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Discussion Starter #1
So I’m in the debate about whether to get a MP5K, or reverse stretch MP5k. Looking at best bang for the buck!! I currently have a flemming sear, HK-53, PTR-91, PTR-32, and a V-51. So I don’t currently have any 9mm parts. I’ve been trying to do some research, but I’ve now gotten myself so confused, I’m not sure what the right answers are. So here I am with the experts!!

Looking to start with a barreled upper from BMP. Cost difference between the two is negligible, so on to the other questions.
1. They both would need the full auto MP5K bolt carrier group right? So cost would be the same for both?
2. Do the barreled upper’s come registered as pistols? or do I need to SBR them?
3. Does either of them come with three lug barrels? They don’t mention it in the description.
4. If I go the real K route, I would need to get a K grip housing? Does my trigger pack fit in one of those? If I go the stretch route, my existing grip and pack should work?
5. On the stock, I just saw that flemming makes a MP5K full stock adapter and would let me use any of my existing stocks with the real K? The stretch would not need the adapter to work with my full sized ones? But with the real K I could go either direction (any K stock, or adapter with any full size stock)?
6. Do they both use a K recoil rod?
Did I miss anything else?
Thanks as always for the great help with all of this.
 

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So I’m in the debate about whether to get a MP5K, or reverse stretch MP5k. Looking at best bang for the buck!! I currently have a flemming sear, HK-53, PTR-91, PTR-32, and a V-51. So I don’t currently have any 9mm parts. I’ve been trying to do some research, but I’ve now gotten myself so confused, I’m not sure what the right answers are. So here I am with the experts!!

Looking to start with a barreled upper from BMP. Cost difference between the two is negligible, so on to the other questions.
1. They both would need the full auto MP5K bolt carrier group right? So cost would be the same for both?
2. Do the barreled upper’s come registered as pistols? or do I need to SBR them?
3. Does either of them come with three lug barrels? They don’t mention it in the description.
4. If I go the real K route, I would need to get a K grip housing? Does my trigger pack fit in one of those? If I go the stretch route, my existing grip and pack should work?
5. On the stock, I just saw that flemming makes a MP5K full stock adapter and would let me use any of my existing stocks with the real K? The stretch would not need the adapter to work with my full sized ones? But with the real K I could go either direction (any K stock, or adapter with any full size stock)?
6. Do they both use a K recoil rod?
Did I miss anything else?
Thanks as always for the great help with all of this.

cant help with the full auto details.
a K does use a different trigger housing , its shorter on the back end and
requires different stocks , thats where the fleming adapter comes in to use your standard mp5 stocks . a stretch
will use the normal mp5 receiver with a k barrel and take the standard stock (as far as i know you cannot put a K
style stock on an mp5 or reverse stretch but with the fleming adapter in a K you can have both )
last time i contacted BMP they said that their K uppers DO have 3 lug barrels , but they have been out of stock

im sure someone with vast knowledge of all things HK and full auto will chime in , this is just what ive learned
lately with my future build
 

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I've been thinking about this too and I've decided on the reverse stretch. You can swap the trigger housing from one gun to the other and the reverse will run slower than the k. The k is really fast. Also, you can move stocks between hosts as well but the k would be different. They also say you can use a full size carrier in the reverse stretch if you turn down the support.
1. Same cost unless you get a full size mp5 upper too and want to swap the carriers between hosts.
2. Pistol
3. I think they are all 3 lug
4. All yes
5. Can't use an A3 because of the k bracing.
6. k is unique and reverse stretch is unique with the k carrier.
 

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hunter,
Most typical set up: the reverse stetch "K" model uses a "K" LP, "K" carrier, "K" cocking tube & support. Re-coil rod length (from a full size rod) is the difference in length between the full size & "K" carrier. A "K" re-coil spring is used.
Dj
 

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So I’m in the debate about whether to get a MP5K, or reverse stretch MP5k. Looking at best bang for the buck!! I currently have a flemming sear, HK-53, PTR-91, PTR-32, and a V-51. So I don’t currently have any 9mm parts. I’ve been trying to do some research, but I’ve now gotten myself so confused, I’m not sure what the right answers are. So here I am with the experts!!

Looking to start with a barreled upper from BMP. Cost difference between the two is negligible, so on to the other questions.
1. They both would need the full auto MP5K bolt carrier group right? So cost would be the same for both?
Yes, both a standard K and a reverse stretch use a k bolt group.

2. Do the barreled upper’s come registered as pistols? or do I need to SBR them?
A barreled upper is a firearm the same as a stripped receiver. The ATF changed the 4473 to add a third option. It used to be that even a stripped receiver had to be declared as a pistol or rifle. Now the categories are pistol, rifle, or receiver. So the barreled receiver would be considered a "receiver" because it has not been built into a complete firearm. A barreled receiver could be made into a pistol or a rifle. It is up to you as the builder to decide what is ultimately built. So unless you want want the gun to be a SBR, you won't have to.

3. Does either of them come with three lug barrels? They don’t mention it in the description.
You can call to check but the BMP barreled receivers that I have seen are all 3-lug barrels.

4. If I go the real K route, I would need to get a K grip housing? Does my trigger pack fit in one of those? If I go the stretch route, my existing grip and pack should work?
Yes you'd need a K housing to use your pack. A HK 9 series trigger pack will work in all 9 series housings. The ejector lever is different for each caliber. In fact my HK53 parts set came with an ejector lever that is shorter than the standard HK33 ejector lever. So you'll need a 9mm ejector for your sear pack to use with the 9mm gun. You might also need to buy a 9mm hammer spring. There are actually three different hammer springs. There are rifle caliber (5.56X45 and 7.62X51 use the same hammer spring), .40 caliber (.40 S&W and 10mm AUTO), and 9mm hammer springs in descending order of strength. If the 9mm host gun is well broken in that 9mm gun might run with a rifle caliber hammer spring. I know I had trouble with my Coharie Arms pistol caliber guns being broken in using a rifle caliber hammer spring. Using a 9mm spring with pistol caliber clones did help with break in. Your existing housing one hole housing for the 53 might or might not work with the reverse stretch receiver. BMP has had some inconsistency with self heights. If the shelf height of of your 53 is the same as the BMP shelf, then it will work. Grip housings are cheap in comparison to your total investment. I have a grip housing for each clone fitted for that specific clone. You'll need to pull the trigger pack out of the housing to change the ejector when changing calibers. So having a housing fitted to each clone works best for me.

5. On the stock, I just saw that flemming makes a MP5K full stock adapter and would let me use any of my existing stocks with the real K? The stretch would not need the adapter to work with my full sized ones? But with the real K I could go either direction (any K stock, or adapter with any full size stock)?
Yes, the Flemming stock adapter will allow use of your other stocks with a K receiver. Something to keep in mind is the Fleming K adapter will increase the length of pull. So if increasing LOP will be an issue for you, I'd think you could find a K stock for less than the $95 + S&H for the Fleming adapter. No the reverse stretch would not need the adapter to use the stock you use with the 53.

6. Do they both use a K recoil rod?
No, the reverse stretch uses a custom length recoil rod because the standard receiver is about 3/4" longer than the K.

Did I miss anything else?
I have a K and a reverse stretch. I prefer the reverse stretch. It has the shorter barrel so my suppressor is closer to my body. So the gun isn't as muzzle heavy with the can. I much prefer the shorter LOP of the early A3 stock, which I can use with a reverse stretch. The high cyclic rate of the K is fun, but that is it for me. The reverse stretch is my favorite 9mm out of my collection (k, reverse stretch, converted 94, and SD.

Thanks as always for the great help with all of this.
 

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Discussion Starter #6
Thanks guys!! I appreciate all the insight. I think I'm leanings toward the reverse stretch. Seems to be more plus's for my situation. Now to start selling stuff to get the money. Anyone have any idea when BMP will have some more of these. I can never get ahold of them. Never return call's and now there mailbox is full. Does anyone else sell these?
 

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So how big of a hurry are you in? Check Gun Broker for BMP/Todd built stuff. His products can be very hit or miss. Maybe you can find a deal. Of course you get what you pay for. You might very well have to send your barreled upper to get it to run right.

Personally, I'd drop a line to "DJF" to see if he has any extra reverse stretches in his build queue that isn't spoken for. Or I'd contact Jeff, Joe, or any number of the smiths here about a parts list for the build and what it would cost. Good luck with the project.

Scott
 

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Hunter,
All of my reverse stretch K's are earmarked with deposits. I do have a few K models available.
Dj
 
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