UMP conversion area of concern

[U.S. Praetorian]

I noticed when cleaning my 9mm conversion that the area in the yellow rectangles on the rear of the the bolt and on the extractor angle .......



...are striking the corresponding areas in the yellow rectangles here....



The softer aluminium has yeilded to the steel and I have since relieved the areas on the receiver block so there is no contact. Has anyone else noticed this on or in any caliber?? (9mm-.45-.40)

[S5TIIOwner]

Only the extractor angle on mine. Mine is a 45 sbr running a USC bolt if it matters.

[fordemti]

Hmm. Here's some pics of mine after 35 rounds.

[Pinion327]

I've noticed the same thing as Praetorian after using my 9mm conversion.

[U.S. Praetorian]

So the next question is: Is the 9mm bolt somehow different dimensions in the rear and is this going to be a problem because it is impacting here instead of just on the buffer. If left unchecked something will fail.

[S5TIIOwner]

I doubt it will cause any issues.

[Lohe]

So with the aluminum out of it's way are you noticing any further wear marks on the stock block?
Because if all it's doing is taking a small piece of the aluminum off and then impacting as usual on the
buffer, there shouldn't be any further problems.

Which bolt are you guys who are noticing this using?

[kabob]

I don't remember my .45 USC bolt carrier looking like that..? I'll take pics when I get home. My main area of concern with my conversion has been the recoil rod nestling against the stock block B metal to metal. It's causing some damage to the stock block's softer aluminum similar to the problem you're having with those metal tabs. Anybody got a solution? I'll take pics when I get home.

[Oreaopla]

So with the aluminum out of it's way are you noticing any further wear marks on the stock block?
Because if all it's doing is taking a small piece of the aluminum off and then impacting as usual on the
buffer, there shouldn't be any further problems.

Which bolt are you guys who are noticing this using?

The photo suggests he's using the Hkparts michaels machines version. I have a factory bolt group in 9mm and 45 but I won't be able to check if I get wear until my conversion comes back from Chop. I imagine it'll stop wearing the stock block down if material is removed by dremel or shooting. The bigger worry would be what kabob suggests but isn't the original lower backplate in factory UMPs made of polymer with the buffer attached? Does that part wear out where the recoil spring sits?

[U.S. Praetorian]

The photo suggests he's using the Hkparts michaels machines version. I have a factory bolt group in 9mm and 45 but I won't be able to check if I get wear until my conversion comes back from Chop. I imagine it'll stop wearing the stock block down if material is removed by dremel or shooting. The bigger worry would be what kabob suggests but isn't the original lower backplate in factory UMPs made of polymer with the buffer attached? Does that part wear out where the recoil spring sits?

It is a MM bolt and I have not shot it since I relieved the areas. I have re-blackened the relieved areas and will re-examine when I shoot it again. It appears compression of the buffer when impacted during recoil is enough that it allows contact in these spots. I also have 2 .45's and have not had a chance to examine those. They are USC bolts and hope it is not doing like. The 9mm has had around 800 rounds through it. I have not seen any wallowing or wearing to the female divot that accepts the recoil rod cone in any of the 3 guns.