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Old November 12, 2019, 11:20   #1
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Scorpion EVO, G36, ARX-160 and plastic trunnions, WTF!!??


I recently bought a Beretta ARX-100 (semi-auto ARX-160), the price was almost a giveaway. I did my normal read the destruction manual and learn to disassemble and reassemble a new gun.

Some unique choices and some obvious cost cutting measures but what killed me the most, like my CZ Scorpion EVO, is the barrel trunnion is plastic! Its simply a barrel held in place by thermoplastic and the bolt carrier slides in plastic.

Has anyone done any destructive testing while full auto to see if there is significant point of aim shift before complete failure?

I think about the G36 issues found while in combat service in high temp environments. I sure as hell wouldn't want to be that Soldier finding out rifle design flaws while bullets are incoming. That had to be a significant emotional event.

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Last edited by Thorack; November 12, 2019 at 12:35.
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Old November 13, 2019, 10:23   #2
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Brother Thorack: Thanks for the heads-up. My kid has a CZ Scorpion and shoots it all the time. I'll let him know to watch out for that trunnion. Are there any after market replacements available?

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Old November 13, 2019, 14:07   #3
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I found this:
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Old November 13, 2019, 17:05   #4
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A member here, Sand Pirate, was the head firearms instructor for the Border Patrol Academy when located in Charleston SC. He told me that the BP had transitioned to the UMP since the MP5 was being phased out. During the firearms training, each rookie was given several loaded UMP mags to shoot at their proficiency course. They shot semi, burst and full auto during this course. And because the BP had a limited number of UMP's but plenty of mags, the rookies just passed the now empty UMP's to the next guys in line. Sand Pirate and the other instructors noticed that as the day wore on, the UMP's started hitting lower on the targets with a couple actually bouncing rounds off the ground in front of the targets. What they found was the UMP had molded in trunions for the barrel. And as the barrel got hot, that thermoplastic started getting soft and the barrel began to droop down. They called HK to see if they had a fix for this and was told to take the UMP's out of service after a number of mags fired through it and place it in an air conditioned room to cool off.

Based upon that, and even though I would probably never do enough mag dumps to get the barrel hot, I will never purchase a weapon larger than a .22LR that has a plastic or thermoplastic barrel interface or trunion. Steel or aluminum (or aluminium is a Brit gun) is the only way I will go.
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Old November 13, 2019, 20:21   #5
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Thanks for the info on this...I have a Scorpion and enjoy it as a range toy. I never run it very hard. I guess I'll fall back on the Uzi for real heavy chores. But here is a video of rebarelling a trunnion. The trunnion seems to be metal:


Last edited by Bubacus; November 14, 2019 at 06:55.
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Old November 14, 2019, 09:03   #6
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The trunnion of the Scorpion is steel, however, there isn't much of it there to soak up heat, and it interfaces directly with the plastic receiver halves.

I would imagine under sustained fire you would see what was described with the UMP.

I know I'm not going to be doing that to mine, so am not worried about that. If I was going to be running an SMG hard use possibly in full auto; I would look to another design.

If I have to reach for a run to shoot until it melts...that's why I have the AK's.
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