Five-seveN Durability Load Rating System

Discussion in '5.7 X 28mm Club' started by Scott60, Mar 4, 2020.

  1. Scott60

    Scott60

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    For those who only shoot FN based factory ammo such as SS195/197/198 and AE5728A this post may not be of interest. For those who hand load, or choose certain types of "performance loads" from various sources, this may be worth a read.

    My first Five-seveN was bought new in 2017 and I've probably shot less than two boxes of factory ammo through it. Everything else has been "uploads" either new ammo I broke down and reloaded to a higher performance level, or "reloaded" ammo from once-fired brass.

    My hand loads have been oriented toward higher velocity and energy and clearly the pistol can handle considerably higher loads than factory, which has always begged the question of why don't the ammo makers provide it? When Speer announced their new offering, testing showed higher velocity than existing main-stream brands - close to the mid-1,700 fps range, but still 200-300 fps lower than can be achieved with a perfectly "safe" hand load....

    I've loaded well over 2,000 rounds of 5.7x28, every one of which has been well above factory. 40 grain bullets over 2,050 fps, 35 grain bullets to 2,200 fps, re-manufactured SS198LF to 2,300 fps, and the latest, 45 grain TSX to 1,900 fps! These are STOUT loads to say the least, and in working them, up I've tried different primers to deal with extrusion and piercing. Along the way I started noticing the Stripping lever (take down lever) seemed to rotate more than when new. The gun works just fine, though I have also noted a small amount of deformation on the Locking cam's slide lugs - they lock into recesses under the slide to firmly "lock" the slide and barrel together during the high pressure phase of firing. The slide recesses look "new" and one would expect the less expensive Locking cam to be intentionally designed to wear first since it's a lot cheaper to replace than the slide. Surely, much of the deformed area - which is quite small, it due to normal breaking in of parts and at this point there's no way to tell if "hot" loads had much affect - directly anyway.

    So the other day I'm out shooting some top loads and noticed the slide wouldn't close when releasing it to chamber a round. This was "new" and I thought maybe I'd finally crossed the line and had the front of a case stuck in the chamber. Turned out to be the Stripping lever. It had finally given up the cause and broken, causing it to rotate completely horizontal and immovable. We take that little lever for granted without ever once considering that it's the key to getting the slide off! With the lever rotated I couldn't even look inside the small oval opening...using a narrow blade tool I managed to get the polymer panel rotated enough so I could see inside, but out in the field was not the place to try and sort out the problem.

    Later, at home, with strong lighting, I first tried to rotate the "button" of the Stripping lever so I could see inside, and eventually twisted it completely off, after which I was able to visualize the Locking cam through the small oval window and use a 1/4" diameter extension to "retract" the cam as I pulled back on the slide, thus pulling if off the frame. The Stripping lever was little more than a few fragments of pulverized polymer. Other than that, everything was fine. Using plenty of light I examined the polymer frame for any signs of cracks and found none. The Locking cam looked fine, so I ordered a new Stripping lever - a $5 part that cost another $6 shipping. When it arrived, I had my first chance to see exactly how the piece is shaped and how it does the job it does. The back side is flat to mate up with the inside of the frame, and on the inside, it has a small ledge with a recessed part, and a long, slender "tail" that extends back underneath the upper frame pin - the frame pin that applies tension to the torsion spring that resists the barrel's rearward movement, and serves as the anchor for the barrel lug. To install the new Stripping lever required removal of all locking components which turned out to be easy. The two frame pins tap out to the right. Getting the new part in behind the Locking cam proved a bit fiddly because the new part was still crisp with no rounding of edges. The way the Stripping lever works is that when forward, the Locking cam is prevented from rotating down beyond a certain point - probably because when the cam is rotated so the center is flat, that's when the barrel can come forward and off! It's this plastic "ledge" that absorbs the impact of the Locking cam with every shot. With mild loads, the ledge probably receives nothing more than a love tap, but with hot loads, the Locking cam is retracting hard and fast and will deform and eventually destroy the Stripping lever's ledge. When the Stripping lever is retracted to removed the slide/barrel unit, this ledge is pulled back, leaving a recess into which the Locking cam can move fully down so the barrel lug can move over the flat center section and lifted off.

    During firing, the barrel is held forward by bore friction as the bullet is traveling through it. When the bullet is gone, energy imparted to the barrel and slide cause the barrel to push back. The slide is still connected or "locked" to the barrel via the Locking cam's twin lugs that index into slide recesses. As the cam rotates far enough down, the lugs drop out of the slide recesses and the slide is free to move all the way back, while the barrel is decelerated by the torsion spring. With standard loads, there is very little strain on these parts, but with hot loads, the barrel lug drives farther back and can impact the polymer frame wall once the Stripping lever has been battered into submission. Seeing the damage to my gun - slight as it was I realized the real limiting factor to 5.7 ammo isn't the chamber pressure, it's the locking system. (cont)
    FsN Cam lock raised.jpg FsN lock cam raised.jpg FsN lock cam retracted.jpg FsN slide notches.jpg
     
    Last edited: Apr 30, 2020
  2. Scott60

    Scott60

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    That's when I started running some numbers - like comparing breech thrust from the original 5.7 loads to current commercial ammo. The original SS190 fired at 32 grain bullet around 2,000 fps. SS198LR fires a 28 grain bullet at a nominal 2,200 fps, though most record lower speeds. SS195LF is even slower - only about 2,050 fps with a 27 grain bullet. But then comes the heavyweights - SS197SR 40 grain an AE5728A 40 grain bullets going much slower - only about 1,625-1,650 fps. Though much slower, the heavy bullet loads produced similar breech thrust to the original spec light bullets. Even today, one can push 30 grain bullets to 2,200 fps without adversely affecting the locking system because that's its design parameter. But when hand loaders such as myself push 40 grain bullets up to 400 fps faster, breech thrust goes WAY up!

    So we start with a nominal "65,000" which is a general approximation of both current 40 grain loads and light bullets. SS198 creates around 61,600, versus SS197 at 65,000, but to hold breech thrust down velocity is also down. At 2,000 fps a 40 grain bullet will generate 80,000 which is a whopping 23% increase! Only because the scale of the 5.7 is so small do we tend to think of such overloading as minor. How many people would start out with a .45 auto loaded to 840 fps with 230 gr. bullet, then bump it up 23%? That's basically .45 Super territory and in an unmodified pistol going to cause functional problems and parts breakage in short order. One of my loads - 45 grain TST at 1,900 fps was a 32% "overload" which is the difference between firing a 9mm 124 gr. bullet at 1,200 fps, then bumping it to 1,584 fps!

    That's when I decided to come up with a personal rating system to alert me to just how "hot" a given load is. Writing down velocity and bullet weight gives performance data but only obliquely addresses the strain on the Five-seveN locking system. By using a "+P" system that goes up by "1" with each 5% increase makes it obvious when a load is either safe, or placing additional strain, should be limited use, or use only for emergencies!
    So if we use 65,000 as the starting point, the next 5% increase is a "+P load. Five percent over that (10%) is a "+2P" load. A load generating 15% greater breech thrust is a "+3P" load.

    Note that his is not at all like or intended to be like the "+P" rating system used for handgun ammo. This rating has nothing to do with chamber pressures, as it only serves to alert the shooter to how much more energy is imparted to the slide. It's also a useful tool in determining a "safe and sane" load. As part of planning a new load, I can multiply the bullet weight by velocity, then divide by 65,000 to quickly see if my planned load is hotter than "standard" as far as the locking system is concerned, and by applying a numeric rating for every 5% increase, quantify how hot the load is. For example, I may choose to load 40 grain bullets to +2P standard or around 1,787 fps which is certainly an increase over factory, and putting more strain on the locking system, but nowhere nearly as hot as a 40 grain bullet going 2,074 fps - +5P!

    When I was working up the 45 gr. TSX load I wasn't thinking about the locking system, only what the chamber could take. So if the round can reach 1,900 fps why not load to that? Well, turns out the reason is because the locking system is taking a SEVERE beating at that 32% overload! SEEING that it's a "+6P" load is very convincing that its far beyond safe if I want the Five-seveN to last more than a few dozen rounds! So now I'm using breech face thrust as part of all load development - for the Five-seveN that is. A +2P load is likely safe for continuous use, whereas anything above that should be limited. Also, if bullet weight is between 27 - 32 grains, loads can easily be pushed to 2,200+ without overloading the locking system. But heavier bullets quickly ramp up breech thrust - even 35 grain bullets can only reach 1,857 fps if staying within the standard locking system stress range, though a mere 5% or +P overload gets it to 1,950 fps.
    FsN lock comparison.jpg FsN_blk_lock cu.jpg Fsn_FDE_lock cu.jpg FsN slide recess comparison.jpg FsN slide recess comparo from frt.jpg
     
    Last edited: Apr 30, 2020
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