Is there any noticeable difference between barrels with a 0.750" and a 0.625" width, if they both have the same gas port size?  Could you tell the difference between a 14.5", 0.625" carbine barrel has a gas port of 0.70", and a 14.5" 0.750" carbine barrel with a 0.70" gas port?  Or, is there a noticeable difference between a 20", 0.625" rifle barrel with a gas port of 0.93", and a 20" 0.750" rifle barrel with a 0.93" gas port?  Internet theory would say the 0.750 might feel less 'gassy' than the 0.625, but I want your experience.

A larger diameter barrel will weigh more which reduces felt recoil.

More gas can result in more felt recoil.


Kind of an odd question you're asking honestly.  Buy an adjustable gas block and enjoy slightly reduced recoil in a lighter barrel.

Interesting thought.  I'm sure there's an actual scientific answer based on physics.

I'm not an engineer, but something comes to mind that possibly has some similarity.  Intake and exhaust dynamics in an engine might have some comparison when you consider intake runner length/diameter and exhaust header length/diameter.  These elements influence the flow of air and gases.  The size and length of the gas port as it's altered by barrel thickness might be too little of an element to actually have an effect...or maybe it does.

I have a somewhat anecdotal experience with a gas port scenario.  I have a 24" SS match bull barrel that was mounted on an AR.  Even with the lighter carbine buffer and spring, the gun would not strip off a new round no matter what...different ammo, standard gas block, another BCG, etc.  Trust me...everything was correctly installed, checked, and double-checked.  I was not getting enough gas...period.

The gas port was what most charts recommend for a 24" barrel.  I eventually drilled out the gas port to the next chart size.  At that point and after, the gun cycled perfectly...every time.

That barrel was a hugely thick artillery piece.  I might consider that the port size and gas flow was somewhat inhibited by the distance the gas had to travel within that thick segment of barrel and therefore reacted positively to the next port size I drilled.  I don't know.  Just a thought along the lines of what you're discussing here.  Hopefully someone who knows some of the science/physics applicable to this issue will chime in.

The larger the diameter of the gas block, the smaller the effective gas port diameter.

The larger diameter barrel journal will have a gas-port of around 0.001-0.002 more, typically.

I'm having difficulty understanding how barrel port journal radius (distance from port opening in the barrel and its exit point on the outside of the barrel) could affect cycling.  The gas block is going to exit the gas at the height of the gas tube.  That will be the same, regardless of port journal thickness, if properly designed.  The gas tube length back to the gas key is the same.

If there is a variation from this, it would be from the failure of a gas port manufacturer to position the opening for the gas tube at the proper height, which should be the same regsrdless of other factors.

Ultimately, despite variations in journal size, the total distance the gas travels from barrel to gas key will be the same, if the gas block is properly designed.   If the height from bore to gas tube remains constant the distance the gas flows does not change. What am I missing?

I think he's overthinking the difference in effective restriction of the thicker barrel wall for a given gas port diameter.  Taken to extremes, yes, the larger the OD of the barrel journal, the greater the restriction of the gas port, all else being equal. Between 0.625", 0.750", 0.875" and 0.936", you're not going to notice any difference that isn't overshadowed by other factors like barrel weight.

JD, can you say where you got that info?  Not challenging it, would just like to know the source to learn more.

I've never seen this discussed anywhere until this post.  That either means there is no issue, or it's one of those things that only comes up once in a great while.

My 24" bull barrel scenario in my earlier post tends to support what you're stating here.

Daniel Defense and BCM. I believe both of them ported their LW barrels (DD is now a 0.750 journal) a few thou smaller than their 0.750 journal barrels. However, you also have to take into account that most of these companies have a 0.001+- tolerance on gas porting. A spec'ed 0.076" port may actually turn out to be 0.077 or 0.075.

FWIW, DD spec's 0.073" on their 16.1" middy, and 0.076" on their 14.5 middy, with 0.750 gas journal.

Realistically, gas-block to journal seal, cam-pin material and finish, and gas-ring drag in the carrier and the natural variance across action springs is likely more significant when added up than 0.001" on a gas port, however, you can certainly tell a difference in feel once you get to around 0.002 one way or another, if all things are kept equal. Remember than the diameter of a circle increases much slower than the area of said circle, numerically.

Many comments posted to the contrary, but if you test gas port sizes you find that .625 barrels generally use a gas port a few thousanths smaller than a .750 gas block, other things being equal. There is a chart floating around of Colt gas port sizes which describes the same thing. The difference is not large.

A long time ago, in a galaxy far, far away (before the 'dissapator' was invented), people used to like to chop rifle barrels (rifle gas systems) right in front of the front sight post.  An A1 profile barrel would usually function, but the heavier barrels (A2 style) usually would not.  This anecdotal comment might be useful if I had some info on gas port sizes, but I don't.  I assumed the gas port sizes were the same, but they may not have been... I walked away with the feeling that barrel diameter had an effect on gas flow... if thats what you are asking.

I'm old, and my memory may be wrong.