Could use some help tuning my new ar9

[OnePivot]

Hey everyone, could use some tuning help with my AR9 I just pieced together.

 

Milspec lower and PSA upper. Bk 16" barrel (probably a BA?). BK 14.6oz bgc, kak k spec 9.5oz buffer and flat spring kit, in a carbine tube. 24.1oz total. 

 

R&D Colt magwell properly installed with divots for the set screws. Tuned the ejector to sit snugly at the top of bcg.  

 

I'm shooting 124gr Everglades plated. They go about 1005 fps in my Glock 17, 125pf on average. Did not chrono in the AR9.

 

Shot 200 rounds today, recoil feels great! I only have a sub 2k to compare to, and the sub 2k kicks like a horse. This AR9 is pleasant and mild.

 

I had 5 or 6 malfunctions total. All were stovepipes, all jammed right at the front corner of the ejection port, locking the bolt shut but sticking out of the gun. Just the brass rim jammed into the port. I had to pry the brass out with a tool. 

 

I wanted to write this off as it being a AR9 and I simply needed to tune the ejector that one last bit... But on a whim, I started monitoring ejection, and my brass is dribbling out! Maybe 2-3 feet max. Lrbho works every time without issue. 

 

Is my bolt too heavy? Spring too stiff? Both? I don't 100% understand the bcg weight relationship, especially since the bolt locks back and ejection is weak anyway!

 

I don't want hotter ammo. Id actually prefer to go even weaker, it's probably going 1200 fps in the rifle, and 1050fps would be more ideal... But I want to take calculated steps to ensure this gun runs and I already have weak ejection.

 

Blowback9 has been incredibly helpful getting this to 90%. Just not sure how tuning goes when minor of ammo gets mixed in. Thanks!

[Alvin]

With all due respect to Blowback9....with my light loads, 131PF, I had the brass dribbling out using an 8 ounce buffer plus stovepipe brass at times.  Went back to 5.2 ounce buffer and it's now throwing the brass about 5 or 6 feet out with no stovepipes.  8 ounce buffer works well with factory ammo, but not with light handloads.  At least that's what I found out with all four of my AR9s

[OnePivot]

55 minutes ago, Alvin said:

With all due respect to Blowback9....with my light loads, 131PF, I had the brass dribbling out using an 8 ounce buffer plus stovepipe brass at times.  Went back to 5.2 ounce buffer and it's now throwing the brass about 5 or 6 feet out with no stovepipes.  8 ounce buffer works well with factory ammo, but not with light handloads.  At least that's what I found out with all four of my AR9s

Thanks! I think that'll be my next step, to pull some weights out. 

 

[OnePivot]

That did it. Removed 2 tungsten weights and replaced with steel. 22.3oz now, and runs great. 

 

Blowback9 has been spot on besides this. You definitely can go too heavy on bcg/buffer mass. 

[Ner]

even better results would be if you leave buffer weight as it is but remove a bcg inner weight if possible. Or move to 115gn @~1170 fps

[dwzuspsa]

When you pull the charging handle to move the bcg back how much room do you have when you lock the bc with ar slide lock.  The bc should have only about 1/4 inch or so  to lock ...any .. more length than that drop some quarters behind buffer spring.

[OnePivot]

On 1/31/2024 at 12:02 AM, Ner said:

even better results would be if you leave buffer weight as it is but remove a bcg inner weight if possible. Or move to 115gn @~1170 fps

I can give that a try, it's easy enough to swap around. Is the difference significant? I figured weight from either would be the same. 

[OnePivot]

2 hours ago, dwzuspsa said:

When you pull the charging handle to move the bcg back how much room do you have when you lock the bc with ar slide lock.  The bc should have only about 1/4 inch or so  to lock ...any .. more length than that drop some quarters behind buffer spring.

I definitely had less than a 1/4 inch. 

 

I ended up short stroking it and don't have lock back anymore, but I don't think I like it. I have a 3d printer and just made a spacer. 

[lgh]

What you want to do is get your 9mm PCC running like a 22lr. Costs a lot but worth it.