Here’s story from a very talented person, 737mechanic has made his own Mossberg 590A1 Mag Extension for 18.5 Barrel. The final result is really impressive.
Thought I would share what I decided to make for my 590A1. I really like the S&J mag extension but at 100 bucks I figured I should give it a try myself since I happen to have a lathe and figured I needed to justify having it take up space in my garage.
I started with a solid piece of 1-1/4 cold rolled 4140 and bored it out to the same size as the mossberg factory tube then I custom ground a piece of HSS for the square thread that it needed and threaded the end so it would screw on the mossberg. I then turned the main body down to 1″ OD and knurled the end. It turned out really nice except the knurling could have been a bit better but overall I spent about 3 hours and 5 bucks worth of metal. All that I need to do now is chuck it up on my rotary table and cut small grooves around it for the spring plunger to engage into and have my buddy at work parkerize it for me and it will be ready for duty.
Here is the finished product, turned out great. I actually ordered a gallon of manganese phosphate and parkerized the extension myself and it turned out perfect. The below pics show it in various stages of creation.
Before milling the 50 grooves:
After milling the 50 grooves: