I decided to build an AR9 pistol for a range toy, this wasn’t ever intended for use for PP/home defense.  I also have no plans to add a suppressor.  I didn’t want to spend a ton of money either.  I have a G19 and a number of magazines for that so I went with a Glock configuration.  I purchased the following from PSA;

1.PSA Stripped billet 9mm lower with the mag catch assembly and ejector installed
2.PSA 10.5” 9mm barrel assembly with M-Lok railed upper and KAK Flash Can, hybrid BCG and charging handle
3.PSA Shockwave Pistol MOE Lower Build Kit that came with the standard carbine buffer and spring

I added a single point sling end plate, sling and Magpul flip sights.  A red dot may come along some day as funds become available.  

The build went without any issues.  The PSA parts all fit well, I couldn’t find a flaw in the billet lower.  

I loaded both Glock and Magpul magazines with some 115 and 124 RN and Federal HST 147 HP’s and off to the range I went.  The first range session was an abysmal failure.  FTF/FTE constantly.  It would feed in the first round but hiccup the next round, shell and new round all slammed together.  It pushed one bullet back into the shell casing on a jam.  It didn’t matter which type of bullet or magazine I used.  I quit for the day, came home and did some more research on this forum.  

There were a number of posts discussing heavier buffers and springs being an answer FTF/FTE problems.  So, after much reading I decided to order a KVP 7.5 oz SS buffer and SS pistol caliber carbine XP spring from NFA.  After their arrival and install, it was off to the range again.  I took along 115, 124, and 147 gr. RN ball and the HST 147 gr. HP’s again. Everything fed and ejected without issue.  I fired off a string of rapid fire and all was well.  I did have one hiccup with the HST’s.  I had loaded six into a new Glock 33 round magazine.  It failed to feed the second round.  I cleared it and fired the rest of the mag without issue.  I then tried the HST’s using a Magpul 15 rounder and 6 rounds.  Those fed through fine.  I’m not sure if it was the new 33 round Glock mag or the HST’s.  I’m going to buy some other brands and weights of HP ammo and see if they work better.  Overall, I'm happy with the project, its a fun pistol to build and shoot after the issues were ironed out.  It will now join my other "black rifles."

I wrote this to possibly help others with AR9 build issues.  In this case, the KVP 7.5 oz. buffer and heavy spring were the answer to my feed/ejection issues.

Regards, Bob

Thanks, I was looking to get the KVP buffer and spring for my build.

my PSA 10.5" 9mm upper came with a steel 5.4oz carbine length buffer.... works fine with my SBR lower with Macon Armory mag well adapter...I shoot reduced loads... 146 power factor 135 Bayou coated lead...


for what it's worth... I wouldn't jump into a heavy buffer until you shoot it  YMMV

A friend of mine had a Quarter Circle 10 lower (similar to PSA/NFA lower) and his would do the same thing. It seemed to do ok with heavier/hotter loads but still suffered from an occasional FTE/FTF. After I took it apart I noticed QC10 used a floating ejector pin that had a bit of side to side movement. I shimmed the sides of the ejector with a peel shim off of a flash hider and now it's 100% reliable with any grain/weight bullet.

I purchased a PSA complete lower that uses Glock magazines and a complete PSA upper with a 7.5" barrel. The lower came with a steel 5.5 ounce buffer installed. The rifle has been SBR'ed.

Works perfectly suppressed and unsuppressed, although I've only used ball ammo thus far but in a variety of Magpul, Glock, and ETS magazines.

Please reread my entire post.  It didn't shoot with any reliability with the steel carbine length buffer and spring.  Only after I installed the heavier buffer and spring... and shot it, did it work.  

This is why I posted this topic.  Some AR9's are fine with the standard carbine buffer and spring and some... not so much.

rbh... sorry, wasn't referring specifically to your post, just a comment at large...I tried a heavy 9oz buffer and .308 spring in my 16" PCC and it beat up the barrel where the bolt face slammed into it