Remington R51: A look at the Pedersen Action

As noted earlier, the R51 utilizes the Pedersen delayed lock impeded-blowback system. There are several things that shooters familiar with the Browning tilting barrel action found in the 1911, Browning High-Power  and many other semi-auto pistols will find very different, even disconcerting.

In most locked breech firearms, when the arm is in battery, the breech is locked. With the Pedersen action, when the breech is locked, the firearm is out of battery. When in battery, the breech is unlocked. It works as follows:

• When the gun is fired, chamber pressure drives the case into the breech block which, along with the slide, is driven backwards until the breech block is stopped by the stop in the frame. Note that when the gun is fired, the breech is not locked even though the gun is in battery.
• The breech block stops and locks the breech while the slide continues to the rear, This delayed lock is the key to the impeded blowback system. The case is partially out of the chamber about .10 of an inch) so the chamber remains sealed unto the bullet leaves the barrel and pressure drops.
• After pressure drops. A lug on the slide cams the front of the bolt down lifting the rear over the block in the frame allowing the slide and bolt to continue together as the case is extracted and ejected.

That's the way it works in theory, in practice, sometimes not so much. There can be problems and the R51 has some.

Headspace/Firing Out of Battery (FOOB)/Fail to Return To Battery (FRTB)

I've seen reports and several videos of failure to return to battery and firing out of battery. Before going to the range, I looked into this.

Most FRTBs were reported on the first round from the mag. I loaded up a mag of dummy rounds I made up using MBC 9Cone and the only way the first round created a FRTB was if I rode the slide down. Slingshoting (pulling the slide all the way back and letting it drop) chambered the rounds every time. So, I decided to test further with other rounds. I had some other dummies made up for plunk testing my High-Power. These would not fully chamber. In fact, the bullet was jammed on the lands with the slide clearly out of battery. So badly jammed that I had to put the rear sight on the end of a piece of 2x4 and lean on it to eject the round. It ejected the case leaving the bullet stuck in the barrel and I had to tap it out with a rod.

I then plunked an empty case, a Federal 115g FMJ, and a Remington 147g Golden Sabre and an older dummy with a different lead cone bullet. and took measurements.

Except for  the initial dummies which I had intentionally seated deep and the empty case, all failed to fully chamber, and the bullets had clearly engraved on the lands. The number below reflect the among of case exposed when chambered and the amount in excess of the empty case.

    Empty Case:                0.120 (0.000) (COL: 0.754)
    9Cone Dummy #1:      0.120 (0.000) (COL: 1.060)
    9COne Dummy #2:     0.186 (0.066) (COL: 1.149)
    Lead Cone Dummy:    0.150 (0.030)
    Federal AE FMJ:         0.141 (0.021) (COL: 1.151)
    Rem Golden Sabre      0.124 (0.004) (COL: 1.148)
    Win SilverTip              0.120 (0.000)
    Rem UMC:                  0.120 (0.000) (COL: 1.105
    Blazer FMJ:                 0.141 (0.021) (COL: 1.151)

Having the bullets deeply engraved in the lands is going to raise pressures. and if the gun is out of battery as well, it is not a good situation. Before I go to the range, I may load up some minimum loads of varying OALs.

Obviously I am going to have to seat deeper when reloading for the R51 than I have been doing for my HiPower. And I think the bullet diameter and shape and OAL of factory loads may be critical. In one of the vids, from either RyeonHam or Tactical Existence, he had FOOBs and FRTBs with Cor-Bon and Speer Gold Dots but no problems with Remington FMJ. Somehow, I have the feeling that Remington's new Ultimate Defense will work as well.

I slugged the barrel (since I already had bullet stuck in it) with the following results:

    Slug diameter:     0.3565
    Lands:                  0.3460
    Grooves:              0.3560

Diameter of bullets tested:

    9 Cone:             0.3565
    FedFMJ:           0.3550
    Rem GS:           0.3470
    Win ST:            0.3510
    REMUMC:      0.3510
    Blazer :             0.3550
   
The Rem Golden Sabre, Remington-UMC FMJ and Winchester Silvertip are apparently close enough to the land diameter that they do not jam on the lands before fully chambering. The lead 9Cone and the Federal AE FMJ are closer to the groove diameter so they do jam on the lands.

I have seen reports online of Cor-Bon and Speer Gold Dot failing to fully chamber, and Remington FMJ chambering properly. I have seen no reports of bulged cases or bulging primers confirmed to be associated with fully chambered rounds.

Standard Case length of a 9mm is .0754". The rim and rebate (extractor groove) is 0.117" That leaves 0.637" for the body of the case. I measured the chamber depth at 0.635 +/- 0.0005" If it is short, is by 0.0015" and my measurement could easily be off that much.

Since this is a delayed-lock blowback system, the case is moving out of the chamber while still under pressure, but because of the delay, it doesn't move far enough to expose the brass walls before the pressure drops, if the round is fully chambered and the slide is in battery when fired.

The chamber design leaves the case head unsupported, but it is solid brass so it really doesn't matter. There is a small area of the body unsupported at the feed ramp. Very similar to a .40S&W Glock. This probably isn't a problem unless the gun fires out of battery which it can do of the round isn't fully chambered, Then the case might be bulged at that point.

The chamber is reamed very closely to spec. Cases fire formed to my BHP will not chamber at all in the R51. As noted, there is no leade or "throat" so where the chamber ends the rifling begins. A cartridge is going to headspace on either the chamber shoulder or the lands. The chamber walls are rough, almost a matte look, and feel like 1000 grit emery. When a case is fired, the walls tend to grip it tightly and when pressure drops, there is very little relaxation so the case is still held fairly tightly.

The angle of the breech block is 4 degrees off of the horizontal bore axis. The breech block face lugs (that contact the barrel) form an 86 degree angle with the top of the breech block, as does the breech block face itself. So the case head and breech block face are perpendicular to the bore and parallel to each other. But their is at least 0..08"-0.10" between the extractor claw and the breech block face (twice the 0.050" rim thickness) which allows the case to feed, but also allows the firing pin to push the case and primer away from the breech block face at ignition. That probably contributes to the primer anomalies.

Factor in the rough chamber walls that tend to grip the case, and it is easy to see what happens when a cartridge is fired. Lets look at the action in a little more detail, focusing on what is happening in the chamber.

When the slide is in battery with a round chambered, the head of the case is under the extractor which holds it loosely against the breech block face. The lug of the breech block face is flush against the barrel, and he breech block is below a recess in the slide with its rear face against a lug in the slide. There is about 0.10" of space under the extractor for a 0.05" rim.

1. When the trigger is pulled, the hammer falls and strikes the firing pin.
2. Because of the space under the extractor, the firing pin pushes the breech block away from the case creating a slight gap (0.050") between the case head and the breech block face while the primer is still in contact with the firing pin.
3. Primer ignition presses the primer against the firing pin and primer deformation begins.
4. Powder ignition expands the case walls, and pressure drives the bullet forward and with the case firmly held by the rough walls, drives the primer backward. (If the case is not fully chambered due to a bullet headspacing off the lands, or failure of the operator to slingshot the slide, the unsupported portion of the case wall exposed above the feed ramp will likely bulge at this point)
5. The primer swells, may be pushed out of the pocket until it contacts the breech block face where it stops and transfers its motion to the breech block and slide which start to move rearward. If the firing pin is still in contact with the primer, the primer may flow around the pin at this point and into the FP hole in the breech block.
6. Pressure should have reached the point that the case is being driven rearward, but if the walls are too right and the chamber too tight, and the pressure curve too sharp, the case may separate at this point. Either way, the case head strikes the breech block face and drives it against the breech block stop on the frame. This stops the breech block and lock the breech while allowing the slide to continue unabated. If the gun was in batter y when fired, the breech has been locked and sealed with the case walls fully supported (except for the small area above the feed ramp) until this point.
7. About this time, the bullet leaves the barrel, pressures drop and lug on the top of the slide contacts  a lug on the top front of the breech block and cams it down, which cams the rear of the breech block up off of the frame. The lug on top of the breech block fits into a recess in the top of the slide which allows the slide to pull the breech block backward as it continues its momentum while the extractor extracts the case until it hits the ejector and is ejected.

The primer is going to absorb a lot of pressure in this system, and harder primers will almost certainly hold up better than thin soft ones. I think I will avoid Federal primers when reloading, but still, there have been lots of reports of primer deformation, but none (that I have seen) of primer perforation. Fortunately, primers are a one use item and whatever abuse they absorbed they take with them. But, a redesign of the firing pin might help here. The pin is tapered so the hole in the breech face may be larger than it needs to be.

Another result of locking the breech out of battery, is that the case may fail to extract. If you take an unloaded R51 and put a dowel down the barrel and push on the breech, you can push it into lock up. But you can move it no further no matter how hard you push. It just locks tighter. This is because the movement of the slide against the lug on top of the block is what cams it out of lock. And in the proper firing sequence, it is the short movement of the block from battery to lock that gives the slide the necessary momentum to then cam the block out of lock. Start out of battery and locked, and there is no momentum imparted to either the block or the slide, so there is the slide does not even cycle.

This action functions very differently from the 1911 and appears to break many of the rules of safe function. The initial reaction of many is that the design is flawed and unsafe. In fact, it is neither.

Next post: The Range