OK got Glock 40...
Now then I was planning on getting a barrel for harder hitting ammo, and also will need a new SS Guide rod and Spring and need weight advice.
Looking at Lonewolf Distributing and KKM Percsion barrels. Both seemed tempered to higher RC hardness.
A lone wolf has a gen 4 that looks like it will fit, and seen adapter washers for others.
I got to much info over load and not well versed right now and could use some advice and ideas.
I outfitted my G40 with a 6" KKM barrel and a Glock Store Gen 4 style stainless steel RSA. I like the KKM barrel, it shoots well and cleans very easily.
I don't personally see any need to convert the Gen 4 to a previous gen type of RSA. If I remember correctly, Lonewolf makes a Gen 4 style stainless RSA which allows you to replace the spring rather than toss the whole RSA. I may go that way next time as the Glock Store version isn't designed this way.
You didn't ask about sights or trigger upgrades but I dropped a Zev Tech Fulcrum trigger in my G40 and Dawson Precision fiber optic front and adjustable rear sight. The trigger is a big improvement over the stock one.