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Posted: 6/1/2010 11:44:46 AM EDT
| I feel like a bit of an idiot for not knowing the answer to this, but can you turn down the gas block area of a barrel after the gas port has been drilled? |
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Quoted: I feel like a bit of an idiot for not knowing the answer to this, but can you turn down the gas block area of a barrel after the gas port has been drilled? Yes. Had ADCO turn down a midlength HBAR for me, to a lightweight profile. I provided them with a .625 FSB to replace the .750 FSB. |
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Quoted:
Quoted:
I feel like a bit of an idiot for not knowing the answer to this, but can you turn down the gas block area of a barrel after the gas port has been drilled? Yes. Had ADCO turn down a midlength HBAR for me, to a lightweight profile. I provided them with a .625 FSB to replace the .750 FSB. Were you happy with it (handling, accuracy, etc)? I'm toying with the idea of doing an SR-15ish clone around a middy LW barrel, or maybe something like a .690 dia before the gas block |
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Quoted: Quoted: Quoted: I feel like a bit of an idiot for not knowing the answer to this, but can you turn down the gas block area of a barrel after the gas port has been drilled? Yes. Had ADCO turn down a midlength HBAR for me, to a lightweight profile. I provided them with a .625 FSB to replace the .750 FSB. Were you happy with it (handling, accuracy, etc)? I'm toying with the idea of doing an SR-15ish clone around a middy LW barrel, or maybe something like a .690 dia before the gas block Man, I'm an idiot.  I had ADCO turn down my 20" AR to .625 under the FSB. They did turn my midlength as well, but it still has the .750 FSB. Both are .625 in front of the FSB. Both rifles shoot great, and they handle much faster. The small amount of weight being removed makes a huge difference. I can't recommend ADCO enough, they do fantastic work. Door to door in about a week, and $100 each for the barrel work, including disassembling my uppers. |
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Quoted:
Quoted:
Quoted:
Quoted:
I feel like a bit of an idiot for not knowing the answer to this, but can you turn down the gas block area of a barrel after the gas port has been drilled? Yes. Had ADCO turn down a midlength HBAR for me, to a lightweight profile. I provided them with a .625 FSB to replace the .750 FSB. Were you happy with it (handling, accuracy, etc)? I'm toying with the idea of doing an SR-15ish clone around a middy LW barrel, or maybe something like a .690 dia before the gas block Man, I'm an idiot.
I had ADCO turn down my 20" AR to .625 under the FSB. They did turn my midlength as well, but it still has the .750 FSB. Both are .625 in front of the FSB. Both rifles shoot great, and they handle much faster. The small amount of weight being removed makes a huge difference. I can't recommend ADCO enough, they do fantastic work. Door to door in about a week, and $100 each for the barrel work, including disassembling my uppers. I'm sold 
My one and only AR (for now) is a middy HBAR, and I get sick of that 1" dia under the handguards real quick
May just keep the .750 gas block to keep my gas block options open and have both sides turned down to .625" |
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A while back, when I was first looking into making AR15 barrels, I found a table on the internet that gave the various gas port sizes for each barrel length and diameter. I also called the author, Rick McDowell, and he was a wealth of information. According to his table, the gas port size changes, if you turn down the gas block diameter. I've never made anything other than a .750 gas block diameter, so I don't have any experience in the area.
According to McDowell, you need a smaller gas port with a smaller gas block diameter, so you'd be taking a risk. I guess it depends on what your port is already. There is a definite range that will work. So, if your gas port is already in the lower range of what will work, you might be fine if you turn down the gas block. I've copied and pasted his chart into this response, but it looks ugly and I don't have the patience to make it look nice. If you're interested in getting it emailed to you, IM me. McDowell's chart: Q: My Rifle has a short-stroking problem - the gunsmith says it may be a gas port problem. What are the correct sizes for the gasport?
A: Courtesy of Rick McDowell of Competition Specialties, NRA Life, USPSA Life, Master ( 105 E. Cass, PO Box 451 , Osceola , Iowa 50213 Phone: (800)369-4481 (800)369-4481 (800)369-4481 (800)369-4481 / (641)342-2011 (641)342-2011 (641)342-2011 (641)342-2011 ) I have an Extraction Problem, is this related? How can they be fixed?The below gas port dimensions are the ones that I have accumulated while working on AR's for years. When someone is having problems with short stroking and their gas port is not within the below ranges, that is usually the main problem. On individual rifles I have varied both bigger and smaller with port sizes, but these were definitely exceptions, or there was no other choice due to poor quality parts we were working with. For example; 11.5" barrel, .750" barrel diameter at the port, 3.850" from muzzle; Port should be approx. .086" to .094" in diameter. The smallest I have seen the port on these short barrels is .081" and it worked with some military ammo but not with some faster burning rate, commercial ammo. With the .086" port the military ammunition worked better but the commercial still wasn't getting enough pressure to work the carrier properly. The .094" port would work with both the high port pressure military ammo and low port pressure PMC ammo. Anything over .094" will cause short stroking/timing problems from too much pressure to the carrier and under .086" might cause short stroking for lack of pressure. To know if the port should be changed or not will depend on a few other things. Is the chamber narrower than it should be? Is the chamber rougher than it should be? Are the rings worn and allowing gas to blow by? Is the carrier key allowing gas to blow by? Etc., etc. Eliminate these variables first! With a smaller diameter barrel at the port, the gas port sizes will usually be a little smaller as well, given the barrel length is still the same. 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 These are of my own compiling and may not be 'spec', but I was never able to find this 'spec', so I made a list of my own from known good shooting, reliable rifles. And just as important, from the unreliable rifles that worked fine after the gas port was opened up or made smaller. Don't let a gunsmith drill out a port because of a short stroking problem that you might have! More than likely it needs to go smaller, not larger. |
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