[DontCome2MyHouse]

Looking into purchasing either HK SP5K or clone (undecided). I noticed that either has the grooves for for what I'm guessing is a retractable stock. I've seen them on the full size MP5, but not so much on the shorter "K" versions as I **think** they were intended to use folding stocks (please correct me if I'm wrong).

If I wanted to SBR, how would I go about installing the classic MP5 retractable stock like pictured on a "K" style? Would any modifications need to be made on the receiver on current HK SP5K?





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[HK556WI]

You will need a stock block extension like the one in the picture.

MP5K to MP5 Stock Adapter, SP89, HK94, HK, Fleming, HKK-1258, RTG Parts

 

[WebStrands]

You're picture is a reverse stretch, not a "K". That's an MP5 with a "K" front end.

 

[scottinthegrove]

You're picture is a reverse stretch, not a "K". That's an MP5 with a "K" front end.

Please take a closer look at the photo. Notice that there are two pins holding the adapter? There is another pin at the rear of the adapter holding the stock on. One pin at the top of the receiver, one in the upper right hand corner of the grip housing. The "K" guns have the lower stock pin in the plastic of the grip housing. Full size receivers (pistol caliber and 5.56) have the rear pin in the tang that slides inside the rear cap.

If an A3 stock is desired for use with a "K" receiver, there are two reinforcement plates, at the rear of the the receiver on the right and left sides, that cover the outside of the channel inside the receiver that the bolt carrier rides on. So those two reinforcement plates would need to be removed and the receiver refinished or a "K" flat would be rolled and welded without adding the two reinforcement plates. Normally to add a stock to a "K" receiver a folding stock is used. If you really want the look in the photo, a reverse stretch (Full size receiver with a "K" trunnion, barrel, and cocking tube/support assemble) would be a cheaper easier alternative to two reinforcement plates removal, and adapter purchase. It would have the same look only without all the extra pins for the adapter. Good luck with your "K" stock project.

Scott

 

[DontCome2MyHouse]

Please take a closer look at the photo. Notice that there are two pins holding the adapter? There is another pin at the rear of the adapter holding the stock on. One at the top of the receiver, one in the upper right hand corner of the grip housing. The "K" guns have a pin in the plastic of the grip housing. Full size receivers (pistol caliber and 5.56) have the rear pin in the tang that slides inside the rear cap.

If an A3 stock is desired for use with a "K" receiver, there are two reinforcement plates that cover the outside of the channel inside the receiver that the bolt carrier rides on. So those two reinforcement plates would need to be removed and the receiver refinished. Normally to add a stock to a "K" receiver a folding stock is used. If you really want the look in the photo, a reverse stretch (Full size receiver with a "K" trunnion, barrel, and cocking tube/support assemble) would be a cheaper easier alternative to two reinforcement plates removal, and adapter purchase. Good luck with your "K" stock project.

Scott

Thank you. Are there any downsides to a reverse stretch?

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[diesel1959]

Additionally, with an SP5K or SP89, you'd have to remove the reinforcing tabs in order to use a retractable stock. And that's in addition to using the adapter previously mentioned.

I've got two Bobcat Weapons BW89s and neither of them have the reinforcing tabs of the genuine HK k guns; therefore, I am able to use the adapter and retractable stock without any modifications needed.

 

[scottinthegrove]

The only downside to a reverse stretch compared to a "K" is the RS would be about 3/4" longer than a "K" with the same style of barrel and HK never made one. So you'd need a HK94 to build one with a HK receiver. I think it is a great step between the full size with an 8.85" barrel with an extra 3/4" in the receiver and the "K PDW" with a 5.85" barrel. The RS will use standard full size grip housings and stocks. Personally, I'm fortunate enough to have all three. The "K" is my favorite, but that is me. They all have their place.

Scott

 

[TooSixy]

Did we not cover this in your other thread? Why repeating the same question?

 

[DontCome2MyHouse]

Did we not cover this in your other thread? Why repeating the same question?

Not really. Glad I asked on here because it looks like a reverse stretch might be the best route.

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[RWO101]

Just get a POF MP5k. It is a reverse stretch and does not have the reinforcement tabs. A retractable stock fits nicely on it and looks great. I got mine for around $1300.

 

[Discogodfather]

A true "K" like my build is something that I will never do again, no point. To get the 3/4" less length you sacrifice a lot in terms of structural rigidity and you loose all the regular MP5 stock options which are plentiful and far less expensive than "K" stuff. The K stock essentially slides into the receiver and thus the need for the reinforcement plates and the two pins, one at the top. Even with the extra pin and the reinforcement plates, I find it far less tight and it still moves around quite a bit, even on real HK made guns.

The K lowers, the real ones, cost an arm and a leg. I got an original SP89 lower for my build and you don't want to know what it cost. With a reverse stretch you get tons of lower options at half the price. Home made K lowers can really suck bad if not done well.

K was designed to fit into a suitcase and 3/4" less length makes no difference in the real world.

 

[scottinthegrove]

A true "K" like my build is something that I will never do again, no point. To get the 3/4" less length you sacrifice a lot in terms of structural rigidity and you loose all the regular MP5 stock options which are plentiful and far less expensive than "K" stuff. The K stock essentially slides into the receiver and thus the need for the reinforcement plates and the two pins, one at the top. Even with the extra pin and the reinforcement plates, I find it far less tight and it still moves around quite a bit, even on real HK made guns.

You are certainly entitled to your opinion, but your experience has not been mine. I find that my HK folding stock locks well and shoulders well. The "K" stuff is more rare. HK parts are pricey and rare HK stuff is even more expensive. That is just the way it is.

The K lowers, the real ones, cost an arm and a leg. I got an original SP89 lower for my build and you don't want to know what it cost. With a reverse stretch you get tons of lower options at half the price. Home made K lowers can really suck bad if not done well.

Like anything, there is suggested retail and what you might find with patient searching. My first "K" build was with a complete HK MP5K parts set and a SW receiver. The complete HK parts set was $1,700. It came with a burst lower. Using that lower, I had a local machine shop modify a couple of G3 "Navy" style SEF grips for use with the "K". I have multiples done as I have many "K" guns in multiple calibers. From personal experience a 10mm "K" is a hand full.

K was designed to fit into a suitcase and 3/4" less length makes no difference in the real world.

I'm fortunate enough to have at least one in "K", "K" PDW, Reverse Stretch, full size and SDs in 9mm configurations. Of all my 9mm roller locks, the "K" is my favorite. I've used that gun in a couple of classes with Teufelshund Tactical. The "K"is more concealable, and faster to deploy than a standard MP5 especially with the HK harness for the "K". I find the "K" is lighter so transitions from target to target is faster and I'm less likely to "swing through" the target with a "K". BTW, it is my understanding that the "K" was developed for conceal ability. The briefcase came afterwards. I'd suggest checking before posting something as "fact".

With the custom sights and finish, why didn't you put the paddle mag release in it? If you don't like it, why don't you sell it and roll the money into something you want? Again, you are certainly entitled to your opinion. My biggest problem with your post is your statements like "K was designed to fit into a suitcase and 3/4" less length makes no difference in the real world." That might be true for you. I for one disagree with blanket statements. In my personal experience that 3/4" makes a big difference. I'm stating my opinion as such. Your statement comes across as statement of fact. That is a disputed "fact" IMHO. YMMV.

Scott

 

[Discogodfather]

I'm saying the K rear end is not worth it, the configuration as a 4" barrel front end is worth it for the conceal-ability. OP is asking for advise on K vs, reverse stretch and I am making an argument the reverse stretch is the way to go for all the reasons listed. You get all the benefits of the K with no drawbacks going reverse stretch. If you need the 3/4" because your an operator and in ninja school then all good, go for it, but be prepared to never be able to use a telescoping stock and be very limited as to options for folding stocks.

I live in CA so there is no point to the paddle release, otherwise I would have it on in a heartbeat.

And the structural rigidity is a fact, it's a compromise because the stock has to essentially put a trunion into the rear of the receiver. It's kind of like a wedge effect, and rough use pushes the receiver outward and expands the receiver. Just less sound than a stock that fits over the top of the receiver. I don't know if you build, but the welding of those little tabs and the lands for the top pin are a pain as well.

 

[axehole]

Just get a POF MP5k. It is a reverse stretch and does not have the reinforcement tabs. A retractable stock fits nicely on it and looks great. I got mine for around $1300.

That's the ticket right there, easy and inexpensive relatively speaking.

 

[Discogodfather]

I did notice this new product, allows the use of a telescopic stock with a K rear end with the reinforcement tabs. Pricey!

https://www.hkparts.net/shop/pc/MP5K-PDWC-Telscoping-Stock-6p17825.htm

 

[jhutchings]

So I have been reading this. I have a reverse stretch and mp5k. I have an HK A3 stock on the reverse stretch. I also have the BT stock for the reverse stretch but rarely use it. I do like the BT stock on the MP5k and love that configuration. Now which is best is up to personal preference. I love the mp5k and always will. But for a really thin and small package the reverse stretch with A3 stock is hard to beat.

I saw the PDW stock from above and that seems like a winner. But if I get the K I want something like the BT stock.

And finally for good measure here is the MP5

All pictures with Fleming registered sear pack.

 

[LongDuck]

Nobody has mentioned it, so I will; the A3 collapsible stock, when collapsed, is difficult to fire the gun as the shoulder extension gets in the way of your wrist. For that reason, I prefer the Choate side-folder for both the HK53 and MP5k when shooting with the stock folded. I'll also add that the Choate side-folders mount will shoot loose over time, as the HK ITD often found with high round counts through training guns. The A3 collapsible stocks tend to get loose, too, as the rails will eventually wear to the point of droop and flop, so both stock types will eventually get loose. It's really more preference, and the A3 stock gets the nod for tradition on the full size MP5.

 

[Discogodfather]

Nobody has mentioned it, so I will; the A3 collapsible stock, when collapsed, is difficult to fire the gun as the shoulder extension gets in the way of your wrist. For that reason, I prefer the Choate side-folder for both the HK53 and MP5k when shooting with the stock folded. I'll also add that the Choate side-folders mount will shoot loose over time, as the HK ITD often found with high round counts through training guns. The A3 collapsible stocks tend to get loose, too, as the rails will eventually wear to the point of droop and flop, so both stock types will eventually get loose. It's really more preference, and the A3 stock gets the nod for tradition on the full size MP5.

Yeah, I can confirm the choate gets looser over time and hard use. That's the wedge effect from closing and opening the folder. I think the best combo for a really rigid setup that would take the abuse a bit better would be a reverse stretch with a B&T folder (the folder that cups around the back of the receiver). That way the stress is being distributed more evenly to the rear of the receiver.

I've thought about incorporating two features on my next MP5k reverse stretch build if I ever get to it: using a BT folder with a reverse stretch receiver, then also welding in two support tabs at the rear. This would give you the best structure possible back there but would kill the ability to use A3.

 

[OlDirtyBastard]

Please take a closer look at the photo. Notice that there are two pins holding the adapter? There is another pin at the rear of the adapter holding the stock on. One pin at the top of the receiver, one in the upper right hand corner of the grip housing. The "K" guns have the lower stock pin in the plastic of the grip housing. Full size receivers (pistol caliber and 5.56) have the rear pin in the tang that slides inside the rear cap.

If an A3 stock is desired for use with a "K" receiver, there are two reinforcement plates, at the rear of the the receiver on the right and left sides, that cover the outside of the channel inside the receiver that the bolt carrier rides on. So those two reinforcement plates would need to be removed and the receiver refinished or a "K" flat would be rolled and welded without adding the two reinforcement plates. Normally to add a stock to a "K" receiver a folding stock is used. If you really want the look in the photo, a reverse stretch (Full size receiver with a "K" trunnion, barrel, and cocking tube/support assemble) would be a cheaper easier alternative to two reinforcement plates removal, and adapter purchase. It would have the same look only without all the extra pins for the adapter. Good luck with your "K" stock project.

Scott

for educational purposes, what is the reason for the reinforcement plates ? my 5rs rear adapter comes off with very little effort, doesn't seem to add any additional "reinforcement" .

 

[Discogodfather]

Plates do two things:

1) They prevent installing an A3 collapsible onto a K (would be too long).
2) They re-enforce the receiver from being tweaked by the inner "trunnion" like part of the K stocks. When you fold the stock or put a lot of pressure on the area, the trunion pushes the receiver outward, it's got a lot of leverage. Worst case it bends the receiver and you get a lot of slop in the rear.

With your extension things are less of an issue since the hinge point is further rearward and the extension creates more stability. On a regular K with a K stock the leverage point is closer to the receiver and cause more stress there.