I need someone to explain to me the purpose of the adjustable gas block

[JBMauser]

I think I am on the wrong path. I am building a 300 Blackout and I was going to put a adjustable gas block on a 16 in barrel. I think or maybe thought that I could open it up and get more gas to cycle cast bullets if I did not have a hot enough load or my bullet was to slick to build up enough pressure to cycle the bolt. Then I read some where it is to limit the gas, not get more.

Before I spend the $$ on a do-dad I will never use I figured I would ask here as Jacketed guys have no clue in most cases. I plan on giving powder coating a go as it seems interesting as I get back into casting.

Thanks in advance on help building my first and maybe last AR... JB

[Ironnewt]

Adjustable Gas blocks allow you to "tune" the amount of gas that bleeds into the tube and works it's magic. Too much gas cause Issue X, Too little causes Issue Y but the correct amount give you "Just Right" Kinda like Goldilocks and the three bears. Another thing you can do is change the setting as needed for Sub sonic/supersonic BUT don't take that one as Gospel. Two of my friends that are pretty good guys with putting these things together are 100% opposed on this one.

[JBMauser]

Thanks Ironnewt, but I guess they do not allow or adjust for more gas than a conventional GB just less. This is my quandary. The post that confused me was talking about turning the gas down and not pounding the bolt group for no good reason. That is why I ask. I will not use a suppressor but I under stand that loads can have cycle problems. My thought was I might find a sweet shooting load that needs a gas tweak to cycle. I thought some cast shooters may do just that.

[Jupiter7]

It is limited to the available gas in the gas system. Port size and tube length are it's limiting factors. It is rifle and situation specific. 300blk is a good example where it can be used. Usually, with a pistol length gas system, Supersonic loads will be over gassed, subs will run fine assuming load is up to the task with above port size parameters being paramount. An adjustable gas block cannot add more gas than the port will allow. A lot of people use them for changing between suppressed and un-suppressed to prevent copious gas blowback to the face. Match shooters use them to reduce recoil and bolt travel speed. I've never "needed" one to date, but have used a few. Don't cheap out, I've had a few cheap ones back out while shooting. I like the ones from Odinworks, BTE, Syrac Ordinance and 2A armament.

[country gent]

I have them on my AR match rifles they will "tune" the rifles performence and ejection. They dont cant allow more gas into the system but can and due restrict the gas to what is needed. Adlusting the gas block would show slight improvements in accuracy at 600 yds when we tested them. Also saved some beating on the action with high use rifles. On a system operating at lower limits of gas volumne there may not be much downward adjustment left. Its a system available to use when needed but isnt always needed. I would test the "standard" style first if ejection seems to brisk or action seems to be cycling hard then try the adjustable block.

[7br]

I enlarged the gas port in the barrel in my 6.8 and then put on an adjustable gas block. I am able run down to under 21 gr of IMR4895 and get reliable functioning. It might take a tweak to the barrel to get the gas volume you need.

[JBMauser]

7br, how did you enlarge the barrel port and not leave a burr extending into the bore? dimensions?

[Artful]

First thing I would do is see if the port is located correctly.
Blow air through the barrel to see if it comes out the gas tube.
Plug the chamber end with a spent case and blow in some compressed air from the muzzle.



Putting something for the drill bit to cut into inside the bore - usually a dowel or lead bullet helps eliminate burrs.

[Jupiter7]

Those numbers in the chart apply to .223. Bore size matters also. Minimum for 300blk at pistol gas port would be .095, personally .100 and up would be my minimum. For carbine length, .11. My buddies factory 300blk with carbine gas wouldn't cycle factory remington subs until .111.

[JBMauser]

I have a carbine barrel and it states it has a Carbine Length Gas System, Gas Port Drilled to .125

[Artful]

I have a carbine barrel and it states it has a Carbine Length Gas System, Gas Port Drilled to .125

That's interesting because the last gas tube I measured was only 0.120" inside diameter, why make the hole in the barrel larger than the ID of the tube? Unless they are providing a gas tube with larger ID (thinner wall compared to the one I measured).

[country gent]

You are also limited to the riflings lands and grooves with the port size as breaking into the edges of lands can b detrimental to accuracy. Match rifles should have the ports thru the groove .

[popper]

Gas port sizes are all over the place. My carbine tube had an undersized port, 070, opened it to 093, works fine now. Put a dowel in the barrel and watched the depth of the drill (tape on the bit for depth), oil well and go slow with the drill speed. Adjustable Syrac GB allows me to tune for gas VOLUME which is different for different powders. Set for H110 load and it won't LBOE for 4227. Use one on my 308 also. Bolt failure typically is crack on cam bolt or hole due to over gassed. Lower can crack at the buffer tube.

[Buckshot]

...............Having nothing at all to do with AR types, I modified the existing gas tap on my Swede Ag42B 6.5x55, which is direct gas impingement operated.



You can see it just ahead of the handguard. This rifle, like it's progeny the Hakim (which HAS an adjustable gas tap) were known for throwing brass into the next time zone, and quite often banging the brass up in the process. Another problem associated with the rather violent action is that the Ag42B has a very narrow ejector, which has on occasion banged a notch on the case rim up into the extractor groove far enough to keep it from fitting into a shellholder.



I removed the screw which closed off the top of the supply passage and then D&T'd the hole 8-40. Then ground the end of a sockethead screw to an angle as shown. I can set it when using cast lead so that the bolt carrier will just unlock the bolt. I can then easily withdraw the bolt and remove the case. With regular jacketed loads it may be set to extract the empty and have it drop on the ground a couple feet to my right, rather then being sent off at warp factor 12

...............Buckshot

[7br]

7br, how did you enlarge the barrel port and not leave a burr extending into the bore? dimensions?

I went very slow and made several passes. Afterwards, I shot it quite a bit will jacketed. Seems to work fine for me.

[Artful]

Good to hear! Now have a happy and safe 4th of July!

[Moonie]

My 6.8 upper (Bison 18" Recon) cycles with 18gr of H4895 using an Ideal 280412 134gr boolit. But I agree, you would normally not need an adjustable gas block unless you are running a 16+" barrel with a pistol length gas system. Running supersonic loads would almost certainly be over gassed.