AR Sponsor
Posted: 2/17/2014 12:25:57 PM EDT
|
I have this customer gun, on which I put together the upper. It is a Mega Maten MTS upper built up with their 16" middy barrel, a Rainier Arms NiB LR308 BCG, and a Slash CAR-10 buffer.
IF this were an AR-15 platform I would say it is over gassed; as it is ejecting forward in the 1-2 o'clock direction. On AR-15s I expect to see 4-5 o'clock with .223Rem and 3 o'clockish ejection with 5.56mm. With the ejection pattern generally moving forward with a can mounted. Should I be expecting similar patterns with the 308 guns? Is this gun kicking brass forward over gassed? It was also sticking cases, but the chamber was coated with what looked like tar (the Mega barrels are melonite) in the bore scope. I did of course clean it be fore shooting it the first time. This crude did not come out with regular cleaning. I had to get aggressive. It is clean and shinny again. And I will being testing it again to determine if I have a problem. Any idea what the nominal gas port should be on a 16" mid-gas .308 WIN barrel? |
|
I've had this issue with new build before...but never with a nitrided barrel. You've given the gun a good cleaning, particularly the chamber and that's the first step. I don't know .308 gas port diameters but I DO know that my 22in JP barrel has a gas port diameter of.093 inches. My 20 inch White Oak, 5.56 barrel has a .095 inch gas port which is .001 high of nominal. (.094 spec at 20 inches on a .750 gas journal ). So, I have only .001 inches of diametral distance between a 7.63 and a 5.56...it almost looks as though the specs overlap to some degree. A 16 (5.56) inch ar gas port has a larger tolerance zone.....063 to .068 on a .625 journal and .070 to .086 on a .750 journal. Can you get some gage pins ( I know...not exaclt something you just go to the store and buy ) or anything like a pin punch that is VERY cylindrical and gage the port? |
|
Quoted:
I've had this issue with new build before...but never with a nitrided barrel. You've given the gun a good cleaning, particularly the chamber and that's the first step. I don't know .308 gas port diameters but I DO know that my 22in JP barrel has a gas port diameter of.093 inches. My 20 inch White Oak, 5.56 barrel has a .095 inch gas port which is .001 high of nominal. (.094 spec at 20 inches on a .750 gas journal ). So, I have only .001 inches of diametral distance between a 7.63 and a 5.56...it almost looks as though the specs overlap to some degree. A 16 (5.56) inch ar gas port has a larger tolerance zone.....063 to .068 on a .625 journal and .070 to .086 on a .750 journal. Can you get some gage pins ( I know...not exaclt something you just go to the store and buy ) or anything like a pin punch that is VERY cylindrical and gage the port? I had put together another Maten upper with the same parts; Mega 16" mid barrel, Rainier BCG, and Slash CAR-10 buffer. I contacted the owner of that upper to see if he had had any troubles. He has not fired it yet, but he is local so he brought it to me yesterday. This new unfired Mega barrel had the same sticky crude (just without the brass bits) in the chamber and bore. I polished and cleaned it out. |
|
I am working on a third upper with a Mega 20" rifle barrel cut back to 17". It too has tarry filth inside. The original gas port on this 20", rifle gas, .750 GB, .308 WIN barrel was .089". This is smaller than any factory 20" 5.56mm gas port I have seen.
The gas port on the original MEGA 16" mid length barrel in question is .076". This matches up with a ST 5.56mm CHF Optimal barrel and a BCM middy 5.56mm barrel. Which to me seems too large for .308. I will find out more when I get to test fire all three rifles. |
|
On my Windam 308 I opened the port to .076, but this was after cutting barrel to 14.5", I never tried it before cutting, my CMMG has same port size, it was 18" before cutting it to 14.5", but it worked fine before and after the cut, brass on both about 3 to 4 o'clock, both are mid-length. Don't know if this was any help or not. |
|
Quoted:
On my Windam 308 I opened the port to .076, but this was after cutting barrel to 14.5", I never tried it before cutting, my CMMG has same port size, it was 18" before cutting it to 14.5", but it worked fine before and after the cut, brass on both about 3 to 4 o'clock, both are mid-length. Don't know if this was any help or not. It makes me optimistic that the signs of over gassing/over pressure I saw in the middy I shot were caused by the tarry crud in the chamber/bore and not by an over sized gas port. |
|
Before you test fire after cleaning the chamber, pull the extractor on that Rainier Arms NiB bolt and see if it has a insert in the extractor spring. Mine didn't and it caused all sorts of extraction problems, including the 1-2 o'clock ejection pattern. Extraction problems went away after I installed an insert. FWIW. |
|
I took the three Matens out this weekend to test functionality. The MKM gun which is has the same make/model of running parts as the MTS gun ran 100% with both CBC and Fed AE ball. The Mk12 gun with the cut down rifle barrel also ran 100%. The ejection patterns on this gun are better at 3-3:30. On this gun I had to open the gas port because the barrel was a 20"/rifle and now it is a 17"/rifle. So it is not over gassed.
The MTS gun, which is the first I built and the one I have had the problems with runs 100% with Fed AE, but will stick every round of CBC M80. At this point I would just call the CBC shit and be done with it, but this gun's owner is of course the one out of the three with pile of CBC M80... |
|
I took the three Matens out last week to test function. The MKM gun which is has the same make/model of running parts as the MTS gun ran 100% with both CBC and Fed AE ball. The Mk12 gun with the cut down rifle barrel also ran 100%. The ejection patterns on this gun are better at 3-3:30. On this gun I had to open the gas port because the barrel was a 20"/rifle and now it is a 17"/rifle. So it is not over gassed.
The MTS gun, which is the first I built and the one I have had the problems with runs 100% with Fed AE, but will stick every round of CBC M80. At this point I would just call the CBC shit and be done with it, but this gun's owner is of course the one out of the three with pile of CBC M80... |
| Problem solved, well diagnosed. These BCGs are the infamous 7.62 Industries brand as sold be Rainier and others a while ago. Plagued with a variety of issues including but not limited to out of spec parts and bad coatings. Swapped the BCG with the spare, and the gun runs fine. Going to try replacing the bolt assembly in the first BCG with a JP .308 Enhanced Bolt. |
|
Just call rainier and tell them that BCG sucks and have them give you a DPMS unit that works
Quoted:
Problem solved, well diagnosed. These BCGs are the infamous 7.62 Industries brand as sold be Rainier and others a while ago. Plagued with a variety of issues including but not limited to out of spec parts and bad coatings. Swapped the BCG with the spare, and the gun runs fine. Going to try replacing the bolt assembly in the first BCG with a JP .308 Enhanced Bolt. |
|
Quoted: Problem solved, well diagnosed. These BCGs are the infamous 7.62 Industries brand as sold be Rainier and others a while ago. Plagued with a variety of issues including but not limited to out of spec parts and bad coatings. Swapped the BCG with the spare, and the gun runs fine. Going to try replacing the bolt assembly in the first BCG with a JP .308 Enhanced Bolt. THIS is good to know!!!! Glad she's running right!!!!! |
AR Sponsor