Ok, I just got finished installing a picatinny rail to the bottom of my HMR, it's pretty easy and it took me about 20 minutes to get everything set up and screwed down. No special tools are really needed, I got everything for this little project from Lowes, except for the rail that is., This is what I used
10-32 stainless steel button head cap screw (2)
10-32 set screw
10-32 tap set
I used 10-32 screws because that's the same thread pitch as the factory sling swivel studs and a 10-32 screw will fit the slot of an arca plate with very little slop if I want to mount a long plate to the bottom in the future. I ended up using an extra BCM polymer rail section I had in my parts bin because it would allow me to use the same tripod mount I across four rifles and the sloped ends would be relatively comfortable shooting off hand or with a sling.
I did all of the locating and drilling from inside the barrel channel going out since you can easily see the location of the skeleton and accurately mark center. I started just forward of the recoil lug pocket and this put my rail section almost exactly in between the front of the mag well and rear sling swivel stud.
It was pretty easy going drilling through the shell and aluminum skeleton, no major issues there. I tapped from the inside going out but didn't realize that in the rear most hole the tap wasn't long enough to fully thread the hole. I didn't figure this out until I went to thread the rear screw in and it wouldn't go. Not sure if would've been a good idea to run the tap from the other way I used the 10-32 set screw to chase the threads all the way through and it seemed to solve my issue. Hindsight being 20/20 I would've threaded from the outside towards the barrel channel. When you do go to tap the hole, use plenty of oil and go slow.
Fit and finish look good and if I want I can swap the rail out for an arca rail. Easy peasy.