Since one of my AR9 carbines in built on a Glock-compatible lower, I thought it might be fun to build up an upper in .40S&W, since the lower would accept the mags without modification.

I picked up an 8.5" Kaw Valley Precision .40S&W barrel from JobBob's Outfitters, along with a New Frontier Amory .40 Bolt Carrier Group. I also swapped out the Spikes ST-9X 7.7oz buffer for a heavier 8.4oz one to counteract the extra umph of the .40. I decided on a regular A2-style flash hider, rather than one of many linear comps. I paired it with a generic carbine-length free-float tube, a set of HK sights, and a Primary Arms red dot.

The final product:



Close-up of the upper:



Hard to see, but the bolt is marked ".40S&W" just to prevent any mix-up.



Here she is ready to rock:



And some testing fodder to see how she runs.



I tried it with 165gr WWB and Federal 185rg. Note that the Federal (right) is a little longer and has a more rounded ogive. That will manifest itself later.





The Federal fed flawlessly, and I used it to sight the red dot in, POA at 25 yards. What surprised me was how easy it was, once sighted-in at 25 yards, to consistently ring the 18" gong at 100 yards. Pretty much POA/POI.

When I switched to 165gr WWB, though, I started to run into hiccups. The shorter/flatter round made of quite the jump into the barrel.



I had some FTF's that wedged into the top of the chamber mouth. It's a pretty good "shuunk" as the bolt slams forward. Hard enough to compress the rounds. Bam, bam, bam, bam, shuunk.



Occasionally you could feel the round sliding against the top of the chamber mouth even on the rounds that fed.

It's really fun to shoot. I like how it runs on the 185gr, and the .40S&W really thumps the falling steel plates on the range. I want to try some other rounds as well, maybe something without a flat profile. If you have a Glock-compatible lower, I recommend trying it out for yourself.

Oh, and here's the way it looks in normal 9mm attire.

You haven't heard 40 isn't going to be around much longer

I like it, nice carbine!

Nice build-up and range report.

I'm thinking that the .40 may have a future as a  PCC round.

_

I've been thinking about doing this lately. Thanks for the write up!

very nice setup

get some 10mm magnum brass (10mm magnum brass has a thicker webbing than 10mm or 40s&w brass)

have it cut down to 40s&w case length

slowly work up some super hot 40s&w using imr 800x

basically along the lines of a 460 rowland

even with that short barrel i bet you could get close to 2000fps with a 135gr bullet

I've got a 10mm blowback carbine whch I've developed some super hot loads for (too hot for any pistol). A extra heavy buffer and stronger spring will help slow extraction. My brass is a little dirty but absolutely no pressure signs on the case from premature extraction before chamber pressure drops.

The down side is light factory loads may not extract/eject/cycle properly.

I am going to finish my .40 upper this weekend. Aero upper, Matrix Aero LRBHO lower. KAW Valley 16" barrel, Frontier BCG. Speaking with KAW valley they like to trim the tab on the bottom of the FA bolt. Ian over there sent me a pic. KAW is working on building their own BCG.

I had the exact same problem with a Kaw barrel, would jam with almost every round.  I even lightly dremeled/smoothed the feed cone and could not get it to work. I switched to a QC10 .40 7.5 inch barrel and it has been flawless. I built a second with a QC10 10.5 inch barrel and it has not had a single hiccup either.  I also use the QC10 lowers and bolts for both.

AR40s are awesome.