I never knew about this, but apparently changing out the locking piece to an 80 degree improves the cycling when running suppressed because it slows down the unlocking of the action. Is there a big enough difference to warrant it?

Also to note, do NOT do this to standard SP5s

My full-size has whatever degree that is 90? 100? IDK. It runs great suppressed and not. No gas, never an issue.

My K ran either great suppressed or great not. Never both with either locking piece I had. Suppressed with turn normal piece it was a little gassy, mind you nothing like an Sig MPX or B&T TP9 or APC9 or blowback AR9 or any of the other guns people who apparently don’t use then say have “zero gas”. It’s possible I’m sensitive to it, but I have shot and competed enough that I think I can tell what a good gun is supposed to feel like. And the K could get there, but apparently not with a wide-enough range to support what I wanted.

The only way I would do another K now I think is if it was a non-ported SD-K or K-SD or whatever you would call it. It just wasn’t as nice to shoot as the fullsize, but if the can was part of the gun this would be better. So if you have a K and are OK considering the can a required part of the gun put that other piece in. If you aren’t, just deal with the yes-definitely-noticeable difference.

Edit: also LOL at the firearmblog, so down much downhill post after post

Oh do we hate TFBTV now? I always liked them

In my experience it's not necessary for semi.  Full auto it makes a difference, especially with different ammo.

A lot of the new MP5 owners read a few things and get convinced their gun will self destruct if they don't use an 80 degree locking piece.

It’s is very useful. But not always necessary. The biggest concern is a high blow back can. It could cause the bolt to reciprocate hard enough that when the bolt is to the rear it actually bottoms out, causing the rollers to protrude and dent the receiver rails.  

One thing to be aware of that I don’t believe was mentioned in the videos. When changing locking pieces-It’s not plug and play. Changing locking pieces often changes the bolt gap, so if you change locking pieces you may also need to change rollers to stay within the .010-.020(?)  spec.

Good info. An MP5 noob like me would never have known that.

This is another Locking Piece video I recently came across.

To be fair I recall a few chewed up buffers a few years ago when the K market blew up and people threw suppressors on them

Want to say even one guy got roller marks on his rails

Ultimately whether you need the 80 degree LP is dependent upon a bunch of factors as there is no one-sized fits all approach.

The 80 degree LP is designed to slow the opening of the action and retard the amount of rearward bolt travel speed in the MP5K firearm in order to reduce the ROF in full auto and prevent damage to the receiver.

The K receiver is not as robust as the full size receiver when it comes to being suppressed or fed a diet of hot ammo.

- You have less receiver length so the gun has to mitigate the recoil from the rearward bolt travel in ~25% less distance (the full size has ~4.5" of bolt travel vs~ 3.5" in the K)
- Most  K stocks go inside the receiver vs. the full size that goes around the outside of the receiver like a clamshell providing additional support on the rear of the gun
- The K rear pins don't have a welded in rear cross bushing supporting the pin like the full size does so the recoil force when the bolt bottoms out is transmitted directly the thin sheet metal holes.
- To the Ks benefit HK added the second upper rear pin holes and the supporting weldments at the rear.

Factors that come into play as to whether a specific MP5K needs a 80 degree LP include:

- Ammo (grain weight and powder charge)
- Suppressor and the amount of back pressure the can generates.
- How fresh/strong is your recoil spring
- Are you running a 9mm hammer spring or a rifle hammer spring.
- Bolt gap to a limited extent.

If you have a low backpressure suppressor, running lightly loaded 115gr ammo, with a new recoil spring, and a rifle hammer spring you may find that your K won't reliably cycle with an 80 degree LP.

Conversely if you are running steady diet of 147gr +P self defense ammo, with a high back pressure suppressor, on a  gun with thousand of rounds down the pipe with a well broken in recoil and 9mm hammer spring and buffer you may find out the hard way you damaged your receiver via roller dents or wallowed out rear take down pin holes over time as well as a highly increased ROF in full auto.

Ultimately its probably the safe approach to test your specific mix of ammo, suppressor, and host gun with the 80 degree LP if you plan to run the gun with the suppressor on a regular basis.  If the gun functions with your combination and doesn't have FTE issues and exhibits a healthy ejection pattern, than stick with the 80 degree LP.  If the gun functions with an 80 degree LP that will save wear and tear on your receiver long term especially as your springs and buffer wear in.

If you put an 80 degree LP in and your combination of gun/ammo experiences FTEs or really weak ejection pattern than go back to the 100 degree LP as  you can damage the extractor spring if you experience a bunch of FTEs plus its no fun to have a gun that doesn't function right.

Its the same as the AR15 world as there is no given recipe as to whether you need to run a heavy H3 buffer and/or an adjustable gas block just because you add a suppressor.  A lot has to do with the suppressor, ammo, gas port hole size, etc.

The full size MP5 is not a sensitive to suppressor use given its longer/stronger receiver construction but it certainly doesn't hurt to run a 80 degree LP in the full size if you are running a steady diet of hot or heavy grain weight ammo along with a high back pressure suppressor in order to keep the bolt from battering the stock and back of the receiver.

In terms of bolt gap change, the poster above is correct that bolt gap will usually change to some degree if you change the locking piece out.  That said if you have a German gun (SP5K) and change the German 100 degree for an 80 degree the change is usually minimal and it won't necessitate changing the rollers.  You may see a 0.001 to 0.003 change if you stick with all German parts and in my experience going down in angle usually result in the bolt gap going up.

If you buy a US made LP than its more of a mixed bag in terms of bolt gap change in my experience.  Either way its prudent to check the bolt gap after changing the locking piece out but it usually a non-issue sticking with all german parts.

Hope this helps.

That is very helpful, thank you for that. I only intend to run mine suppressed so I'll have to look into that.

This is what they have over at HK Specialist

Do these things really run $200, or is that a bulk pack? It looks like there's 4 in there, I only need 1

Unfortunately that is the cost for ONE #28 locking piece.  They have become very desirable lately and are hard to come by.  At least the German ones are.  
  Just a heads up, Ive read where some people installed the #28 locking piece in their new SP5K-PDW’s and experienced FTE/FTF issues.   This is with 147gr subsonic ammo w/ a suppressor.  I guess every gun is different and it depends on the combination your running.  Sucks that its a $200 experiment/gamble.

My SD receiver with K-PDW barrel would chew through extractor springs every few hundred rounds before I changed the locking piece.

Unfortunately the German 80 Degree LPs are in pretty high demand right now and are even more expensive than "normal" given all of the guns the roller locked 9mm firearms HK is importing.

That said you do have a $3500 to $5500 German K host (depending upon if its a SP5K or PDW) and presumably a $700 suppressor between the can and stamp.   $200 for the correct part to make sure the system is running right isn't terrible investment.  As has been mentioned they are in pretty high demand and not readily available so I would presume you could get most if not all of your money back out of a lightly used 80 degree LP should you find you don't need/want it.

A couple of options if you are on a budget.

You could also try an RCM LP for a much reduced cost as well and it may very well work out fine or at least give you a better idea if you need/want the German version.

Another option that G3Kurtz (who worked for HK) would do to determine if a MP5 had enough locking piece angle was to put some modeling clay about 1/4" think in the back of the stock and fire a round to see if the carrier was mashing into the clay and if so how hard. He used this method to determine if a MP5/10 needed the High or Low locking piece.  I personally do the same thing on M16 receivers to make sure the gas carrier key doesn't impact the buffer tube.

I am not sure how well the modeling clay method would work for a MP5K given the reduced operating margin,  but its another cheap option to try and see if and/or how hard the carrier is impacting the stock and if $200 is warranted for an 80 degree LP.  If your setup doesn't doesn't result in a carrier impact into the clay or just a light touch into the clay you should be good.  If the carrier mashes itself all the way through the clay and stops at the stock, well maybe you should try an 80 degree LP.

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I was getting damage and could feel the bolt hitting the buffer until I changed my locking piece to a #28. This is a Zenith K sbr. Octane 45. 147 gr Lawman. Not as in depth as JB but this is my experience.

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Hard to imagine a higher backpressure can/ammo combo

Understood and agreed.

With a .45+ caliber bore?

I guess a 9mm octane would be worse.  But the .45 is pretty bad.

Interesting

I guess I just figured K baffles would make more backpressure than the octane baffles but I really have no idea

My tirant in 9 has so much that the gun doesn’t run. Even with the lock piece.

No problems on my older Ati mke but I haven’t run my Sp5k pdw suppressed yet.  I just run 147gn so we shall see.

Suppressors can ad varying degrees of extra back pressure.The only real way to know if the 80 degree locking piece is necessary is to watch the buffer for signs the bolt carrier is hitting it.Also adding a stock changes things as well.Again you gotta watch the buffer.

Are the 80 degree locking pieces interchangeable between the sp5k and the sp5?

Yes, the locking pieces fit either, but the 80 degree LP almost always isn’t needed in the SP5 due to the longer receiver.  It’s really only a potential issue in the short K receivers.