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Posted: 6/19/2026 1:52:10 PM EDT
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Maxim Defense has a clever 9mm "Roller Delayed" buffer conversion for AR9s that are direct blowback. It seems rather novel and simplistic conversion. I tried one and it actually is noticeable improvement. Not that blowback 9mm carbines are bad, just improved. In any case I got to thinking People Generally complain about the UMP recoil being very abrupt, particularly in 45. I got to thinking "could such a roller delay system function as the recoil spring system in a UMP?" |
| I tried the Maxim RDB buffer in the 9mm DD PCC, and had to take it out. It was far too hard to pull back the charging handle. Aside from requiring more power to manually pull the bolt back, the big problem is that the diameter of the recoil spring in the UMP is far smaller than the AR buffer size Maxim RDB buffer. I think it would be difficult to make a similar system small enough to work in the smaller diameter UMP recoil system. A better option might be the Nexus Evo type system with spring loaded rollers in the receiver just behind the bolt, but that would be problematic in the UMP polymer receiver. You’d probably need to make an aluminum receiver like Nexus did. But then, I personally don’t find the UMP recoil a problem. |
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I think a barrel with an extension like an MG42 could work. It would drop in from the rear and be pinned in an unmodified UMP receiver, and it would have the cuts for the rollers in the barrel extension. The bolt would have to be completely different (more like an MP5, but squared up). |
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