I bought one to convert my 40 XD to 9mm. I can't recall what the problem was, probably it wasn't accurate, but I wasn't happy with it at all... so I sold it and bought a Storm Lake and am much happier with it. Looking at my load data spreadsheet, the SL has a lot better accuracy than the EFK did, but this was back in 2016 so YMMV...
I'm surprised they discontinued the XD line of barrels... any report as to why? maybe they figured Glock was their mainstay and it wasn't worth the extra production time. I would do an extensive google search to see if anyone has one leftover in stock...
ETA...
According to their website...
https://www.storm-lake.com/products.html All of their XD barrels are still available... the 4.05" 40 to 9mm conversion is product #34110... however the 'Buy Now' button is missing... it's also missing from their Glock and S&W products. The only one they have a buy button for is the 1911's... I wonder if they had to stop for some legal reason like the patented barrel lockup/pivot system?
So far all the retailers I have checked show it as discontinued...
Looks like they are not an option any longer...
IF that is the case then EFK may be the next to disappear... better buy something now if you want it...
ETA... TopGun has some in stock - mostly Glock and one XD 9mm non-conversion barrel...
BoTach says they have 4 XD 40 to 9mm conversion barrels in stock...
https://botach.com/stormlake-conversion-gun-barrels-springfield-xd-40-s-w-to-9mm-4/