just built an AR 300blk with a 9inch barrel need help

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austinar15
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just built an AR 300blk with a 9inch barrel need help

Post by austinar15 » Sat Mar 04, 2017 9:35 pm

I just brought my new build to the range for the first time today and I ran into cycling/feeding issues with subsonic ammo. I built the Pistol with a 9inch DIY 300blk kit from Ground Zero, a rainer arms lower, magpul grip, KAK blade, eotech EXPS3-4, and a PA 6x magnifier & flip to side mount. First Mag I attempted to run through it ran was a 15 rounder (unknown brand) with 220 grain subsonics sig ammo the first round fire and the second round didn't feed then I tried the same ammo in a hexmag then same issue. Next, I loaded a hexmag up with hornady 110vmax factory ammo and it cycled flawlessly through all 20. then, I tried Remington subsonic 220 grain in the hexmag and unknown brand 15 rounder 1st round fired and second wouldn't feed. After trying all the factory ammo I had I loaded up the hexmag with Hornady 125gr sst over 16.5 g, 17g, 17.4, & 18g of H110 in groups of 5 out of time respectively and 3x for each load. throughout the 12 test all but 3 or 4 round fed perfectly. those 3 to 4 rounds where not fed into the chamber by the bolt. All the rounds that didn't (with the exception of 1 subsonic that was fed weird from the 15r mag) where sitting in the mag perfectly almost as if the bolt did not get fully cycled back to pick up the next round sitting in the mag.

I'm betting its got to be an issue with the gas system. I don't think its the because the mag full of factory hornady fed perfectly. Also, sometimes the bolt catch wouldn't engage after the mag was emptied. I wasn't running a suppressor just so you guys are aware (the gun did have a muzzle brake on though) but from what I read a 9 inch barrel should have no problem cycling subsonics with or without a suppressor.

with all this being said what should I do to figure out where the gas problem is coming from. Any and all suggestions would be appreaciated. One more thing I forgot to add, the brass deflector on the upper had marks on it from the brass hitting it so hard. I have not seen this before I feel like it could be a clue to whats happening.

thanks in advance for the help.
Austin

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austinar15
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Re: just built an AR 300blk with a 9inch barrel need help

Post by austinar15 » Sat Mar 04, 2017 9:58 pm

the gun has the the buffer tube, buffer, and spring that came with the ground zero precision DIY kit (not the KAK blade buffer tube kit) figured that bit of info might help if you guys think its a buffer issue.

this is a link to the kit I got from GZP with a list of parts which might help.

https://www.groundzeroprecision.com/collections/ar-15-pistol-kits/products/ar-15-pistol-kit-3ba9?variant=23142506759

I also put a bunch of lube on the bolt and all moving parts before I used it.

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Walkers Bay
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Re: just built an AR 300blk with a 9inch barrel need help

Post by Walkers Bay » Sat Mar 04, 2017 10:01 pm

First 300blk hit the brass deflector, check your cases they will all have dent in the side.
Use velcro to stop the receiver marking.

Second is your gas port and block lined up perfect.

austinar15
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Re: just built an AR 300blk with a 9inch barrel need help

Post by austinar15 » Sat Mar 04, 2017 10:34 pm

Walkers Bay wrote:First 300blk hit the brass deflector, check your cases they will all have dent in the side.
Use velcro to stop the receiver marking.

Second is your gas port and block lined up perfect.
Thanks for the reply brother!

First, As long as the brass from a 300blk hitting and marking the deflector is normal in a mechanically sound and reliable 300blk AR15 pistol I could care less about the aestofhetics of a marked receiver but thanks for the suggestion.

Second, when I was building the gun I used a pencil to mark the low Profile blocks centerline and the top of the barrel and tried to line the block up that way because it was the only way to think of lining the port and block. I did and do have a concern that they aren't lined up perfectly and that was going to be an issue when I was building it and is the issue when I was having the problem today. with that being said I don't know how to line up the block and port perfectly. Do you have any suggestions and techniques to check the alignment and/or align it perfectly if its out?

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Re: just built an AR 300blk with a 9inch barrel need help

Post by IMGrunt » Sat Mar 04, 2017 11:03 pm

HB Industries makes a fool-proof alignment tool if you don't mind spending the $10.
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Re: just built an AR 300blk with a 9inch barrel need help

Post by GSO » Sat Mar 04, 2017 11:35 pm

My theory is that the gas port itself is undersized. According to their website, the 9" barrel has a port size of 0.076". I believe this is fine for supers, but a bit small for subs.

https://www.groundzeroprecision.com/col ... 5511332033

I can't recall what the port size is on my AAC barrel or the two PSA CHF pistol barrels that I've already built. I think I have a spare barrel somewhere in the shop and can measure it tomorrow if I can find it.

I did discover the article below that supports the need for a larger gas port for subs.

http://micromoa.com/300-blk-port-size-testing/

Edit: Since I had to let the dogs out before bed I went ahead and grabbed the barrel and checked it with calipers. It's an 8" CMMG and the port looks to be about 0.095"...so I'm sticking with undersized gas port for subs. Rather than disassemble it and drill out the hole, I recommend getting a suppressor! That will up the gas pressure and you should be good then...
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Re: just built an AR 300blk with a 9inch barrel need help

Post by austinar15 » Sun Mar 05, 2017 12:27 am

GSO wrote:My theory is that the gas port itself is undersized. According to their website, the 9" barrel has a port size of 0.076". I believe this is fine for supers, but a bit small for subs.

https://www.groundzeroprecision.com/col ... 5511332033

I can't recall what the port size is on my AAC barrel or the two PSA CHF pistol barrels that I've already built. I think I have a spare barrel somewhere in the shop and can measure it tomorrow if I can find it.

I did discover the article below that supports the need for a larger gas port for subs.

http://micromoa.com/300-blk-port-size-testing/

Edit: Since I had to let the dogs out before bed I went ahead and grabbed the barrel and checked it with calipers. It's an 8" CMMG and the port looks to be about 0.095"...so I'm sticking with undersized gas port for subs. Rather than disassemble it and drill out the hole, I recommend getting a suppressor! That will up the gas pressure and you should be good then...

What are the pros and cons of drilling the port out to .095ish? I am not really currently in the financial position to shell out the money for a tax stamp and then a can(I'm currently in my last year of law school and so broke). Also, I live in Philly, PA idk how the process of getting a suppressor works but I am going to look into it.

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Re: just built an AR 300blk with a 9inch barrel need help

Post by smustian » Sun Mar 05, 2017 12:44 am

Could have been a break in issue. From your statements, you started by shooting subs first then moved to supers. Should have been the other way around. Parts need to wear and mate to get the smoothest operation. I usually say the first 100 rounds should be supers and run "wet" with oil.

Before doing anything to the gas port hole, I would try the following. 1) Plug the chamber tightly with a rag. Loosen the gas block. Use an air compressor or blow through the muzzle end of the barrel and listen for air flow. Move the block back and forth to find the position where air flow is greatest the tighten the gas block. Reason for this is some gas blocks are designed to be offset from the shoulder .025. 2) Find 2 boxes Remington 120 grain UMC ammo at Walmart and shoot them first on your next range trip. For the first 10 rounds, load one at a time in the magazine and check for bolt hold open. Shoot the rest normally checking for function. You should now have over 100 rounds through the rifle.

Now try your load testing again, including subs. Make sure that you have a standard carbine buffer when testing the subs. If the subs cycle then supers will probably be over gassed slightly.

Let us know what you come up with.

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John A.
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Re: just built an AR 300blk with a 9inch barrel need help

Post by John A. » Sun Mar 05, 2017 1:20 am

There's another easy way to make your own gas block alignment tool. And is re-usable and especially handy if you are building several uppers.

The gas block placement is designed to be used in conjunction with a handguard cap. Though most folks installing gas blocks are using floated handguards a lot of times so the cap is not installed.

So, if you take a hacksaw or dremel and cut a handguard cap straight down the middle and make two pieces out of it, you can slide the handguard cap in position between the barrel shoulder and the gas block as a shim, and that ensures proper alignment (forward and aft) and all you have to do is draw a line on the barrel with a pencil indicating center for the gas port before putting the block on so you can align it left/right.

Once the gas block is tightened or clamped down, you simply slide the handguard cap half shim out and that's it.

Yes, I came up with that little trick all by myself. Sometimes as a gunsmith, necessity is the mother of invention. I call it "the shim". :lol:
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Re: just built an AR 300blk with a 9inch barrel need help

Post by Boolit_Head » Sun Mar 05, 2017 1:43 am

An old fashioned blade type feeler gauge will do the same thing without having to cut up a cap. Probably .025 will be about right.

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