Increasing port size on 8.5" CMMG barrel

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LiberalGunToater
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Increasing port size on 8.5" CMMG barrel

Post by LiberalGunToater » Tue Nov 25, 2014 9:13 pm

I built my SBR 300 Blackout rifle from scratch and on my first outing to the range with subsonic round that I loaded, I wasn't achieving the reliability I felt like I should have. The gas port size from the factory was .094 in. I loaded a lot of variation of the 220 gr Nosler HPBT and I felt like it was taking too much powder to get the round to cycle reliably. Just to give an idea it would take 8.6 gr of Lil Gun with a OAL of 2.220 and light crimp to get reliable cycling.

So I decided to take the plunge and increase the gas port size. First I stumbled over to my local harbor freight an purchased an numbered drill bit kit for 20 buck with a 20% off coupon.
http://www.harborfreight.com/60-piece-t ... 34627.html
Then I found the drill bit that fit in my gas port hole #41 and worked my way up utilizing every drill until I got to #37. When I measured my gas port it ended up being .106". I wanted to stop here and try this size and see if I noticed an improvement.

Boy did that small difference make a huge improvement. Now I can reliable cycle the same loads down to 8gr of Lil gun without a suppressor and Im sure I can go lighter with my suppressor attached. I tested 3 powder with various charge rate and they all function flawlessly even the ones that didn't work on my last outing.

I won't be increasing my port diameter anymore but I am please with my results.
Also I made my increase with a standard drill and my upper in a vise block. As long as you go slow this worked great for me.

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smustian
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Re: Increasing port size on 8.5" CMMG barrel

Post by smustian » Tue Nov 25, 2014 9:34 pm

Good to see someone actually testing and doing the work themselves instead of whining and bitching. Nice job !!!!!!!!!!

Sharkbite
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Re: Increasing port size on 8.5" CMMG barrel

Post by Sharkbite » Tue Nov 25, 2014 11:15 pm

Is this a "sub only" gun? What does a .106 port do with Supers?

LiberalGunToater
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Re: Increasing port size on 8.5" CMMG barrel

Post by LiberalGunToater » Wed Nov 26, 2014 1:28 am

I don't expect it affect my super too much. I will get some more shooting time in Friday. Ill report back.

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MMA10mm
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Re: Increasing port size on 8.5" CMMG barrel

Post by MMA10mm » Wed Nov 26, 2014 2:06 am

My 10.5" bbl. came with a .105"-.110" port, and it runs supers fine.

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r.tenorio671
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Re: Increasing port size on 8.5" CMMG barrel

Post by r.tenorio671 » Wed Nov 26, 2014 5:25 am

Sharkbite wrote:Is this a "sub only" gun? What does a .106 port do with Supers?

....increase the cyclic rate and maybe a little more "oomph" in recoil, but not enough to damage anything.....

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300Blk
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Re: Increasing port size on 8.5" CMMG barrel

Post by 300Blk » Wed Nov 26, 2014 2:31 pm

It depends on what buffer you have. You can't talk about port sizes without talking about buffers.

0.094 strikes me as being ok for a standard buffer and too small for a heavy buffer.

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Re: Increasing port size on 8.5" CMMG barrel

Post by eastern_hunter » Wed Nov 26, 2014 3:25 pm

The CMMG 8.5" barrel in my SBR came with a 0.089" port ... didn't work with subs at all.

Increased it to 0.120" and used a Paladin Machine Services 3 position gas block with an H buffer and CS spring. Works fine.

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Re: Increasing port size on 8.5" CMMG barrel

Post by RNT » Wed Nov 26, 2014 10:24 pm

Sharkbite wrote:Is this a "sub only" gun? What does a .106 port do with Supers?
I use the micro moa on my 8.5". Supers suppressed and unsuppressed beat the gun up and the cycle rate was FAST, and mine was drilled ~.104!!! With the micro moa now I run everything suppressed by a mere sliding of the plate.

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Pink_Vapor
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Re: Increasing port size on 8.5" CMMG barrel

Post by Pink_Vapor » Fri Dec 26, 2014 10:35 pm

I've a 10" pistol gas barrel that HAD @ a 0.075" port and I didn't get BHO with 17.8G of H110 behind 125g SST without the can. Just drilled it out to 0.094" and the supers BHO each time now, even with the adj gas block screw turned in considerably now.
I need powder for the 208 AMAX to work now.
Hogan billet, Geissele SSA-E, 10" SS 300BLK McGowan, Seekins MSCR V2, NiB BCG, TBAC
Mega billet, 6.5 Grendel 18" SS McGowan, Geissele NM Hi Speed, NiB BCG, Magpul PRS, MCSR guard, TBAC :)

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