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Thread: Mk VII 50AE Modern Barrel Issue...

  1. #21
    Join Date
    May 2018
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    Hmm... My new DE50SRMB has the same ~0.3984 measurement (as measured with $30 general purpose digital calipers). It ran flawlessly through 60 rounds (using 3 different mags) yesterday at the range, but this piston clearance issue still makes me nervous. I've only tried it with Underwood 300 gr JHP, which is on the warm side at a claimed 1580 fps.

  2. #22

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    I understand... With Hornady ammo, and a super clean/slightly lubricated slide rails, my gun with function with a standard piston, most of the time, but the case ejections are weak. The ammo toward the lower end of the pressure spectrum will not come close to functioning the slide. If I pop my original barrel and piston on there, the ejection distance is close to three times the distance with the American made barrels.
    With a few thousand rounds through all nine barrels at this point, it is clear that the Israeli barrels are superior relative to function. One consistency is the diameter of the gas cylinder. They are all exactly the same. The American barrels I have are all over the place in gas cylinder diameter, and their function reflects this. I will replace each of my American barrels with Israeli models as I find them because of this.

    As long as it is functioning, there is no concern in my opinion. If you buy some bulk American Quality or Armscor 50AE (.86/Rnd stuff of which I shoot thousands of rounds per year) you will quickly find out of your gas system is within spec. These ammos are at the fringe of what will function in the US Barrels I have, and will not function in my US .50AE barrels without my custom made piston(s).

    MrBlackCat

  3. #23
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    May 2018
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    Just curious, how are you measuring the gas cylinder diameter and how much variance are you seeing with the american made barrels? I'm not sure how well my calipers take measurements like this and I'm thinking about checking it with something else. By the way thanks for sharing all this information about your DE's and experiences with MRI / Kahr.

  4. #24

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    I am using a set of Gauge Pins which are two tens under (-0.0002) and I am using two different sets of bore gauges which I measure with two different sets of micrometers. Measuring in to the tens is not an absolute for many reasons, which is why I use second and third opinions with different tools/systems.
    Calipers are only hitting one area and aren't accurate below a thousandth or two, with disregard to roundness, so that is limited in some ways. Gauge pins check for roundness, but only go in whatever increment you have gauge pins in. Bore gauges are relative to the users precision. I do use clutched micrometers on the bore gauge to be as consist as I can. When I get under/within four-tens (+/- 0.0002), with two different sets of micrometers, I record that average as accurate.
    Also, you can measure the slack in a gauge pin (movement at its furthest point from insertion) and get a good idea of the relative clearance of a gas piston in a given gas cylinder.

    More later... got to run.

    MrBlackCat

  5. #25
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    May 2018
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    Wow that sounds quite thorough. I messed around with a wood dowel and might revise my estimate to something in the .393" to .395" range, assuming the gas cylinder is actually round, which it probably isn't. I might try some reloads this weekend and I'll be curious to see how the suggested starting load of 29.7 gr. of H-110 does.

  6. #26

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    Here is a quick clip of a small part of a spreadsheet I did to look at the gas system with math... this is messy and for my own research, and this is just an input section to show some dimensions I pulled a while back. This will show you the amount of clearance and what it could mean to volume. I used the sheet to calculate the fluid loss that would result from the extra clearance. Even in the volume produced by the impulse at 30,000PSI takes a huge hit in potential slide popping energy from the clearances shown here.

    Piston Dia 0.3910 Clearance
    Cylinder Dia. Clearance Over- Value Increase
    .50AE Original 0.3952 0.0042 0.0000 0.000%
    .44 Mag/US 0.3970 0.0060 0.0018 42.857%
    .50AE US 0.3985 0.0075 0.0033 78.571%
    Not sure if the format will be preserved enough to be clear... keep in mind the Clearance Increase is just that... not a measure of volume reduction. The volume reduction is fairly complex to calculate because of pressure over time with the factor being clearance volume. Pure volume loss isn't enough to accurately access this either, as this isn't a gas supplied from a mass volume like an air-compressor... because of the low volume from the high pressure impulse, pressure drop is not linear, but grows exponentially as the clearance increases. PI is not your friend with clearances like this under the conditions which the Desert Eagle works. They need the low clearance to really work well... the Israeli IMI/IWI seem to believe in this, as every Israeli barrel I have has virtually identical clearance in the gas cylinder.

    I don't reload, but I can tell you this I have learned from use of a chronograph... with the same weight projectile, sometimes the faster round will NOT cycle the slide, and the slower cartridge will. Must be rate of burn creating a pressure over time. I can't recite these from memory, but I save almost all of my chronograph data.

    MrBlackCat

  7. #27
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    May 2018
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    Very interesting, especially the part where you mention ammo with a lower muzzle velocity can function better than higher velocity ammo. I don't recall the exact wording but the comments in the reloading book I have seem to suggest the recommended powders have a slower burn rate than typical pistol loads.

    After seeing your gas cylinder diameters and after measuring the O.D. of my '5 in 1 tool' I have zero confidence in the numbers I've reported so far for my barrel. I'll wait for a more reliable measurement before throwing out another diameter estimate.

  8. #28
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    May 2018
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    After meticulously narrowing down a brass feeler gauge to find the width that matches the diameter of the gas cylinder I'm calling it 0.397". More important than my measured diameter is the fact that the gun seems to work fine with medium power reloads. It didn't have trouble picking up the next round until muzzle velocities (300 gr.) dropped below ~1125 FPS and by that point the gun had 25 rounds through it and was a little dirty. After that I went to a slight hotter batch and shot 10 more rounds at 1175 - 1250 FPS with no issues. Well, my 3-4 foot groups at 100 yds are an issue but I can't blame the gun for that.

  9. #29

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    That sounds about right for a .397 in my experience. In my .44 magnum barrel with about a .397, I often demonstrate how it won't function reliably at all with a specific target load (Precision One 210gr standard, NOT XTP) using the standard size gas piston. Then I swap the piston for the one I made to adjust the cylinder to piston clearance down to Israeli spec... and it functions absolutely reliable. Note that Precision One makes a Desert Eagle specific 44 magnum round guaranteed to cycle properly in the Desert Eagle... this is not the round I am shooting... I am shooting their target ammo in .44

    If I can group 3 ft at 100 yards, I am good. You can always just blame the ammo like they do in videos on the internet... "yeah, this ammunition isn't all that accurate beyond 15 yards..." I have yet to find a gun that shot SO bad I could actually say it wasn't accurate... but that is just me... and some days I can be pretty bad.

    MrBlackCat

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