How do I know gas port sizes?

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gun'sRgood

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Dear smart people. I've got a .875 diameter barrel and I see plenty of gas blocks for it. But I don't see where they state port size. Other than an adjustable block, How do find the port sizes?
 
Are you looking for a gas block the AR15? You didn't provide too much information. The port size in the gas block probably doesn't matter. It should be large enough. The port size in the barrel does matter.
 
The limiting factor is the gas tube ID which is .125". The gas block port has much larger hole size than the barrel gas port. For set screw gas block, they drill the gas port when drill the set screw hole, I think the set screw is #8, I don't remember now. #8 would be .136" dia #29 drill bit, bigger than barrel gas port or gas tube ID!
 
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Use a precision pin gage set, like starretts and you will know precisely the diameter of the hole.
 
A lot of assumptions can be made with varying degrees of accuracy. Barrel diameter has no effect on gas port diameter. Things that do affect gas port diameter - barrel length, gas system length, caliber... All have an effect on port size selected by the maunufacturer. As stated, measuring is best. You can buy a small selection of gauge pins in the range you need on Ebay or from McMaster-Carr without breaking the bank.
 
Port size is based on length of gas system.

20" rifle length 0.935"-0.980" (Torque shoulder to port 12.125")
16" mid length 0.078"-0.081" (Torque shoulder to port 8.75")
16" carbine length 0.0625"-0.070" (Torque shoulder to port 6.75")

The length of barrel after the port has a big impact on function. The time the bullet passes till exiting is all the time it has to function. For most reliably of operation you need a min of 5" after the port, longer the better. With short barrels and shorter gas port location, the pressure is higher and calls for a smaller port.

Pin gauges are the best for measuring small whole sizes. # drills can be used to get you close. If the gun has been fired a lot, carbon build up will reduce the hole size you measure. This normally happens when you restrict the flow.
 
A lot of assumptions can be made with varying degrees of accuracy. Barrel diameter has no effect on gas port diameter. Things that do affect gas port diameter - barrel length, gas system length, caliber... All have an effect on port size selected by the maunufacturer. As stated, measuring is best. You can buy a small selection of gauge pins in the range you need on Ebay or from McMaster-Carr without breaking the bank.

Actually barrel diameter at the gas block journal can have an effect. A gas port in a .625 journal will be slightly smaller than a larger diameter journal like .75 or .875. The gas port in the barrel is the single most influential choke point of the gas system and thus the diameter and length of that diameter of that choke point have effects.

The best way to measure a gas port diameter is with gauge pin set but in a pinch a good set of Number Drill can be used to get a close enough estimate for most work.

But to the OP original question the port in the gas block has very little effect on the gas system assuming it is not a regulating gas block. Pick the gas block that you like the looks of and fits you application/build.
 
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