XCR Factory (and Custom) Port Specs

[Sean K.]

This information is ONLY for Type 1 Gas Systems.

It may be applicable to Type 3 as well in terms of sizing to get you in the ballpark, but I have NO IDEA if that's the case. It's just a guess.



XCR Factory Bbl Port Specs:
5.56 XCR 16” bbl: .070"

5.56 XCR 9" Mini bbl: .086"

7.62x39 XCR 16” bbl: .116”

XCR Factory Gas Block Port Specs:

5.56 XCR w/16” and 9” bbl: .136”

7.62x39 XCR w/16” bbl: .136”


Factory 5.56 XCR 16” bbl Gas Dial specs:

S=.046”
1=.059”
2=.063”
3=.070”
4=.120”

Factory 7.62x39 XCR 16” bbl Gas Dial specs (same for 6.8 SPCII 12” bbl):

S=.046’
1=.067”
2=.081”
3=.093
4=.120”

My Stnd.10.5”& Mini 8.3125” bbl’d 5.56 XCRs: Gas Block .140” (9/64), Bbl Port .098”

Gas dial sizes for the 10.5” and 8.3125”:
.140" or .136” (5.56 #4 dial position)
.093" (5.56 #3 dial position)
.081 (5.56 #2 dial position)
.067" (5.56 #1 dial position)
.046” (5.56 S dial position)

[Whiskey three six]

Thanks Sean K for posting this is very good and useful information.

By chance do you have or know where to find the port and regulator specs for the 12" barrel ?

[Sean K.]

Thanks Sean K for posting this is very good and useful information.

By chance do you have or know where to find the port and regulator specs for the 12" barrel ?

Unfortunately, the only 12" gas dial numbers I had, I posted for the 6.8 SPC.

It *seems* the 16" 7.62x39 and 12" 6.8 SPC II dials work well in shortened 5.56s.....I accidentally bored mine out to almost the exact same specs and my 10.5" 5.56 and 8.3125" 5.56 are running 100% on any ammo without cleaning or oil.

HTH,
Sean

[Sean K.]

BTW, the posted information is NOT set in stone. It's what I found when I measured the dials and bbls/gas blocks I had. There was some variance...some of that may be due to port erosion, or getting clogged up.....these are just general guidelines.

Hope you find them useful to your project or diagnosis.

Sean

[cjt50]

Thanks Sean, excellent info. Definitely a "sticky" candidate.

[Whiskey three six]

I picked up a 16" barrel that had been cut back to 12" with pinned and welded CAR15 flash suppressor for 16.125 length measured from closed bolt face.

Barrel gas port was .070 function was less than 100% on setting #4

Barrel gas port was drilled out with a #44 drill bit and is now .086

The intent was to drill the port out to .760 with a #48 drill bit first, test and go up in steps as needed to .086

A moment of distraction and the wrong bit was in the drill chuck resulting in a .086 port right off the bat.

On setting 1 function is 100% with XM193 ejecting and landing in a 3 foot circle 8 to 12 feet away steel case at 4 to 10 feet.

I got lucky this time.

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Might be possible to tap the gas passage/ gas port in the front sight base gas block and use a set screw that has been center drilled as a gas bushing to choke down the gas in cases when needed.

[Sean K.]

Cross reference to Chris' (Aziator's) thread on getting 9" bbls to run:

https://www.xcrforum.com/index.php/topic,9236.0.html

It has some drill sizes for 7.62x39 for 9" bbls IIRC.

[Sean K.]

7.62 Factory Bbl Port Specs for 10" mini:
.111

didn't measure gas block port

7.62x39 bbl Gas Dial specs:
4= .120
3= .063
2= .059( can't get the bit all the way in so maybe .055 also)
1= .055
s= .040

[wombat338]

Very good information in this thread. Can we sticky it?

It would be great to develop a list/spreadsheet of detailed specs -- measurements for barrel profiles, etc.

[wombat338]

Reported.