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I finally got my 9MM MP5SD out of ATF jail and took it to the range. The first two boxes I ran were 147 grain Winchester and the gun ran perfectly. I changed to 115 grain and after the first couple of rounds, it would fail to eject the casing. Clear it, and the second new round would FTE. I assumed it was just running hot so I let it cool and came back to it. Same problem, different magazine same rounds ran fine through the SP5K. Went back to the 147 grain and it spun like a top. Any one had this problem before?
 

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Merchant of Death (Admin)
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If the barrel is actually ported, you should be running 115's and hotter rounds. 147's would be a no-no. Ideally, with ported barrel, you should be running regular rounds (not sub-sonic) and an SD locking piece. I would also check and likely replace the extractor spring. Cheap part, easy fix.
 

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If your 115 grain ammo is Winchester White box, that’s the likely culprit IMO. Make sure you have an SD LP in it. Try some quality 124 grain and see what the results are.
 

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I finally got my 9MM MP5SD out of ATF jail and took it to the range. The first two boxes I ran were 147 grain Winchester and the gun ran perfectly. I changed to 115 grain and after the first couple of rounds, it would fail to eject the casing. Clear it, and the second new round would FTE. I assumed it was just running hot so I let it cool and came back to it. Same problem, different magazine same rounds ran fine through the SP5K. Went back to the 147 grain and it spun like a top. Any one had this problem before?
@straightgrain is correct that 147gr ammo and any sub sonic ammo should be avoided as the design of the gun is to take supersonic ammo and make it subsonic. If you use subsonic to begin with you risk suppressor damage due to baffle or end cap strikes depending on your SD suppressor manufacturers specs such as dia, length, number of baffles etc.

Your gun was designed to shoot nato spec 124gr. Other loads may work but 124gr nato spec will always cycle the gun reliably. Also some SDs need a bit more lovin than others so a 120 degree #5 lp can help to cycle lighter 115 gr loads if you must run them.

Older original SD locking pieces were 120 degree and marked with a #5 or just SD, Newer ones are 115 degree and marked on one side with both MP5 Action and MP5 SD. Also FYI some builders like Ralph at RDTS use a RCM 120 degree locking piece now in their builds.

I have seen various SDs that have the F style carrier and recoil rod that do not function well with 115gr ammo, so try a standard action 3 carrier and recoil rod. Also bolt gap spec can play into spotty function with 115gr ammo depending on your chosen ammo. I detailed most of this info before in post #14 of the following thread so give it a read and report back what fixed your issues if you are intent on running 115gr ammo instead of the 124gr nato spec that the gun was designed for.

https://www.hkpro.com/forum/hk-nfa-talk/517778-mp5sd-bolt-carrier-question-2.html
 
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If your 115 grain ammo is Winchester White box, that’s the likely culprit IMO. Make sure you have an SD LP in it. Try some quality 124 grain and see what the results are.
Funny, I was about to post the same thing about Remington budget ammo. I have found those to be some of the most anemic performing factory rounds.
 

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I load all my 9mm 115 FMJ to about 1225 FPS. The SD lowers that by about 200 FPS. I have no issues. I’ve found Fiocchi 115 gr to basically match my specs. It’s my favorite factory 9mm. I don’t think I’ve ever run 124 gr.
 
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