Buffer for 9x19mm AR

No, standard buffers are not heavy enough for blowback 9mm, blowback operated firearms need adequate buffer weight to hold the bolt closed during firing prevent OOB discharge, bolt bounce, breaking bolt latches, extra wear, etc..

You want the combined weight of your BCG and buffer to be somewhere around 20-23 oz. I think my buffer is 7.5 or 9 oz. I can't quite remember. I can weigh it when I get home. Another important factor is the buffer spring weight which also aids in holding the bolt closed.

Another factor you want to consider is when you put together your AR9 is pull the charging handle all the way back and see how much distance you have from the face of the bolt to the bolt catch. You want to minimize extra space there to prevent a 'running start' the bolt gets on the bolt catch, this can cause bolt catch breakage. One can place quarters in the bottom of the buffer tube to take up that space.

In my carbine stocked 9MM Colt pattern ...I use a 9MM extended buffer ( about 7.5ish oz ) and a Tubbs 308 Recoil spring.( Yes, a 308 recoil spring... Even JP recommends a 308 recoil spring )
https://www.kakindustry.com/ar-15-parts/lower-parts/buffer-tubes-and-parts/ar15-9mm-buffer-long

https://www.larue.com/products/tubbs-buffer-spring-ar10-sr-25-buffer-springs/

Or this configurable 9MM extended kit...
https://www.kakindustry.com/ar-15-p...tubes-and-parts/ar15-config-9mm-buffer-system

As mentioned, you do NEED to make sure the distance between the compressed into the buffer tube bolt face to bolt catch is no more then 3/16"... otherwise sooner or ( later... more sooner then later ) you will break a bolt catch.

Here is a 9MM spacer for using typical AR15 buffers... BUT... you install it at the bottom of the buffer tube, so IMHO, it would be easier to just buy the different buffer and spring and change as you change calibers.
https://www.spikestactical.com/collections/lower-parts/9mm-buffer-spacer/

Changing to a heavier buffer weight and stronger buffer spring weight... really took the bark and blast right out of the 9MM blowback system.

I highly recommend it.

I have a solid buffer, I think it’s 9 ounces, and an AR10 extra power recoil spring which works pretty good for my setup. I also have a rubber buffer tube spacer but some people just use quarters. Mine still throws brass 15 feet.

It really depends on the bolt/carrier. If the carrier has enough mass, adding too much weight to the buffer will give you carrier bounce and ultimately lead to out of battery excitement.

I have found that New Frontier bolt/carriers do not need anything other than a standard buffer.

I have a New Frontier (plastic) buffer spacer in one of my carbines, and a Spikes (aluminum) spacer in the other.

The dedicated CMMG pistol lower (Colt pattern) came with a 5.5oz buffer. It works just fine with the CMMG 9mm bolt

It’s all about the right bolt, buffer and spring combo. I have the New Frontier Armory spacer in my pistol build. I have the Rock River buffer in my carbine and pistol.
I built my carbine a few years ago before everybody and their mom were building AR9s. I tried different buffers until I found the right combo.
https://www.jsesurplus.com/rockriver9mmbuffer.aspx
Some people get bulged cases from the wrong combo, but that’s not the most common problem. The most common problem is the gun beating itself up from having to light of weight combo.