In fact it's not a modification at all, but a return to the original configuration as designed by John Moses Browning; and changed at the behest of the Army Ordnance Board.
So, what is one and why do you use it?
In a word, timing.
These are two versions of the firing pin stop; the 1911a1 type with a heavily radiused bottom edge, and the earlier squared type, with a lightly relieved bottom edge (pics blatantly stolen from Firehand):
I put an explanation of what the squared bottom does, and why it works in the comments there on "Irons in the Fire"; but the question has come up again, so I'm going to expand and refine the answer here.
Essentially, the squared stop alters the lockup timing and lockup strength of the gun fractionally; by increasing the camming angle of the pin stop (and therefore the slide), against the hammer. The slide has less mechanical advantage against the hammer, so the mainspring takes more of the slides force to compress.
Primarily, this causes the barrel to stay locked up just a bit longer, which has several beneficial effects.
First, there is a slight, but measurable increase in mechanical precision, as longer lockup and slower but crisper unlock (there is stronger resistance to unlocking until the bevel cams over , and then the resistance is suddenly let off; vs the smooth progressive roll with the “conventional” pin stop ) means the follow-through of a shot will involve less struggling against the inertia of the slides motion.
Because the slide is delayed in it’s motion a bit; the pressure in the chamber when the slide does move will be lower. This also slows down the peak slide speed somewhat; and slows and alters the felt recoil of the weapon slightly; again allowing for better follow-through on the shot.
Remember, the total recoil impulse remains the same, it’s just that the delivered energy distribution is altered. The initial character of the recoil will be less effected by the mechanical motion of the slide (as that motion is resisted marginally more), and more effected by the energy of firing.
This slower sliding action also reduces the inertial tendency to flip, because although again the same total force is there, it is spread out over time, and thus is lesser in peak magnitude (technically a reduced "magnitude of impulse").
The sum of these effects is to have a somewhat “straighter” recoil push; meaning the recoil pushes closer to straight back into the shooting hand rather than a rotating upward.
Illustrating what I mean a bit better (in terms of reducing the peak impulse, and spreading the force over time); imagine a graph showing the distribution of recoil energy from a shot:
With the conventional firing pin stop you’ll see a large steep spike with a sharp peak, and a very rapid, curved falloff… kind of like looking at a childs playground slide from the side. This is because the distribution of the impulse from the firing, and the unlocking and recoil of the slide, are very close to simultaneous (and thus added together), and fall off in a steep smooth curve.
With the squared stop, you’re going to see more of a stepped shape in the initial energy distribution. There will be a medium sized steep ramp from the initial explosion, which will plateau or fall off very slightly, then after a short time another steep spike; but the peak force will be lower than the steep spike of the shot from the A1 pin stop, and the peak will be more rounded and longer, with a steeper falloff. More like looking at a mesa in the southwestern desert; or a ski slope with some moguls, and a cliff at the back side.
The total energy being distributed will be the same (the area under the curves will match), but the PEAK force for the squared stop will be lower, and the force distribution will be longer.
Now, there are lots of side effects that occur because of this change in the character of energy distribution.
Remember, all of these effects are VERY slight; I would wager that in most guns shooting most loads, they wouldn’t be noticeable to the shooter in a double blind test; but in precision auto pistols, there are a lot of ways to induce imprecision, so every little bit helps.
It seems unlikely that changing the pin stop would increase the velocity of a shot very much if at all; because in a properly timed gun the bullet should have left the barrel, and all the propulsion that’s going to happen should already have happened by the time the gun unlocks; therefore the unlocking isn’t robbing any energy from the shot…
...unfortunately not all guns are properly timed.
If you have very inconsistent shot to shot velocities from known good ammo, then your gun is probably mis-timed. If you are ejecting all over the place as well, you’ve probably confirmed it. There are other problems that could be occurring, but in an otherwise properly functioning pistol, timing is the most likely culprit.
Upping the weight of your recoil spring, and installing a squared firing pin stop could help with fine tuning the timing of the gun quite a bit. However they are no substitute for checking your locking lugs for proper fit and angle, and making sure you have the right length link.
One major effect I WOULD expect, even in properly timed guns, is a significant reduction in wear of the locking lugs. I would expect that the change in timing radically decreases the pressure on the edge of the lugs as they unlock; which would reduce the tendency to round or crack the edges of the lugs (sharp corners are good for lockup, but corners are natural stress risers; reducing shear force on natural stress risers is a GOOD thing) . This would also have a similar; though lesser in magnitude, effect on the barrel link and link pin.
Another significant effect; the squared stop would also ease extraction slightly (and thus reduce extractor and brass wear), as obturation (the pressure of the expanded case walls against the chamber walls) would have subsided more in the chamber at the time of unlock.
Oh and as slide velocity is somewhat decreased, and made more consistent; I would expect it to shorten your ejection distances, and make ejection patterns more consistent....
...though actually I’m not 100% on that one. Though peak slide velocity will be reduced; I think average slide velocity over the full stroke may be increased; because of the change in the character of impulse distribution… It will certainly be more consistent, but the slide may actually be traveling slightly faster when the round strikes the ejector than it would be with the rounded stop (because with the rounded stop all the impulse occurs at the “front” of the stroke, and by the time the casing hits the ejector the slide will have slowed somewhat from the peak). This would cause ejection distances to increase somewhat, but they would be closer together.... honestly I’m just not sure, I’d have to run some simulations or do a highspeed camera test.
Have I mentioned that the timing of auto pistols; especially short recoil or delayed blowback auto pistols; is a complicated business? It makes timing an AR gas system seem like childs play in comparison… of course you also don’t expect a 1911 to make 1/2” groups at 200 yards…
I should also note, that ALL of these factors are part of the several reasons why I recommend using the heaviest recoil spring that will reliably cycle with your chosen ammunition. Though the recoil spring doesn’t effect the unlock timing of the gun in relation to other parts (the Link length and lug depth are the primary determinants there), it does reduce overall slide speed which slightly effects the overall timing of the gun (along with the pin stop); and has other benefits.
In my custom commander, I have a double captured recoil spring setup, with a 24# main spring, and an 8# secondary spring. The gun is rated for .45 super, but cycles well with all but the weakest white box loads (which sometimes fail to completely eject).
Of course, my wife can barely cycle the slide on the gun, which is a bit of an issue.
Which actually brings up why the Army Ordnance Board changed the pin stop in the first place. It was felt that soldiers with less hand strength were having problems actuating the slide with the original stop; so Browning came up with a way to reduce the force required.
Browning also reduced the strength of the original recoil spring all the way down to 12lbs in testing. ON the original 1911 prototypes he used an 18lb spring; then went to a 14lb spring (which the 1911 was later restored to after the 12lb experiments didn’t work out); and in my opinion was a mistake, he should have stayed with the 18lb spring. The 14lb spring is just fine if you’re only shooting 230gr hardball at 800fps, but doesn’t work out so well for 200gr XTP hollowpoints at 1100fps.
Honestly, the only negative of the squared pin stop is the increase in force required to cycle the slide... at least presuming the stop is beveled or radiused properly. It must be absolutely even, or you can induce side loading to the hammer and hammer pin, which can cause either or both to break prematurely.
This is also why I don't generally recommend people fith them on their own, unless they are competent enough in metal work to ensure that the bevel or radius is cut properly.
I personally recommend a squared stop for anyone who's going to be firing a lot of hot loads; or for 10mm, .45 super, .38 super, 9x21 and 9x23, .460 rowland, and other high pressure chamberings of the 1911.
Actually, I'd say it's good for anyone; unless you have a problem cycling your 1911 with a normal strength recoil spring already.
