AR Sponsor
Posted: 2/25/2013 11:48:16 AM EDT
| Can anybody give me the exact distance of the gas port from the rib that is behind it on a standard m4 barrel? Long story short I have a barrel with the gas port a hair too far forward and I need to get the standard measurements so I know how far forward to move the gas block on mine (so the port and gas block hole line up) if I put the gas block all the way back to the rib, it will not line up. |
|
Quoted:
Can anybody give me the exact distance of the gas port from the rib that is behind it on a standard m4 barrel? Long story short I have a barrel with the gas port a hair too far forward and I need to get the standard measurements so I know how far forward to move the gas block on mine (so the port and gas block hole line up) if I put the gas block all the way back to the rib, it will not line up. Sounds like the gas port is in the correct place. You never jam your gas block up tight to the shoulder. The space is by design and to allow the placing of a handguard end cap. Get some LCD calipers (they're cheap and a must-have for the bench) 1) Measure shoulder to gas port (center) on barrel. 2) Measure rear edge of GB to gas port (center) on GB. 3) Subtract the smaller number from the larger number 4) This is the distance from the shoulder to the GB |
|
Quoted:
Quoted:
Can anybody give me the exact distance of the gas port from the rib that is behind it on a standard m4 barrel? Long story short I have a barrel with the gas port a hair too far forward and I need to get the standard measurements so I know how far forward to move the gas block on mine (so the port and gas block hole line up) if I put the gas block all the way back to the rib, it will not line up. Sounds like the gas port is in the correct place. You never jamb your gas block up tight to the shoulder. The space is by design and to allow the placing of a handguard end cap. Get some LCD calipers (they're cheap and a must-have for the bench) 1) Measure shoulder to gas port (center) on barrel. 2) Measure rear edge of GB to gas port (center) on GB. 3) Subtract the smaller number from the larger number 4) This is the distance from the shoulder to the GB ^ This is true, but the gas hole in the block should be large enough to accommodate small variances and the lack of a hand guard cap. |
|
Quoted:
Quoted:
Quoted:
Can anybody give me the exact distance of the gas port from the rib that is behind it on a standard m4 barrel? Long story short I have a barrel with the gas port a hair too far forward and I need to get the standard measurements so I know how far forward to move the gas block on mine (so the port and gas block hole line up) if I put the gas block all the way back to the rib, it will not line up. Sounds like the gas port is in the correct place. You never jamb your gas block up tight to the shoulder. The space is by design and to allow the placing of a handguard end cap. Get some LCD calipers (they're cheap and a must-have for the bench) 1) Measure shoulder to gas port (center) on barrel. 2) Measure rear edge of GB to gas port (center) on GB. 3) Subtract the smaller number from the larger number 4) This is the distance from the shoulder to the GB ^ This is true, but the gas hole in the block should be large enough to accommodate small variances and the lack of a hand guard cap. I just did one that wanted a .093" space. Would not have worked slammed up to the shoulder. |
|
From the gas journal (shoulder) on the barrel to the nearest edge of the gas port it is exactly .255"
From the gas journal (shoulder) on the barrel to the farthest edge of the gas port it is exactly .316" leaving me with a .061" port That .093" space for a hand guard cap? That is way too much, most hand guard caps are a third of that or less. I measured some coins (thickness at rim) just for shits and giggles. Dime = .051" Penny = .057" Nickel = .079" |
|
Quoted from elsewhere:
From Randall (AR15barrels.com),
I have measured and documented dozens of different factory barrels to include diameters and lengths of every step on the whole barrel. From the barrel torque shoulder where the barrel extension torques against, it's 6.75" for carbine, 8.75" for mid-length and 12.125" for rifle to the step at the rear of the gas block seat. Breach face is 3/4" back from the torque shoulder. Gas port is usually 0.275" to 0.295" forward of the rear step, depending on if there is a handguard cap in the design or not. You should be able to figure everything else out from that... Unknown, but sounds good: On a 20" service rifle barrel the critical dimensions are as follows: Length from tang shoulder to gas block shoulder: 12.150" (That's the 1.000" diameter) (from there the barrel shoulder is angled back at 10* for 0.85" The gas port is 0.310 from the barrel shoulder (0.310" + 12.150" = 12.460 from barrel tang shoulder) *The diameter of that section is 0.75". *The 0.75" diameter is 1.875" long. *The rest of the barrel is 0.740" End is threaded 1/2-28 for .600 from muzzle. * |
|
Quoted:
From the gas journal (shoulder) on the barrel to the nearest edge of the gas port it is exactly .255" From the gas journal (shoulder) on the barrel to the farthest edge of the gas port it is exactly .316" leaving me with a .061" port That .093" space for a hand guard cap? That is way too much, most hand guard caps are a third of that or less. I measured some coins (thickness at rim) just for shits and giggles. Dime = .051" Penny = .057" Nickel = .079" Not too much as long as the GB is over the port. I was expecting about a .030 gap though. Should always measure, regardless. |
| Quite a few different answers in here lol. The barrel was sold to me with the description that the gas port was too far forward. From comparison photos I can see that it is. Just trying to set it up so I can make a custom gas tube and shoot the damn thing:-). And what is LCD calipers? |
|
Quoted:
Quoted:
Quite a few different answers in here lol. The barrel was sold to me with the description that the gas port was too far forward. From comparison photos I can see that it is. Just trying to set it up so I can make a custom gas tube and shoot the damn thing:-). And what is LCD calipers? Different answers because every one is a little different. That is why you should measure and deduct to determine your gap. LCD Calipers (as opposed to Dial Calipers) 
About $20 for the grade that works fine for this stuff. Harbor Freight has them even cheaper. If you have to have the best get Mitutoyo or Starrett Brace for sticker shock, though. |
|
LCD Calipers I think he means digital readout calipers.
If the barrel was advertised as having the gas port drilled too far forward it probably is... With that said, you can alter the gas block to capture the gas port of it isn't off by a whole bunch. The diameter of the hole (in barrel) is what dictates the cycle impulse, so leave it alone. Alter the underside of the gas block to enlarge the hole to fit over the gas port in the barrel so if you were to slice the gas block in half length-wise, the gas port (in block) would go from looking like this || to this |\ so the bottom of the hole is larger and now over the gas port in the barrel. If possible, and if you have the time, make the slope as smooth as possible so as not to obstruct the gases anymore than necessary to function. If you have a drill press or mill, you can do this easily. ETA - A person reading this over my shoulder asked why not slide the gas block forward? Then you're shortening the length of the gas tube and it may not seal in the carrier key if you do. Mount the gas block where it is supposed to mount, alter the connecting hole in the block so you don't have to move it. |
AR Sponsor