Cotton Arms Adjustable Gas Block - .750"

Manufactured by Cotton Arms
SKU: 750AJ
MPN: 750AJ
Manufactured by Cotton Arms
8 Reviews
No Longer Available
For individual Officer, First Responder, and Military pricing please make sure you are logged in. for details



Cotton Arms takes great pride in bringing precision to their work in an industry where fractions of a thousandth matter. They understand the precept that accuracy influences how often one can hit the target, while precision dictates how small that target may be. Expect nothing but the finest craftsmanship from Cotton Arms.

This Cotton Arms Adjustable Gas Block is precision machined for AR-15/M4 pattern firearms. The end of the adjustment screw pocket is deliberately left unfinished on the last ½ turn. Upon adjustment, this unfinished half turn imparts a bite, eliminating the need for applying thread locker or gimmicky mechanisms.

  • Precision machined
  • 4140 chrome moly steel
  • Black oxide 5/64 Allen head set screw
  • 0.309” 3/32” Allen head adjustment screw
  • Inner diameter: 0.752”
  • Wall thickness: 0.1615”
  • Length: 1”
  • Height: 1.345”
  • Gas port diameter: 0.181”
See More


BrandCotton Arms
Gas Seat Diameter.750
MaterialSteel - 4140 Chrome Moly
TypeSet Screw
California Prop 65 warning

 WARNING: This product can expose you to chemicals, which are known to the State of California to cause Cancer and Reproductive Harm. For more information go to


by -

Bought this due to price. Installed it and it quite literally blew up on the first shot. The adjustment was at approximately 3\4 open and it was on a pistol length system firing 55gr M193.
<br>From PA: Very sorry to hear this! One of our reps will contact you.

Cheap but it simply worked for me

by -

After seeing the posted review I got nervous as I hadn't shot my rifle with this block on it since finishing the build several months ago. Didn't need to worry. I am running a lightweight BCG, and 1oz recoil system so the block was adjusted to be barely open and I had no issues. After the first 20 shots had to open a little more to account for build up but otherwise worked perfect. The no set screw wasn't an issue as my dialed in setting stayed out and didn't walk out. For the money it'll do the job.

Gritty threads

by -

This block feels solid and durable. However, upon installation I noticed some burrs inside the block that hindered a smooth transition into the barrel. Once this was mitigated, I installed the set screw for gas regulation. The screw stopped as soon as it hit the threads. I felt like I was going to strip it out as I torqued down on this stainless colored screw. Luckily I was to push the gritty threads alone and seat the set screw properly. This is a decent buy however only if it is on sale. My rifle appears to be firing flawlessly as I am using a lightweight bcg. With 200 rounds down range and hopefully more to come.

Solid piece of junk.


I wound up with one of these (not purchased from PA), and feel compelled to write a review here after seeing it offered. This GB fit so tight, it had to be tapped on to get it to seat properly, this after de-burring it inside. The gas tube was difficult to insert, and the roll pin hole was mis-aligned, preventing the pin from passing cleanly through. I would not recommend this item to anyone.

Good design, bad QC

by -

First one of these was incorrectly manufactured, the roll pin hole and adjustment screw hole were in the wrong place as if the block was placed in the jig to drill those hole backwards or something. I exchanged it with Primary Arms and the new one is well made.
<br>This method of adjustment retention is a good low tech solution that is easy to adjust and easy to clean should it ever seize up from fouling. I have some pricey detent-style examples that I'd honestly trade out for these if I could get this in a melonite finish.

Small budget option

by -

Installed for someone who purchased due to price. This is the simple way to close your GP. It works and should do fine for a hunting or range gun. Pros - cost... Cons - HEAVY, needs to be driven or pressed on. No set screw to bind the adjustment screw. Use of lock tite is mandatory so make sure it is set correctly the first time.

Surprisingly good for so cheap.


I bought this on sale for a super cheap 7.5in 5.56 build. The pistol length gas system was causing violent recoil from the BCG slamming back, the barrel is apparently WAY over gassed. Starting with this fully closed there is still some gas making it through the system, not enough to extract a case but enough to drive the BCG back about half way. I was able to adjust it to the point where it locks back on empty reliably but is dramatically less violent. I have about 100 rounds through it after adjusting and the witness mark I made on the adjustment has not moved. It fit the barrel and gas tube perfectly, all screws were good, and for $20 on sale I don't think you can beat it.

Fixed the Problem


Been working with a BCA .308 AR10 upper that is way over-gassed (brass and primers showing pressure signs; actually blew a couple of primers). This adjustable gas block solved the over-gas issue and along with a heavy buffer and spring smoothed out cycling. Ejection went from erratic 1-3 o'clock, 8-12 feet away to consistent 4 o'clock and no more pressure signs in brass and primers. Block worked perfectly (125,150 and168 grain bullets loaded light to heavy) and was bought at a great price. Quick shipping as usual with Primary Arms.