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Discussion Starter · #1 ·
Ok, I know this is an unusual query, but I took some overall measurements on an HK MP5 locking piece and compared
them to the same measurements on an MP5/40 locking piece and they appear to be the same height, width and thickness.

Has anyone modified an MP5 locking piece to be an MP5/40 locking piece? I was thinking about using an SD locking piece
and moding it to be and MP5/40 one. I already looked at the RCM web store the MP5/40 SD locking piece they made is out of stock.

The 40 locking pieces have some material removed at the edges of what I would call the underneath side and also the
underneath side has a strip of material removed running down the side. Click on the links below I borrowed from the HKParts.net web site
to see what I have tried to describe.
Thanks

https://www.hkparts.net/shop/pc/catalog/lockingpiece24-2.jpg


https://www.hkparts.net/shop/pc/catalog/mp5standard-4_1834_detail.jpg
 

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The only one that I'm aware of who is known for anything remotely like this is Ralph (RDTS) who modifies standard 94 locking pieces for his SD builds. Since he also makes a 40 cal SD build, you might check with him what he uses.
 

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I'd also drop a line to Mike at RCM and ask when he is planning a run of .40 SD locking pieces. I could be wrong, but I thought the final step to making a locking piece is heat treating to harden the surface. So if you machined the surface wouldn't that remove the hardened surface?

Scott
 

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+ scott,

yes, the surface is hardened, depending on how deeply the original was hardened, and how much you grind off, will determine if re-hardening is needed.

this is most likely not the sort of job a garage gunsmith and dremmel tool are going to succeed at doing.

OP if you need a interim use locking piece a 10mm low might interchange for a 40, depending on the power level of the 40 S&W ammo.
 

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I'd also drop a line to Mike at RCM and ask when he is planning a run of .40 SD locking pieces. I could be wrong, but I thought the final step to making a locking piece is heat treating to harden the surface. So if you machined the surface wouldn't that remove the hardened surface?

Scott
I'm no technical guy, but I also think it depends where the metal is removed and whether that area was specifically hardened. In other words, I thought a piece isn't necessarily hardened uniformly, but hardened in areas that are required because of contact. Or, I could be wrong.
 

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OP if you need a interim use locking piece a 10mm low might interchange for a 40, depending on the power level of the 40 S&W ammo.
To my knowledge that is not correct. The "#24 LOW" is the current standard LP of both the full size MP5-40 and MP5-10. The #26 is out of production. Like putting a 100 degree LP in a 9mm SD, a #24 in a .40 SD I would think would cause over function. I'm sure "Bronco75" could enlighten us on LP hardening. If it were my .40 SD project, I would get the right LP. If it is a RCM .40 SD barrel, then I would get the LP from RCM. Yes, Ralph does make a .40 SD. Does he use a different port configuration in .40 S&W like that he uses in 9mm? Does Ralph's 9mm SD use a custom LP he makes or can a standard HK SD LP be used in one of Ralph's SD guns? I don't know, I'm asking.

Scott
 

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I didn't see the OP say he was building a 40/SD, I read that to be he wanted a mp5-40 LP, couldn't find one and wanted to grind a mp5-9 to make one.

I agree completely if this is a SD build, then a #24low, is going to be wrong for that application.
 

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Discussion Starter · #9 ·
I didn't see the OP say he was building a 40/SD, I read that to be he wanted a mp5-40 LP, couldn't find one and wanted to grind a mp5-9 to make one.

I agree completely if this is a SD build, then a #24low, is going to be wrong for that application.

Jdonovan

That is what I was thinking. I have a MP5/40SD built by Terry Dyer from a 40 and an HK 94. I am using a Low 24 Hk locking piece in it.
I put a sear 4 position lower on it and it will fire full auto and 3 shot bursts for a mag or two and then have a failure to eject after it has fired a round.

Just put a new extractor and extractor spring in it and planning on taking it to the range in a day or two to see if it offers a solution.

I sent an email off to Ralph at RDTS and he came back saying that he uses the standard HK Lo 24 locking piece in their 40SD.

Thanks to everyone for the comments they have opined.
 

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Lots of variables.
What barrel? Was a 40 MP5 barrel made to the 9mmSD spec?
Amount of ports in barrel etc.
Whose can? Does it turn std. ammo subsonic like the 9?
You might just need to find the right angle.
MM
 

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Discussion Starter · #11 ·
Lots of variables.
What barrel? Was a 40 MP5 barrel made to the 9mmSD spec?
Amount of ports in barrel etc.
Whose can? Does it turn std. ammo subsonic like the 9?
You might just need to find the right angle.
MM
Hummm... That's a thought. I have been using 180 Gr ammo which is subsonic to begin with. Maybe the ported barrel, a std. Hk 40 barrel cut
down to SD length and alternating 2 and 3 holes drilled into the groves of the rifling, is mitigating the recoil impulse sufficiently to cause the
intermittent failure to eject. The suppressor is a S&H Arms 9mm one I filed a Form 1 on and registered as a new caliber 40 S&W and had the
baffles opened up slightly to accommodate the slightly larger diameter of the .40 round. The suppressor itself is 12 inches long and fills up
the cage completely so I am guessing its about 1.6 inches wide.

I think I am going to get some hotter / lighter .40 ammo and try that.

Thanks for your input.
 

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Just be careful not to over do it.
Check ejection distance and recoil as to not get receiver dents etc.
The ports might be taking off to much energy and causing the issue and the sup not making uf for it like cans on other HK's.
Hard to say as 2 mags fine then fail.
I would first try slow steady semi and check ejection pattern....see if it starts falling short as you go.
MM
 

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Hummm... That's a thought. I have been using 180 Gr ammo which is subsonic to begin with. Maybe the ported barrel, a std. Hk 40 barrel cut
down to SD length and alternating 2 and 3 holes drilled into the groves of the rifling, is mitigating the recoil impulse sufficiently to cause the
intermittent failure to eject. The suppressor is a S&H Arms 9mm one I filed a Form 1 on and registered as a new caliber 40 S&W and had the
baffles opened up slightly to accommodate the slightly larger diameter of the .40 round. The suppressor itself is 12 inches long and fills up
the cage completely so I am guessing its about 1.6 inches wide.

I think I am going to get some hotter / lighter .40 ammo and try that.

Thanks for your input.
@HkCHUCK I might have an extra @26 LP.....Let me check and I PM you if I do
 
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