I thought I'd write a bit about the Remington R51, because a lot of friends really like the look of it (and the original Remington model 51 it was based on) when they brought it out a few years ago, and they were very disappointed when it proved so unreliable that Remington had to discontinue production and recall all the shipped pistols.
Recently, Remington brought out a revised version of the R51, and it has come back to mixed reviews; some stating it had fine reliability, and some that it had better than the first production run, but still poor reliability.
TFB-TV, the video production side of the Firearms Blog, ran a full test of the weapon, on video, with 600 rounds of 8 different kinds of ammo... and really, the quote speaks for itself:
"After 600 rounds of testing, the [second gen.] Remington R51 is looking like the kind of gun I'd want my enemy to have in a gunfight" -- TFB TVWatching the testing, I think I have a pretty good idea of what's wrong... and I thought I'd break it down here.
Basically, the only ammo it would run cleanly with, was the hot German 124gr NATO round nose FMJ stuff. It had just one total failure in about 200 rounds of that ammo.
That's very good ammo by the way, and my favorite factory load for breaking in difficult 9mm pistols, and for shooting through subguns and 9mm carbines. You used to be able to get it pretty cheap by the "battlepack", but I haven't seen any available lately.
The remainder of 8 ammo types tested were averaging one malfunction per every other mag of brass ammo, and the weapon wouldn't run any steel cased ammo at all, with one or more malfunctions per magazine.
Essentially all the malfunctions with brass cased ammo were nosedives, most of which the slide overrode. This normally suggests that the magazines are the primary issue... as is generally the case with MOST self loading firearm malfunctions.
Given the 124gr nato ran very well, and all of the other ammo did not... most of it was lighter, and if it wasn't lighter it was hollowpoint... It seems the weapon is extremely sensitive to cartridge OAL and nose profile.
The German 124gr nato ammo has a long ogive nose profile, making it a couple millimeters longer over all, than either a 124gr JHP, or lighter FMJ or JHP loads (its also longer than 124gr winchester white box, which has a shorter ogive and a rounder more spherical nose profile).
That NATO spec ammo is also hotter than most ammunition... in between an American +P and +P+... It will result in higher slide speeds and energy, even with the "Pedersen Hesitation Lock" blowback action, because it's a modified blowback system that locks the barrel and breech block together until pressure drops, but it still has much more energy hitting the slide earlier in the timing cycle, than a Browning or Walther style locked breech operation (and because it pressure dependent, it is much more sensitive to ammunition variability).
What this suggests to me (as an engineer, a gunsmith, and a shooter) is a combination of timing, and magazine issues.
Hesitation lock weapons are almost always going to run better with hotter ammunition, and may be unreliable with lighter loads. Also, cartridges that obturate differently, or that have different lubricity and stiction (such as steel cases vs. brass), may dramatically alter the timing of the weapon compared to brass test loads.
If you tune an action to run reliably with lighter pressure ammunition, with hotter ammunition it's more likely to unlock too early, and possibly malfunction, accelerate wear, or even have a catastrophic failure.
If you tune it to run reliably with hotter ammunition, though it will be less likely to have safety and durability issues, and will wear less; it may be unreliable with lower pressure loads.
It's a delicate balancing act that's difficult to manage even with all other factors being perfect. Adding steel cases which obturate in the chamber very differently, and which have dramatically different friction characteristics, just adds another set of issues to the tolerance stack.
That's the timing issue. But, while the other +P ammunition also ran better than the standard pressure, it still had problems... thus the magazine issues.
Its likely that the magazine lips and follower are presenting the cartridge too far back, and too low, at too shallow an angle, and with too much friction on the cartridge.
This combination would cause the feeding cartridge to tilt nose downward too far... without moving forward, or at least before it moves far enough forward... thus a cartridge with too short an OAL may fail to engage the feed ramp, and instead slam into the front of the magazine.
Since the slide is overriding the cartridges instead of jamming on the rims, it also means it's pushing the cartridge stack down far enough, with little enough effort, to continue its stroke.
The failures with steel cased ammo were mostly nosedives as well, but there were also FTI failures, from light primer strikes.
They had nosedive failures every few rounds with steel, which doesn't surprise me, because steel is going to have higher feeding friction, against the mag lips, mag follower, and the rounds under it. Thus the nosedive problem would be exacerbated.
The light primer strikes are lightly due to inadequate force exerted on the firing pin, rather than any more complicated issue.
Both of these issues indicate to me that they used springs that are either too light by spec, or too soft by QC, both for the internal hammer, and for the magazine.
These spring issues are common failures in quality control for any manufacturer, and common problems with any self loading pistol... and frankly, are not surprising given the state of Remingtons manufacturing and quality control issues over the past few years.
When the geometry and timing of a design are just barely on the edge of being reliable, a slightly soft spring can easily make the difference between a weapon that runs, and one that doesn't.
So... what do you do to fix it?
As a manufacturer or a gunsmith, there some relatively simple changes that might help.
First, I would try a slight revision in the mags and mag catches, to make the magazine sit just a little bit higher could improve feeding function; as could a slight easing of the feed lips, reducing friction on the feeding cartridge, and presenting it in a slightly more nose up attitude, allowing it to move further forward with less pressure; thus making it easier for the cartridge to engage the feed ramp and jump up under the extractor, rather than nosediving (particularly for shorter OAL or steel cased cartridges).
If one has an R51 that is overly load sensitive, and has similar issues, and changing to a stronger mag spring doesn't resolve them, then one could try easing the feed lips slightly themselves, as well as polishing them with emery cloth, to reduce feeding friction.
Other than that... There's not much you can do, except to internally blueprint the gun... polish metal mating surfaces, deburr anything that would be in a feed path etc...; and make sure that your springs are good.
Self loading firearms are all going to be sensitive to timing, ammunition variability, magazine geometry, quality control in springs and mags, and quality control in general; and pistols doubly so over rifles, since you've got much less energy and much shorter distances and time windows to deal with.
Honestly, it's pretty easy to make one unreliable, and pretty difficult to make one reliable.
Using a locked breech design helps reduce the variables, and make them less sensitive, which is why you don't see many full power centerfire pistols that don't have a locked breech mechanism.
Even the original Remington model 51 was only available in .32acp and .380acp. There's good reason why blowback pistols... even modified blowback pistols such as the R51... above .380acp in power are rare.
You're already cutting your tolerances close and stacking them high... when you add questionable quality control on top of it... well... the results are... predictably unpredictable shall we say?