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adjustable gasblock

 
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  #1  
Old 08-15-2011, 10:39 AM
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Join Date: Jul 2011
Location: Bethlehem,Pa
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adjustable gasblock

Has anyone taken the time to make your own adjustable gasblock? If so how did ya do it and how well does it work. I have an extra gasblock that i am gonna try it on just wanted some input from the guys that have done it and if they would change anything that they did. Thanks for the input and pics if you have any.
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  #2  
Old 08-16-2011, 09:25 AM
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Join Date: Oct 2007
Location: Patagonia Mountains, Arizona
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Re: adjustable gasblock

Quote:
Originally Posted by AaronVan View Post
Has anyone taken the time to make your own adjustable gasblock? If so how did ya do it and how well does it work. I have an extra gasblock that i am gonna try it on just wanted some input from the guys that have done it and if they would change anything that they did. Thanks for the input and pics if you have any.
Tell us more about what you're trying to accomplish. I have not made an adjustable gas block but I've considered it and have a JP adjustable block on one rifle which I've found to be less than satisfactory. . A lot depends on why you want an adjustable block.

A gas block must be initially able to handle the weakest cartridge which you want to cycle the rifle. For example, if you want to cycle with both subsonic and supersonic cartridges the gas port needs to be located for the subsonic cartridge. The adjustment then further reduces the flow to allow operation with full energy cartridges. Further reducing the flow (to zero) may be desirable to eliminate action noise and breach blast when using a suppressed firearm With that setting the action must be cycled manually.

Things to consider in selecting an adjustabel gas block.
1. what range of cartridge energy should it cycle properly.
2. can any gas block do the job or are other changes necessary.
3. do you want t to stop action cycling or not.

Then you need to decide what method to use to restrict the gas flow. Some designs use set screws. Some use a ported rod which rotates to select or vary the flow. This is a cross rod with either holes or ground sides. When the rod is rotated (up to about 3/4 turn) different flow rates are selected.

Then you need to determine how to select and tune the gas flow for various cartridges.
A gas block like the JP has a set screw can be adjusted over a wide range, but setting it in the field to change from full velocity rifle and a subsonic setting where both will cycle the action is a tedious process requiring an allen wrench and counting turns on the set screw. The one I have has no lock. Though I haven't had it change when firing it doesn't give me confidence that it won't. Better might be having a lever with detents to switch between settings. ( sub - full - off ). There are many possible designs. The pressures and temperatures the port has to withstand aren't as extreme as at a rifle's barrel throat, but they are still high.

Of course it has to be practical to make, withstand hot propellant gas, fit on your rifle around the barrel and not interfere with the hand guard, and not have loose parts which will cause inconsistent accuracy. Beware of mixing different metals (like stainless and aluminum) which have differential expansion and will either go loose or tight when the gas block gets hot.

What i've decided generally in is to not use adjustable gas blocks, rather tune each rifle for a specific bullet and load and stick with it. After several attempts I no longer believe it's possible to build a rifle which can perform well with both supersonic and subsonic ammo. The problem isn't just cycling. The barrel lengths, action sizes, case capacities, twist rates, and sights all need to be different.

AR-15s/10s do have a fairly forgiving gas system. They should cycle ok with up to a 2 to 1 gas port energy range. Normal supersonic loads vs subsonic loads are typically 4 to 5 times different both in muzzle and gas port energy. Muzzle energy is a function of velocity squared.times bullet weight and the energy available in the powder charge must be roughly proportional.

Last edited by LouBoyd; 08-16-2011 at 11:34 AM.
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  #3  
Old 08-16-2011, 10:43 PM
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Join Date: Jul 2011
Location: Bethlehem,Pa
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Re: adjustable gasblock

I want to be able to turn the gas system off and use as a bolt action at the range and afeild in the spring and summer for groundhog hunting and then for deer hunting i want to be able to adjust it so it doesnt slam the bolt and bang up the brass. I wont be using subsonic loads only full loads. If it helps i have a RRA LAR-8 mid length 16" barrel. I also have a rifle length hand guard on it so i know i might have to do some modifications to it to get an altered gas block to fit and to get to the adjustment (screw or lever). I am not a gunsmith but i do have a good background in machining. I like the simplicity of the m14 valve that is on my dads m14. Just looking for something that is reliable and functional and hopefully will allow me to neck size my brass and not have to full length size everytime. I have some ideas in my head but need to pull my rifle apart and get some measurments and do some drawings with my grand father who is a draftsman for a machining company to go over opitons. Any inputs will be thankfull and any changes you think might make one better will be helpfull as well( two heads are better than one and some times three is better) Thanks again
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  #4  
Old 08-17-2011, 09:34 AM
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Join Date: Oct 2007
Location: Patagonia Mountains, Arizona
Posts: 757
Re: adjustable gasblock

If you only want to reduce (not increase) the gas flow of your existing gas system you might consider an MGI adjustable gas tube. It's just an adjustable valve inserted in a standard gas tube. You would only need to drill a small hole in your handguard to access the adjusting screw .
MGI Adjustable Gas Tube Rifle Length AR-15 - MidwayUSA
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  #5  
Old 08-17-2011, 10:13 PM
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Join Date: Jul 2011
Location: Bethlehem,Pa
Posts: 16
Re: adjustable gasblock

Thanks for the suggestion. I looked at the tube and if it had a ball valve instead of a set screw i think it would be better. I guess i have some research to do and possibly some designing to do. Thanks for the input.
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