Short version. If you have a problem with last loaded round from the magazine "stove pipe" feed jamming or completely exiting the pistol, you may need stronger magazine springs. Glock 43 magazine springs fit the R51 magazine. You may or may not need to clip a single coil from the bottom. I used the +15% springs purchased from Ghost Inc. It appears to have fixed this problem for me. All of my magazine followers already had the legs trimmed about 1/8", prior to this.

https://ghostinc.com/43-plus-15-magazine-spring-for-glock/

Posting this for others who may benefit.

My two R51s occasionally had last round "stove pipe" feeding failures, where a loaded round would end up with bullet out of the ejection port. Occasionally the bullet would fly out of the gun completely. I found this to be vexing.

I assumed it was soft magazine spring, but couldn't find report of an alternate. So I lived with it. A range trip 2 weeks ago produced this loaded last round stove pipe a few times, and once I  got home I started a-Googling for a solution again.  This time I got a hit to an old thread on this very forum, which said Glock 43 magazine springs from Ghost work in the R51 magazine, if you clip a coil.

So ordered a batch. I've tested moderately well, started with just a few mags with the G43 springs, and have had no "last round stove pipe feeding failures". I was able to reproduce the stove pipe with a few stock spring mags. One mag needed adjustment to G43 spring to load all 7 rounds.  I've now switched 9 magazines to G43 +15% springs and no stovepipe or other problems. Feeds a full mag of 7 rounds with 1 in chamber. A few times I tried the full G43 spring and it worked great, others I clipped one coil. One needed a little adjustment to bottom coil so 7 rds would load. It looks like a legit solution at this time.

I am happy.

5 stages, about 70 rds. It ran great except for 2 fail to feeds on first shot at 7+1. I think these happened with the same magazine and need to confirm that one and adjust the mag. .

I picked up a $235 shipped gray R51 from Gun Pro this month. Have been eyeing these pistols for several years and never did it because it served no role other than "I want it". This deal was too good plus I'd had a few bourbons and impulse got the better of me.

On arrival, one mag was good and the other tight with 7 rds, so I sanded the legs of the follower a bit and tried again for about 90 mins until there was enough rom for the 7th rd to depress when the slide was operating. Stripped and cleaned and oiled it, not much difficulty there. Trigger clean, about 5.75 lb on my trigger scale.

At the range, the 1st 4 mags shot perfect. Then some problems cropped up. Had the next round in mag come flying thru ejection port, that was a new one to me. A stove pipe. A fault to feed or 2. Fired about 80 rds mixed of reloads and Ranger T, had 4 fails. Took it home, stripped and cleaned it.

Next session I shot a variety of reloads and some 147 gr American Eagle. I also shot up about 30 different  case-gauge fail reloads I had accumulated over the years and they all fired and ejected. American Eagle ran fine. Fired about 100 rds and they all worked. The trigger measures 5.25 lb after cleaning and oiling again. Fired 7+1 many times, no problems. Seems to shoot low, but I need to put it on paper and not just plink at small steel at 50 ft. Have been short on time pst 2 range trips.

Feels good in hand. Looks cool. Have a threaded barrel coming and will stick my old Poseidon can on it and see how she does before messing with sights.