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ManufacturerModelGas SystemBarrel LengthsBarrel SteelChrome Lined/SSGroovesChamberBarrel RatingGas Port LocationM4 FeedrampsThread SizeOverall Rating

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Addax TacticalGPUPiston16"Wilson 416SS1:11"6SPCIIBMidYes5/8x24B

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Armalite (old)6.8SPCDI
16"MIL-B-11595-B
Chrome Moly Vanadium
Chrome Lined1:10"4SPCIIBMidYes5/8x24B

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Armalite (new)6.8SPCDI
16"MIL-B-11595-B
Chrome Moly Vanadium
Chrome Lined1:11"4SPCIIAMidYes5/8x24A

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AR15Performance6.8SPCDI
16"/18"4150 CMNitro-Carburized
1:11.25"5R6.8x43AMidYes5/8x24A

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AR15Performance6.8SPCDI
Various416 SSSS1:12"3DMRAMidYes5/8x24A

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AR15PerformanceHunterDI16"416 SSSS1:11"4SPCIIAMdYes5/8x24A

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AR-Stoner6.8SPCDI
16"/20"416 SSSS1:12"5RSPCIIAMid/RifleYes5/8x24A

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Barrett
Rec7Piston16"4150 CMChrome Lined1:10"6SPCIICMidYes5/8x24C

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Bison Armory6.8SPCDIVarious416 SSSS1:11"4SPCIIAVariousYes5/8x24A

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Black Hole Weaponry6.8SPCDIVarious416 SSSS1:11"3/5 PolySPCII1:11"VariousYes5/8x24A

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Bushmaster (old)6.8SPCDI
16"4150 CMChrome Lined1:10"6SAAMIDCarYes5/8x24D

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Bushmaster (new)6.8SPCDI
16"4150 CMChrome Lined1:11"4SPCIIACarYes5/8x24B

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Cardinal Armory6.8SPCDI
Various4140 CMChrome Lined1:11"4SPCIIACarYes5/8x24A

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CMMG (old)6.8SPCDI
16"/18"4150 CMChrome Lined1:10"6SPCIICMid
Yes5/8x24C

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CMMG (new)6.8SPCDI
16"/18"4150 CMChrome Lined1:11"4SPCIIAMidYes5/8x24A

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Denny's Guns (new)ReconDI
VariousWilson 416SS1:11"6SPCIIBVariousYes5/8x24B

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DPMS (old)6.8SPCDI16"/20"4140 CMChrome Moly1:10"6SAAMIDCar/RifleNo1/2x28D

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DPMS (new)6.8SPCDI16"/20"4140 CMChrome Moly1:11"6SPCIIBCar/RifleYes1/2x28B

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LMTMRPDI12.5"/16"4150 CMChrome Lined1:10"6SAAMIDCarYes5/8x24D

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LMTMRPDI18"UNKSS1:10"6SAAMIDCarYes5/8x24D

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LMTMRPPiston16"4150 CMChrome Lined1:10"6SAAMIDMidYes5/8x24C

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LWRC
PSD/M6PistonVariousHammer ForgedNitro-Carburized
1:10"PolySPCIIBVariousYes5/8x24B

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Model1 Sales6.8SPCDI16"/20/24"4140 CMChrome Moly/Chrome Lined1:9.5"6SAAMIDCar/RifleNoTarget CrownD

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Noveske (old)6.8SPCDIVarious416R SSSS1:10"PolySAAMIDVariousYes5/8x24D

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Noveske (new)6.8SPCDIVarious416R SSSS1:12"PolySPCIIAVariousYes5/8x24A

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POF6.8SPCPiston14.5"/16.5"4150 Mil-B-1159F Nitro-Carburized
1:11.25"5R PolySPCIIAMid
No5/8x24B

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PRI6.8SPCDIVarious416 SSSS1:10"6SPCIICMid
Yes5/8x24C

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Rainier Arms6.8SPCDIVariousWilson 416SS1:11"RatchetSPCIIAMid
Yes5/8x24A

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Robinson Arms (old)6.8SPCPiston12"/16"UNKChrome Lined1:10"4SPCIIBN/AN/A5/8x24B

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Robinson Arms (new)6.8SPCPiston12"/16"UNKChrome Lined1:11"4SPCIIAN/AN/A5/8x24A

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RRALAR-6.8DI16"4140 CMChrome Lined1:10"6SPCIICCar/MidYes5/8x24C

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SAOD6.8SPCDIVariousLW50SS1:11.3"PolyPolyAVariousYes5/8x24A

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Shilen6.8SPCDIVarious416 SSSS1:11"4SPCIIAMid
Yes5/8x24A

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Stag (old)5HDI16"4150 CMChrome Lined1:10"6SPCIICCarNo1/2x36C

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Stag (new)5HDI16"4150 CMChrome Lined1:11"4SPCIIACarM21/2x36B

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Stag7HDI20.77"UNKSS1:11"4SPCIIARifleNoTarget CrownB

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Spike's Tactical6.8SPCDI16"LW50SS1:11.3"PolySPCIIAMid
Yes5/8x24A

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Wilson Combat6.8SPCDI16"/18"Wilson 416SS1:11"4SPCIIAMid
Yes5/8x24A

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WOA (new)6.8SPCDIVariousWilson 416SS1:11"6SPCIIBMid
Yes5/8x24B

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WOA/Denny's (old)6.8SPCDIVariousWilson 416SS1:11"6SAAMICMid
Yes5/8x24C

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Yankee Hill Machine6.8SPCDI16"MIL-B-11595-B
Chrome Moly Vanadium
Chrome Lined1:10"6SPCIICCarYes1/2x36C

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ColorExcellent

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ColorGreat

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ColorGood

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ColorFair

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ColorPoor

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Milspec Barrel Steel

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Milspec Barrel SteelBarrels are typically made from two grades of chromoly steel, 4140 and 4150, with the latter being a slightly higher grade that withstands heat slightly better than the former. The long version involves very specific types of steel, much longer numbers, and is in fact generally considered to be of little consequence. There are, however, other grades and/or types of steel that meet or exceed the properties of 4150 and are therefore acceptable.

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Chrome Chamber and Bore

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Chrome Chamber and BoreChrome-lining of the chamber and bore serve to protect both from corrosion due to the heat of combustion of the gunpowder in the bullet as well as "neglect" in humid or other harsh environments. The chamber and bore are directly related to the accuracy potential of a firearm, and damage to either in the form of pitting will negatively impact the accuracy potential. The trade off is that chrome is often applied unevenly, at the microscopic level, meaning that it may negatively affect the accuracy potential in and of itself. The potential for damage due to other factors is generally considered greater than the small amount of uneven application, and so chrome-lining is generally considered desirable. No, it cannot be added after the fact as barrels intended for chrome-lining are first slightly overbored with the lining then reducing the internal diameter to the proper dimension. One thing that you can use to greatly improve your Chrome-Lined Barrel is to break-in your barrel with the Tubb's Final Finish System. This will greatly help with accuracy and help if you have a tight bore.

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Twist Rate

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Twist RateTwist Rate has very little effect on pressure when compared to the Chamber and the Land to Groove Ratio.

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6.8 SPC Chambers

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6.8SPC ChambersThe 6.8 SPC was designed as a Combat Cartridge. One of the most important things about the AR is reliability, especially if you plan to use the 6.8 SPC for serious work. The SPCII and 6.8x43 Chambers are reliable and very accurate. The SPCII and 6.8x43 Chambers have a longer throat which allows you to shoot the hotter loads without worying too much about pressure signs. For anyone that has a SAAMI Chamber, the one thing you could do is have someone lengthen the throat to the Improved Chamber specs.

Remington 6.8x43 SPC-(SAAMI Chamber): Has a .050 freebore, 45 degree cone angle, .278 diameter freebore.
6.8 Remington SPCII (SPCII Chamber): Has a .100 freebore, 45 degree cone angle, .278 diameter freebore, .3085 neck.
6.8x43: Has a .095 freebore, 45 degree cone angle, .2775 dia, and a .3085/.309 neck.

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M4 Feedramps

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M4 FeedrampsM4 feedramps refers to the feed ramps in the barrel extension being matched up to feed ramps cut into the upper receiver. The alternative is Rifle feedramps which stop at the end of the barrel extension and do not continue into the upper receiver. Longer projectiles, soft-point projectiles, and carbines with faster cyclic rates tend to benefit from the extended M4 feedramps. There are no known downsides to having the extended feedramps. There have been some reported problems with guys shooting the Sierra 90 Gr. bullets and other similar shaped bullets in 6.8SPC Ar's without the M4 Feedramps. M4 Feedramps are pretty much a must with the 6.8SPC. If you don't have them, you can file or dremmel them in yourself or have someone do it for you.

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Carrier Key Staking

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Carrier Key StakingThe gas key on top of the bolt carrier is the part that the gases (which have been re-directed through the gas port, then the gas block or front sight base, and into the gas tube) impact on in order to push back on the carrier, unlock the bolt and cycle the firearm. As such it is under tremendous pressure and is critical to the continued operation of the firearm. The key is held on to the top of the carrier by two screws, typically allen but sometimes torx, that are tightened to a specified torque. After tightening the metal of the key should be "staked" in such a way as to prevent the screws from loosening. In order for the staking to perform it's job properly it must deform the metal of the key sufficiently to make contact with, and perhaps even deform a bit, the attachment screws. Use of Locktite is not sufficient, as virtually all versions of Locktite are weakened by heat.

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Black Extractor Spring Insert

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Black Extractor Spring Insert
The crucial element here is, in fact, the 5-coil extractor spring which the black insert indicates as there is some debate as to the actual chemical or physical properties of the insert itself as compared to the rifle version which comes with the 4-coil spring and is blue. The shorter gas system of the carbine makes for a sooner an more violent operation of the BCG which can cause the extractor on the bolt to jump over the rim of the case and not properly extract the empty case. To some degree this can be mitigated by gas port size, but beefing up the spring tension to cause the extractor to clamp more tightly on the case ensures proper function.

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This Chart was completed with permission from rob_s. This chart is meant to be used as an aid for the new guys when purchasing a 6.8SPC. Some information was also obtained from rob_s' thread on M4Carbine.net. http://www.m4carbine.net/showthread.php?t=6642