SP5 Modifications for SBR?

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  • smdub

    Active Member
    MDS Supporter
    Nov 14, 2012
    3,875
    MoCo
    How about replacing the buttpad of a HK33 A2 stock with a lengthier one onto an SP5 like this one?
    I wouldn't know the exact dimensions so I'm not sure what the OAL would be but I wonder if that'll be anywhere closer to 29.
    Thats for a G3/HK91 A3 stock (They are a nice upgrade!). Those stocks have two vertically spaced holes near the butstock that it clamps onto. (which are used to store your take down pins when you pull the stock off.) The MP5/33 A2 stocks don't have those holes. Their buttpads don't just pop off.
     
    Last edited:

    Jason22492

    Junior Member
    Mar 28, 2013
    31
    Montgomery County
    Thats for a G3/HK91 A3 stock (They are a nice upgrade!). Those stocks have two vertically spaced holes near the butstock that it clamps onto. (which are used to store your take down pins when you pull the stock off.) The MP5/33 A3 stocks don't have those holes. Their buttpads don't just pop off.
    Thank you for the reply and clarification.
    May I then add and ask if that buttpad would be compatible with this item which would then go on a SP5? (Or is this stock also a no-go?)

     

    smdub

    Active Member
    MDS Supporter
    Nov 14, 2012
    3,875
    MoCo
    No.

    This is a G3/91 stock that that heavy butt pad is designed for:
    You can see the two cross pins in the rear that the rubber butt pad clips onto. The MP5/33 stock you show doesn't have those. That wood stock w/ rubber buttpad you linked must be screwed together somehow. You could probably cut off the thick butt pad's clips and drill for screws? Nothing is impossible but its going to take some fab work.
     

    smdub

    Active Member
    MDS Supporter
    Nov 14, 2012
    3,875
    MoCo
    I went downstairs and pulled the heavy butt pad off a G3 stock (it also has a snap on cheek riser on it - ignore that.) You can see the cross pins the clips hold on to. MP5/33 stocks don't have those.

    Edit: I bet you could take a G3 stock and fit it w/ the MP5/33 front metal one hole cap. Not sure the OAL difference that would provide (I can measure tomorrow.). But you'd be able to snap on a thicker butt pad.
     

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    Jason22492

    Junior Member
    Mar 28, 2013
    31
    Montgomery County
    No.

    This is a G3/91 stock that that heavy butt pad is designed for:
    You can see the two cross pins in the rear that the rubber butt pad clips onto. The MP5/33 stock you show doesn't have those. That wood stock w/ rubber buttpad you linked must be screwed together somehow. You could probably cut off the thick butt pad's clips and drill for screws? Nothing is impossible but its going to take some fab work.
    I went downstairs and pulled the heavy butt pad off a G3 stock (it also has a snap on cheek riser on it - ignore that.) You can see the cross pins the clips hold on to. MP5/33 stocks don't have those.

    Edit: I bet you could take a G3 stock and fit it w/ the MP5/33 front metal one hole cap. Not sure the OAL difference that would provide (I can measure tomorrow.). But you'd be able to snap on a thicker butt pad.
    Thank you so much for going out of your way to educate me! That answered a lot of my curious questions that I wish I could've known but simply couldn't due to not having seen them personally physically.
     

    Eastwind

    Nice!
    MDS Supporter
    May 3, 2020
    116
    I went downstairs and pulled the heavy butt pad off a G3 stock (it also has a snap on cheek riser on it - ignore that.) You can see the cross pins the clips hold on to. MP5/33 stocks don't have those.

    Edit: I bet you could take a G3 stock and fit it w/ the MP5/33 front metal one hole cap. Not sure the OAL difference that would provide (I can measure tomorrow.). But you'd be able to snap on a thicker butt pad.
    Do let us know! Thank you as well.
     

    smdub

    Active Member
    MDS Supporter
    Nov 14, 2012
    3,875
    MoCo
    No problem.
    Looks like the G3 & 33 stocks are the same LOP so not much advantage to be had swapping parts around. The heavy HK21 butt pad is ~1/4" longer than a stock G3 pad at the tip. Its main advantage is that its softer and wider so distributes the recoil forces better. A little grippier too so it stays in your shoulder pocket better. Off topic but a heavy recoil buffer and HK21 pad really make a tuned G3/HK91 shoot nice.
     

    Jason22492

    Junior Member
    Mar 28, 2013
    31
    Montgomery County
    No problem.
    Looks like the G3 & 33 stocks are the same LOP so not much advantage to be had swapping parts around. The heavy HK21 butt pad is ~1/4" longer than a stock G3 pad at the tip. Its main advantage is that its softer and wider so distributes the recoil forces better. A little grippier too so it stays in your shoulder pocket better. Off topic but a heavy recoil buffer and HK21 pad really make a tuned G3/HK91 shoot nice.
    May I request one more measurement information? I'm interested in knowing the total length from the middle of the rear takedown pinhole to the end of the buttstock backplate where the rubber pad begins on a G3/33 stock.
    I'm trying to do some math here without having any of the equipment on hand; from what I've been able to research online,
    If the LOP of the G3/33 stock is the same, then the OAL of the SP5 on an A2 stock should be 26.8 inches. I've read somewhere online on a forum that the "MP5 A2 stock measures 9.5 inches measuring from the buttstock pin hole to the buttplate."

    If that is correct, and the G3/33 stock measures the same on the SP5, (perhaps without the buttpad which can be replaced?)
    that means that the LOP needs to be increased (assuming no changes are made on the muzzle end) by a whopping 2.2 inches...
    the best information I could find online where that could be possible with a G3/33 stock was E&L manufacturing, where they make stock extensions by 1.5"...
    https://www.elmfg.com/store/stockext.html
    500.jpg


    Even then, assuming all the measurement is correct, that would still bring it to 28.3"... am I right?
    Unless the G3/33 stock measurement from the pinhole to the buttplate is significantly larger than 9.5, like 10.5, I.... I don't see this happening and I'm all out of ideas. Trying to find a way to add 2.25" is way more ridiculous than I thought.

    the B&T UMP Folding stock has been known to have a significantly larger LOP than most others; HKParts lists that "* Total length from middle of the pinhole to end is 11″", and it can also be seen here from these pictures:

    Tommy Built UMP Brace LOP Comparison.jpg


    Tommy Built Brace comparison.jpg


    Unfortunately even then, with a length that has 11" from the middle of the pinhole to the end of the buttstock; If my math is correct that would still come up 0.7" short like the 1.5" spacer on a 9.5".....

    As you can tell, I've been doing alot of research online and data surfing with things I don't physically have in hand. Haha.

    Honestly though, The G3/33 buttstock with E&L stock extender looks deceivingly longer and I wonder if that just might make it.
     
    Last edited:

    smdub

    Active Member
    MDS Supporter
    Nov 14, 2012
    3,875
    MoCo
    Yes, you are going to need to add ~2" to get a MP5 to MD legal length. My pre-2012 SBR is 27" long w/ an A2 fixed stock. This has been covered pretty extensively on this forum. Several folks like Squaregrouper have figured out how to make spacers. You're going to end up w/ a long LOP.

    I tried to take pics w/ the lens over the butt end to minimize parallax but it makes the other end of the tape measure seem off center. It is lined up w/ the middle of the pin.
    G3 A2 (10.25")
    G3.jpg

    HK33/MP5 A2 (non buffered) (10")
    HK33_UB.jpg

    HK33 A2 buffered: (10.25")
    HK33_B.jpg

    MP5F A3 (10.5"-10.75" depending on what you measure to)
    MP5F.jpg

    All together compared: (this one the lens was over over the center of the stocks, but you can still see there is very little difference w/ exception of the A3)
    All.jpg

    Notice HK made a liar out of me. The metal cap HK33 buffered stock does indeed have a removable G3 style end cap. It can take the heavy HK21 butt pad w/ no modifications. I forgot these existed until I dug some out of the boxes (I have at least 2 of those.)
     

    squaregrouper

    6.5 Man Bun
    MDS Supporter
    I am waiting on a stamp for an MKE AP5 (MP5 clone). The link below would be the easiest, least expensive route, but wouldn't look correct. Add to that, your fellow H&K folks would think you're one of "the poors" for needing to use AR furniture on your SP5.
    Easy Peazy method to be MD compliant...
    But the adaptor linked above and then buy one of these, add an AR stock and you will be over 29" with ease.
     

    Jason22492

    Junior Member
    Mar 28, 2013
    31
    Montgomery County
    Yes, you are going to need to add ~2" to get a MP5 to MD legal length. My pre-2012 SBR is 27" long w/ an A2 fixed stock. This has been covered pretty extensively on this forum. Several folks like Squaregrouper have figured out how to make spacers. You're going to end up w/ a long LOP.

    I tried to take pics w/ the lens over the butt end to minimize parallax but it makes the other end of the tape measure seem off center. It is lined up w/ the middle of the pin.
    G3 A2 (10.25")
    View attachment 382029

    HK33/MP5 A2 (non buffered) (10")
    View attachment 382031

    HK33 A2 buffered: (10.25")
    View attachment 382030

    MP5F A3 (10.5"-10.75" depending on what you measure to)
    View attachment 382032

    All together compared: (this one the lens was over over the center of the stocks, but you can still see there is very little difference w/ exception of the A3)
    View attachment 382033

    Notice HK made a liar out of me. The metal cap HK33 buffered stock does indeed have a removable G3 style end cap. It can take the heavy HK21 butt pad w/ no modifications. I forgot these existed until I dug some out of the boxes (I have at least 2 of those.)
    You're a hero. Thank you so much.
    Yup, I'm actually aware of the other extensive threads of people making rod stock extenders. But this definitely answers all my curious questions. I really appreciate it.
     

    smdub

    Active Member
    MDS Supporter
    Nov 14, 2012
    3,875
    MoCo
    No problem.

    SG just posted an excellent solution above. Something to consider: a collapsing stock is going to be a lot more comfortable to shoot and give you LOP options than a fixed stock w/ a 2" spacer. (That is unless you are super tall like SG & I and can handle a long LOP.)
     

    Jason22492

    Junior Member
    Mar 28, 2013
    31
    Montgomery County
    No problem.

    SG just posted an excellent solution above. Something to consider: a collapsing stock is going to be a lot more comfortable to shoot and give you LOP options than a fixed stock w/ a 2" spacer. (That is unless you are super tall like SG & I and can handle a long LOP.)
    Good point, duly noted. I appreciate SG's input as well. That does seem like the most painless method to achieve it.
    You probably were able to tell however that I was trying to find possible solutions while trying to maintain the classic look of the SP5 with an HK A2 stock for aesthetic purposes (I'm sure you'd understand as a collector of multiple A2 stocks); But alas, post 2013 it does not seem possible (in MD), haha.

    On a more bright side, AR stocks have way more options; I'm sure I'll be able to find a stock brand/combination to suit my taste.
     

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