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Posted: 12/29/2013 3:08:45 PM EDT
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I have been fighting this issue for some time. Typically the spent shell ejects fine, but the cartridge picked up from the magazine doesn't get al the way into the chamber. The bolt jams against the side of the cartridge If I pull back just a little with the charging handle, the bolt picks up the cartridge and properly locks it into the chamber. I shot 100 rounds this weekend and the rifle jammed about 6 times. Twice the bolt did not lock back with an empty mag.
I have a 20" Shilen barrel that was shortened from a 24" barrel. I am shooting Nosler Ballistic tips (55gr) with 24gr of Behcnmark at a velocity of 2950 fps. I am using 10 round P-Mags. I am using a rifle buffer and have switched out the lower with one using a carbine buffer - no change. I recently changed the gas block and gas tube with no change. Any ideas would be greatly appreciated. |
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Barrel Length (in) Barrel Diameter (in) Distance from Muzzle (in) Min Port Size (in) Max Port Size (in) 11.5 .625 3.850 .081 .089 11.5 .750 3.850 .086 .094 14.5 .625 8.375 .063 .078 14.5 .750 8.375 .070 .086 16 .625 8.375 .063 .078 16 .750 8.375 .070 .086 20 .625 6.875 .086 .093 20 .750 6.875 .093 .096 24 .825 N/A .089 .089 According to this, you'll be under gassed if you simply cut the 24" barrel down to a 20" barrel. What you're experiencing sounds like short stroking. The bolt carrier isn't traveling far enough back to cleanly strip a cartridge off the mag. |
| Thanks johencon2001! I've seen this chart other places in the forum. I failed to mention that I have a bull barrel with a .936 ID gas block. It appears that barrel size affects the port size slightly. There is not a .936 barrel listed in the table. Am I making too much out of this or should the port size be different than a .750 barrel? |
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Quoted: Thanks johencon2001! I've seen this chart other places in the forum. I failed to mention that I have a bull barrel with a .936 ID gas block. It appears that barrel size affects the port size slightly. There is not a .936 barrel listed in the table. Am I making too much out of this or should the port size be different than a .750 barrel? |
| Thanks again for the great advice. I just noticed in the table that a 20" barrel with a diameter of .0625 calls for a port size of .086 - .093 and a .750 barrel calls for a port size of .093 - .096. I am trying to decide what size I need to make the port. Also, can I drill this myself with a cobalt bit or do I need to take it to a gun smith? |
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I think I have this resolved. I read somewhere that Shilen has a stepped gas port and I can now confirm that. The top of the port is larger and at the very bottom it is necked down. I am not sure what size the bottom was, but I drilled it with a 3/32 drill bit which is .0938. This only cut the very bottom of the port where it was restricted. The top part of the port was slightly larger than the drill bit. Shot 60 rounds though it and zero issues.
Thanks for the help everyone! |
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Quoted:
One thing I am wondering about is the gas port size. Would it be different from a 24" barrel to a 20" barrel since this was originally a 24" barrel and shortened to 20". I couldn't find any info on the gas port size on the Shilen site. Yea the ports are different sizes. Sounds like you're undergassed. |
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