliably, 4-9 turns ejects but still but won't cycle or lock back. The screw does seem to be loose at all positions besides completely closed.

Everything is tightened down to specs with torque wrenches. I've lubed the in all the standard places, all the screws and barrel have been tightened down to spec with torque wrenches. Pulling the charging handle to cycle the bolt, it feels pretty heavy and a touch gritty.

Could someone please look at the attached pictures to see if I am doing something obviously wrong. Could it be that this isn't rated for 7.62x51 ammo? Could the gas block be slightly off centered, limiting the gas to the bolt carrier group? Is my spring setup right? Should the Gas screw ever be unscrewed almost out of the gas block? Why would it cycle by hand reliably, but when shot, eject and not cycle or lock back?

In photos 3 (between the gas tube and barrel extension on the frame) + 5 (on the buffer face) there seems to be wear already even though I have shot 30 rounds total.

Any help would be great, as I'm really frustrated after the months and thousands of dollars that it took to put this together. And as the icing on the cake, someone shooting on an adjacent range accidentally shot me while skeet shooting. I'm not seriously injured.









 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

If you don't mind me asking, what kind of buffer and spring are you using?
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Looks like a beautiful build. From what you describe, I'd be inclined to think the same thing as you, not enough gas. If in fact that is the problem, higher power ammo should help,also, a lighter spring or buffer could also allow it to cycle further with the same ammo. I don't have the JP adjustable GB but have used others. Once your adjustment screw is past the gas port hole in the GB, it's no longer restricting the gas, your not getting any more gas by unscrewing it any further than that. I don't know what other parts you have on hand but if you have a lighter buffer or spring, I'd try that, as well as different ammo as suggested by the other poster. The logical thought would be that the light weight JP BCG should cycle easier and should require you to restrict gas somewhat to avoid slamming the BCG into the buffer tube, if thats not happening for you, it might be because the gun is new and doesn't cycle smoothly. You should really lube the cycling parts excessively, until the oil drips off it. I had a .308 AR that initially cycled but didn't lock back on an empty mag, once I oiled it down until the gun was dripping and got about 100 rds of full power loads through it, the gun worked 100%. My gun had the CAR XH buffer from heavy buffers, so that might have contributed to my initial cycling issues. Good luck with it.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

I have a Mega set also w/18.5" rifle length and a JP adjust block but a regular BCG and it runs smooth.
I use SW Ammo 175's and the screw on the block is barely backed out but I did try using SSA 175 otm's and it started doing exactly what you described and I did have to back the set screw almost all the way out.

Ive tried some other ammo and she is pickey as hell which is fine with me cause we both like the FGMM and SW 175's the best(565yds-1k)


Hope this helps

AL
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Let me share my assessment of what the problem is.

You have an 18" barrel with 12" gas tube and you are using a 7% plus power spring. I really like the JP springs, but they dont work with 18" barrel 12" gas tube configs.

I just fixed this problem for one of the top gun writers. He cut his DMR-10 barrel down two inches and guess what... We've run into this consitently with 18" barrel 12" gas tube guns. On 18" barrels I have the gas port located at 10" and 12" on 20" barrel guns.

The fix is to use a standard power or reduced power spring. Just use the standard DPMS AR-308 spring or get Wolf to do an RP spring for you if the DPMS doesnt work.

AR's run on gas and springs and your problem is that with the 18" barrel and 12" gas tube you dont have enough dwell time, therefore not enough gas to run the system with the 7% plus power JP spring.

The spring is a qick and cheap fix.

Your gun looks like you did everything else right. Very nice!

Try it out and report back.

-E
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Thanks for the advice everyone.

I took the gas tube out and and tried to blow through it like a straw and it wouldn't budge then I stuck a small hex key in the end to see if I could unclog it. after a bit I can now blow through it very easily. I'm going to take it back to the range (extra lubed up) and see if it was just clogged from the get go.

I think there might have been something blocking the gas from flowing since the original installation.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Good luck today at the range and watch out for crazy skeet shooters!!!!!
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Very interesting can't wait to find out if that worked.

Also replace that spring with a standard.

And I've ran some of the AE 168s threw an Armalite AR-10 and they cycled fine and were hammers.

Also something about building and choosing stuff from the get go that is different from normal like the 7%+ spring, a lighter weight BCG...is alright but not knowing if it would cycle or function on a Regular spring and BCG is time/money wasted to me. Now if you went with the "normal" and then tried the "different" and it didn't work...well then you would be fine and just switch back.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Running the JP Low mass in my MA-TEN with the standard spring with 100% reliability.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Another thing to add is that the MA-TEN's have cycling issues under the first 200-300 rounds. I don't think this is the exact same problem, but my brother's build and my build both would shoot 10-20 rounds, then stovepipe or fail to feed. The problem was much more pronounced with PRVI and surplus while FGMM was able to cycle reliably for the day.

Personally, I'm not very impressed with the MA-TENs, or at least the idea of mixing and matching parts on AR308 platforms. When I build a rifle, I want it to function with 100% reliability as my Armalite AR10 did, not be finicky with ammo and such.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rchman</div><div class="ubbcode-body">Another thing to add is that the MA-TEN's have cycling issues under the first 200-300 rounds. I don't think this is the exact same problem, but my brother's build and my build both would shoot 10-20 rounds, then stovepipe or fail to feed. The problem was much more pronounced with PRVI and surplus while FGMM was able to cycle reliably for the day.

Personally, I'm not very impressed with the MA-TENs, or at least the idea of mixing and matching parts on AR308 platforms. When I build a rifle, I want it to function with 100% reliability as my Armalite AR10 did, not be finicky with ammo and such. </div></div>

I don't see how the upper/lower receiver can affect cycling/gas issues. I wouldn't listen to this. Like the others have said, sounds like you're short stroking, I'd try a regular buffer spring and turn your gas all the way up. If those don't fix it, you might have to get the gas port opened up a bit.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dlxcupid</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rchman</div><div class="ubbcode-body">Another thing to add is that the MA-TEN's have cycling issues under the first 200-300 rounds. I don't think this is the exact same problem, but my brother's build and my build both would shoot 10-20 rounds, then stovepipe or fail to feed. The problem was much more pronounced with PRVI and surplus while FGMM was able to cycle reliably for the day.

Personally, I'm not very impressed with the MA-TENs, or at least the idea of mixing and matching parts on AR308 platforms. When I build a rifle, I want it to function with 100% reliability as my Armalite AR10 did, not be finicky with ammo and such. </div></div>

I don't see how the upper/lower receiver can affect cycling/gas issues. I wouldn't listen to this. Like the others have said, sounds like you're short stroking, I'd try a regular buffer spring and turn your gas all the way up. If those don't fix it, you might have to get the gas port opened up a bit. </div></div>

I totally concur with this advice.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

I've put about 500 rounds now through my MA-Ten build. Not even one failure. Failure to feed with this system will be a function of how well put together the rifle is, rather than the upper lower receiver you choose.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

I agree with Mute I have over 800 through mine no issues
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Range day two with the gas port at full open all day:

I bought some 150gr and brought the rest of the 168gr and fired about 30+ rounds. One cycled, all ejected, but I am narrowing down the problem. The system is still 100% reliable when hand cycled, and the action locked back every time on an empty magazine when hand cycled, 50% of the time when fired.

I think that I have a feed issue, but I still could be misunderstanding the problems. When the rifle fires, the round is ejected and the the next round in the magazine gets gashed by the bolt but doesn't get scooped out. It gets scooted forward a bit, and it las a linear scrape down the whole length of the round. This happens the same with 2-20 rounds in the magazine.

Do the pictures below illustrate that the BCG is being pushed back far enough to cycle the rifle?

It shoots really nice groups, it just isn't cycling.







 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

I had the same problem in a 556(short stroking). I put in a larger gas port and it runs really smooth now.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Go find you a normal DPMS rifle length buffer spring. See what that does. I'm more curious as to why your rounds are getting gashed up like that. Did you lube the crap out of the BCG? Make sure you're using the screw for the PRS stock and not a regular A2 stock screw.

Also did make sure your gas ports are lined up? Take off the gas block and measure the port on the tube and on the barrel, make sure those line up.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

I double checked that I am using the PRS screw. Would short stroking do that to the cases? Would switching to a 7% less powerful spring really change that much?

I did lube the hell out of it. It made the hand cycling much easier. No gritty feeling now.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

JW,

Yes, the +7% spring does make a differnece. I use the JP srings quite a bit and they are a very good product.

However, my experience is they work fine with 20" barrels and not with 18", where the gas tube is a 12"

I just went through this with a customer who is a prominent gun writer. He cut his 20" to 18" on his DMR-10 and it turned into a single shot gun. In that case we put a 20" barrel back on it. On another AR I built for him he wanted the 20" barrel cut to 18" and using the JP spring it was a single shot gun. Changed the spring and it worked perfect. He then went to a training with DoE Nulclear Security and ran the rifle 14hrs a day for a week and he said it ran flawlessly.

So there you go, a spring can make a big differnece. It could be you have some other problem, but odds are high its just the spring.

On your cases short stroking can cause those scratches. The bolt/carrier should be going fully to the rear and then stripping out a new round from the magazine on its way forward, and it engages the base of the cartridge to feed it. If you get a short stroke the carrier/bolt doesnt move all the way to the rear and then slides forward over the top of the next round in the mag.

Try a standard rate DMPS 308 spring first. It problably will fix the problem, and if not, then there are several options for reduced power springs.

I'd like to see you get your gun running. Looks like you put a lot of care in its build.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

You have some main issues here to look at. The main issue is the BCG is not traveling the whole distance to strip off the next round.

So thats is two things:

1. not enough gas to force the BCG to the rear (or gas related equipment not to standard)

2. The BCG is not being allowed to travel its full distance.

Gas:
If you have done all the basic gas system checks and nothing has helped, its the port size. No enough value of gas flowing back to move the BCG the whole distance of travel.

BCG travel (non-gas):
To much spring tension behind the buffer, or the buffer is to long. Or you have something dragging down the speed of the BCG.

so we go to:

The magazine is to high/low in the magazine port.

Next time on the range, push the mag away from you in the mag port and shoot a round. See if it strips the round out. Then pull the mag towards you and see what happens.

The mag catch could be cut to high (thats what I think) and its causes the bolt to drag over the next round, then the bolt slows down due to the drag and short strokes.

By moving the mag back and forth as much as you can, you might fine that it feeds.

Let us know.

John
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Load only 1 round in your mag, fire it, if it doesn't hold back on the bolt catch your gun is under gassed. I would find out from the barrel manufacture what gas port hole size is in the barrel, you might wont to have it opened up just a bit, sounds like you are almost there.....
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

I just talked to JP Rifles and asked their opinion, they stated that the specs are wrong on their website and that the springs they send out in 308 are actually underpowered instead of 7% overpowered. It is a typo.

Now I have no idea what to do because going towards a standard spring would be making the spring more stiff.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

JW,

I had the same problem with my MA-TEN, as did my brother. That is a short stroking issue which resolved itself after a couple hundred rounds.

Have you tried FGMM 168? AE and other sub 168 grn rounds are on the anemic side in the beginning and won't fully cycle the action all of the time.

Again, it took a couple hundred rounds to "wear in" the rifle.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Try some FGMM 168's or 175's just to be sure.
I'd check the alignment of the gas block again
Or on a hunch try adding weight/mass to the BCG w/Tubbs CWS
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

If you Turned your back while at the range a 556 AR may have ran over to lay with your MA10 it may be a simple case of it cought the coodys...?

Two fellas in this thread have went threw the possible Fix's for your problems ...

one just fixed the same issue with about the same exact parts,I would listen to him...Or get some coody shampoo and do some Voodoo candles ....

Love the MA-10 Upper/Lower wish mine Had the MONO
grin.gif
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems



I have a new standard spring on order. I tried a new magazine today, that didn't fix anything. Also I only have 1/4 inch of additional travel for the BCG to ride past the point where it needs to be for it to lock back on an empty magazine. I always assumed that there would be more room for it to travel past that point.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Tolerance stacking...hand cycle the bcg a couple hundred times and apply lube as it burns off. You mentioned that it felt gritty to charge, so break it in until it feels slick. Then go to the range and see if you are still short stroking.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Thanks. It isn't gritty anymore, smoothed out. I'm not sold that the gun needs to be broken in. Something besides that is going on. I'm waiting on the new spring. Will it allow the bolt to travel farther back into the stock, or just easier?
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

JW,

The buffer is what stops the reaward travel of the BCG. The spring only slows the BCG velocity.

If you have dummy rounds (or just use fired cases) hand cycle the action and if it ejects the case and picks up the next one, then make sure that ofter the last round it will lock open, if so you should have enough clearance. If the BCG doesnt go back far enough to feed a new round or lock open, you may have the wrong buffer in there.

-E
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems


Well I went to the range with my new 'DPMS standard recoil spring' and 180gr Remington corlock. I closed the gas all the way and loaded one shot in the mag, then shot. Backed out the screw less than a 1/4 and shot. I repeated this until the screw was over 4 turns out (full open).

No setting locked the BCG back. Hand cycling still worked 100%. I am so frustrated, and have wasted over $125 in ammo costs trying to find a way to get it to cycle. What do I do next?

Below is a 5 shot group at 100 yards. Hand cycled.

 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JW Snydes</div><div class="ubbcode-body">


No setting locked the BCG back. Hand cycling still worked 100%.

What do I do next?


</div></div>

If it were mine, I would want the gas port in the barrel opened up a few thousandths. Even if overdone slightly, the adjustable gas block should allow correction. At a minimum, I would measure the gas port and compare against a known standard for your barrel length and gas port location.

Obviously, I think the flaw is in the gas system. This assumes that chambered unfired rounds extract easily.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

My gas port is 82 thousandths. That should be adequate right? I think it might be over gassed and the window is so small I can't hit it because the BCG is a lomas. It is chunking the next round in the magazine still when there is more than one in the magazine.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

I sold an Eagle Arms 10T upper to a member here. He cut the barrel back to 18" w/ a rifle length gas system, just like you have. Ultimately he replaced the buffer spring with one for an AR-15. That and getting the round count up to around 300 on the upper worked for him.

If you need an AR-15 spring to test and don't have one, PM me and I'll send you one to use. Check out what I lifted from the NJ Highpower shooters website:

Buffer spring - too stiff
If not using an x-08 based case, try using an AR15 buffer spring. Some AR-10'ers have had good results with Tubb's CS AR-15 springs.


I don't see how you could be over gassed if the adjustable block is functioning as designed and from what you've said, you can shut the gas completely off. The gas port sounds like the proper size. I believe they normally run .078 - .082.

If I had a rifle that shot MOA for 5 rounds w/ Rem. 180 Core Lokt, I damn sure wouldn't give up on a gas problem.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Sometimes it feels like adjustable gas blocks are troublesome. Would a fixed block help?
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

FNGMIKE I'm going to call around to some local shops to see if they have ar-15 springs. I'll see if that does anything, thanks for the advice.


ROUS I got it originally so I could put a can on it later and just make the adjustment to have it function. I am shocked at the problems I have been having. If I had another option, I would really look at it. But after dumping 3+ grand into this rifle, I really want it to work!
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

^^^^ I went to the range over the weekend and popped in a Tubbs CWS (heaviest weight)<span style="font-weight: bold">not to fix a problem </span>but
to try and reduce recoil even more so then I already have.
The BHO would only work with just 1 round in the mag as soon as I tried 2 rounds it wouldnt work no matter how I had the GB adjusted.
No problems stripping rounds though.

As soon as I swapped out springs,modified AR10 from slash to a Tubbs CS flat wire the BHO functioned perfectly no matter how many rounds were loaded in the mag.
GB screw is set 2 1/4 turns out from close.
Shoots even smoother and brass is coming out in pristien condition

Im using the Tubbs AR15 spring since I have a UBR stock.
From what Ive read Tubbs 308ar springs wont work with carbine buffer tube.
Using SW Ammo 175's
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Gas Ports- .093"

Also...get a New Gas Tube ya never know might have a hole in it. Alse check to see how tight the JP GB is around where the tube enters could be something wrong with that and too much gas escaping out around it. The Gas Block could have been built on a Friday at quit'n time...
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: USW.260</div><div class="ubbcode-body">Gas Ports- .093</div></div>

Yup, after working through everything, I have it at a local gunsmith that is going to open up the gas port in the barrel. This after a few conversations with JP (who has been very helpful) narrowed it down to being under gassed from the beginning. I should have it back in a week or so. Thanks again for all the help so far. I'm trying not to get discouraged, and hopefully someone else can avoid all these steps that I have had to go through when this thread is finished.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

I just got my gun back after having the gas port opened up by the local gunsmith.

At the range I put the JP polished 308 spring back in. I closed the adjustable gas block, then unscrewed it 2-1/2 turns, loaded 5 rounds in the mag...



Bang-bang-bang-bang-bang!

I'm so pumped that it cycles and I hope that people having similar problems can use this thread to help them through it.

I'm now 100% satisfied with this build, The Mega and JP people have been great to work with and buy from!
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Glad you got it figured out. That is one sick build. I just got my Mega billet AR-15 lower that I plan on putting their monolithic upper on.
 
Re: Mega Arms Monolithic 308 JP Low Mass Problems

Glad that it's running for you. Looks like a great build.
 
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