Adjustable vs Non-adjustable Gas Block

[Idon'tCareAtAll]

Before I pose my questions, I would like to explain my reasoning for the question in the first place.

I have three ARs, one 5.56mm carbine with a carbine length gas system, one 5.56mm 20" barreled with a rifle length gas system, and one .224 valkyrie 22" barreled with a rifle length gas system.

Years ago I was talking to a gentleman that has pretty much spent his whole adult life reloading ammo. He spent 8 years in the military loading specialized ammo for rifle teams and special operators and nearly 50 years running a reloading shop and teaching those who would be teachers reloading.

The gentleman was telling me about how vibration, harmonics, and several other things about the frequency of a barrel affecting accuracy. To be honest, as someone with a fairly decent grasp of physics, he was talking way over my head.

Now, I have changed out the A2 sight/gas system on my carbine and built the rifle with the 20" barrel with non-adjustable gas blocks. I built the 224 with an adjustable gas block.

I tuned the gas block on the 224 so that it cycles the ammo that I fire from it reliably. It is very accurate. I don't have any photo examples of my groups, but, I find them very satisfying.

I changed out the A2 gas system to a non-adjustable gas block on my carbine and put on a handguard that would make the barrel free floating. I did it mainly because I have long arms and the carbine grip was too short for my liking. Also, I thought it would improve the accuracy. It didn't. It actually deceased it a bit.

Now, finally, my questions. Is it possible that changing out the gas system on the carbine some how changed the harmonics, frequency, and whatever else to the point of affecting the accuracy negatively? Would it behoove me to put an adjustable gas block on it and adjust it as I did the 224? Will the 20" barreled rifle benefit the same way with an adjustable gas block?

 

[hlee]

Yes
Maybe
Maybe

 

[MtnCreek]

I'd lean toward the barrel nut/handguard install rather than gas block changing your carbine accuracy.

 

[drewthebrave]

I'd lean toward the barrel nut/handguard install rather than gas block changing your carbine accuracy.

This is my inclination as well. Is the rail mounted securely? Is it possible the gas block or gas tube are making contact with any part of the handguard or mounting hardware on the new forearm? There are lots of variables here that need to be checked before I'd assume the gas block was the culprit.

 

[UpSideDown]

I'm with MtnCreek and drewthebrave, going from an attached handguard to a free floating and losing accuracy sounds like there may be an install issue; too low torque on the barrel nut, or a loose barrel-receiver fit. What comes to mind as the most immediate possiblity is that the new handguard might be touching the gas block when the gun fires and the barrel moves. Is the handguard over the gas block?

Adjustable gas blocks are nice because you can tune the gun to properly shoot the ammo you want to run. I only run adjustables now, but they aren't a requirement. Whether or not they'll have a major affect on accuracy, I don't think so. Most of the accuracy of the gun comes from the bullet/cartridge load, and the quality of the barrel.

 

[alamo5000]

A gas block might change accuracy, but that is waaay down the list of possible things that can be happening or potential fixes.

 

[Evintos]

Now, finally, my questions. Is it possible that changing out the gas system on the carbine some how changed the harmonics, frequency, and whatever else to the point of affecting the accuracy negatively? Yes it's possible. Your A2 FSB has a different weight and profile than a low profile gas block and would alter the harmonics of your barrel. Your new free float handguard and barrel nut and torque spec on it also altered the harmonics. You will have to go through the process of finding or developing the right load again for that barrel. Changing the muzzle device can also affect this. To what degree each individual component affects the barrel harmonics is unpredictable.

Would it behoove me to put an adjustable gas block on it and adjust it as I did the 224? If you want an optimized gas system (for recoil impulse and unlock timing) you can but gas flow adjustment in of itself doesn't necessarily affect accuracy unless your unlock timing is really off.

Will the 20" barreled rifle benefit the same way with an adjustable gas block? Depends on gas port sizing and what you want to accomplish with the adjustable gas block. Do you want to just optimize the timing of the system or do you want to also tune recoil impulse?

 

[Idon'tCareAtAll]

I'll take a look at all the things listed so far.

I greatly appreciate the advice given thus far. I will post what I find out here.

 

[javierortega72]

Hmmm from my understanding adjustable gas blocks are for adjusting gas flow to cycle the bcg properly when using subsonic ammo while using suppressor/silencer. U close it adjusting clock wise fully shoot it and back off counter clockwise til it stops cycling then clockwise 1 click. And that would be properly adjusted for that ammo u currently are using. You would have to recheck all ur build torque specs and make sure it is where is should be. To me sounds like something is loose. I'm no expert, but been around the block a few times. Started reloading just like 2 years now. Done about 15,000 rounds between 9mm, 45, 5.7x28, 7.62x39, 5.56 or 223, and 308! Ammo consistency is must!

 

[the monk]

Do you still have the A2 gas block? If you do I have seen this in the past where because the way the A2 and barrel mate up when you take that away you change the harmonics. I would generally recommend to shave down the A2 to fit under a free float handguard. This has the benefit of keeping the harmonics and is a more robust system with it being pinned.

 

[javierortega72]

I'm new here I apologize if I mentioned something I shouldn't. I was unaware of the rules! If you may can u list them for me? And what in the world is 'px'

 

[javierortega72]

In real world I know it means post exchange, but here it may be different!

 

[Evintos]

@javierortega72 https://www.snipershide.com/shooting/account/upgrades/
https://www.snipershide.com/shooting/threads/2021-snipers-hide-forum-rules.6920466/

https://www.snipershide.com/shooting/#post-exchange.20

 

[Idon'tCareAtAll]

I've examined the rifle thoroughly. I retorqued the barrel nut, installed the handguard back on the rifle, checked to see if it was loose, and checked the space between the gas block and the handguard. Everything went back together as well as can be expected. I'll send pictures. I was really hoping that I had just had a brain fart and installed the gas block and handguard improperly. I don't see anything wrong.

I'm going to take it to the range again and see how it does. Hopefully I did make a mistake on the original install and it will shoot as well as it did. I know that it sure feels much better in my offhand shooting, even if it wasn't shooting as accurately.

I'll post my progress.

 

[Idon'tCareAtAll]

I didn't see a difference in accuracy after checking that the barrel was seating completely and reinstalling the handguard. I have tried every 5.56 and .223 ammo that I have with the same poor results.

I may not get an accuracy increase just from floating the barrel, but, I would expect it. I'm going to pull the handguard and barrel nut and put a barrel nut shim in to make sure that there is not some imperceptable barrel movement. Should have done that before. Duh.

In a last ditch effort, I will put an adjustable gas block on it to see if it helps.

I'll report my progress.

 

[javierortega72]

I bought an adjustable gas block .750 from mmc for 10.99 each (got 3 of them) and wondering if it would mate up good with my 6.5mm creedmoor cal ar10 from Daniel defense 24" stainless steel barrel .750 gas block non adjustable. Want to put one in my Adam arms witch doctor 5.56 cal nitride barrel. Would I have any issues. Adjustment is what I'm worried about since it simply has one locking set screw and a set screw with no positions and just in and out. I figure it will be a little optimistic dialing it in! Any suggestions?

 

[Evintos]

Be aware. Set screw AGBs like those will inevitably seize up and make it harder to adjust once it seizes. If the gas block is machined correctly and the gas block journal on your barrels are properly machined to the 0.750" diameter, you're good to go.

The below method is what you would do to save time and ammo when finding a good AGB setting. This will be done before you even head to the range.

Before you even swap gas blocks, check each gas block to see how many revolutions it takes to go from wide open gas to completely closed. Save that number.

After you remove your old gas block figure out what size your gas port (ideally with pin gauges however calipers will get you close enough), measure out your reciprocating mass (bcg, buffer) total in ounces, then identify what gas length you have (carbine, midlength, rifle length, other). You use this information to formulate a starting point. For example -

Your combo (pseudo MK18) - 10.5" barrel carbine length gas, barrel gas port 0.083", H2 buffer + BCG = 16.2oz

US Military MK18 (off the top of my head) would be a 10.5" carbine length gas, gas port 0.072", H2 buffer + BCG = 16.2oz. You can find the military equivalents online for the most part.

Since your gas port is oversized by about 15% but everything else is similar to the US military spec, 15% reduction will be needed.

If your AGB takes 10 full turns to go from full open (100%) to complete close (0%), that's 10% per 1 full turn. (100 ÷ 10 = 10). Therefore to get around 15% reduction, you need 1 1/2 turns close from full open to achieve the rough equivalent of a 0.072" gas port size (0.083 ÷ 0.072 = 1.1527777; 1.15277 to percentage is 115.277%) That would be your starting point before. You reduce gas or increase gas for every difference against the US military baseline standard.

You use military spec as the baseline since they've tested it at a far greater degree to find a suitable gas port size for combat reliability. US military tends to overgas for reliability purposes so if you want to optimize further, you'll reduce gas further.

If your gas port is smaller than the baseline standard and you run into cycling issues, buffer mass changes or drilling the gas port since 100% open on an AGB behaves exactly like a non-adjustable gas block.

 

[Idon'tCareAtAll]

I bought an adjustable gas block .750 from mmc for 10.99 each (got 3 of them) and wondering if it would mate up good with my 6.5mm creedmoor cal ar10 from Daniel defense 24" stainless steel barrel .750 gas block non adjustable. Want to put one in my Adam arms witch doctor 5.56 cal nitride barrel. Would I have any issues. Adjustment is what I'm worried about since it simply has one locking set screw and a set screw with no positions and just in and out. I figure it will be a little optimistic dialing it in! Any suggestions?

I think that you really can't lose installing an adjustable gas block. If you read up on the topic, I can't recall any articles off the top of my head, you will find that most manufacturers that are installing a non-adjustable gas block do so intentionally providing more gas than is necessary, as @Evintos pointed out, to increase reliability. BTW, I am extremely impressed with his math. Reminds me of all those years as an engineer.

I don't know the exact math, but it just seems logical that if there is too much gas expended at the gas port, as the projectile passes the port, that it would cause more pressure drop instantaneously than is needed. This would cause a sort of mini shockwave that would have to affect the barrel frequency and barrel whip, thus accuracy.

Forgive me if my explanation seems too redamentary. I can't think of any scientific theories that are related to my hypothesis. In engineering there are times when you have to use your gut when making decisions. This is one of those times that I am going to trust my gut and put an adjustable gas block on a rifle that I feel will benefit in increased accuracy in doing so.

And, @javierortega72, I think that you can't go wrong in installing adjustable gas blocks on your gas rifles. If I show improvement in my carbine, I'm going to put one on my other AR that doesn't have one. It's already a shooter, but, since I don't reload, maybe it's something that I can do to increase its accuracy.

Good luck.