Hey guys, just got my SB MPX brace a few days ago.  Installed it per instructions (not complicated).  Took it to the range on Sunday and noticed after about 30 rounds that the brace had loosened despite the dab of loctite used on both set screws.  Got home later that day and removed the brace; the set screws are gouging the aluminum lower pretty badly.  I don't want to torque these things don't for fear of permanently damaging the area that prevents other attachments from sliding over.  

So... it appears that the braces designed for MPX/MCX compatibility are completely different than the prototype photos shown last year.  I believe this was confirmed by SB Tactical themselves due to issues with the machine shop charged with putting out the attachment blocks.

But, the design of the new block, while simpler isn't ideal.  If you don't tighten the set screws, the brace will end up moving loosening. In my case, there was quite a bit of lateral movement up and down as the large bolt isn't wide enough to fill the channel it rides in.  I'm curious as to how you guys with the same setup are counteracting this?

Ideally, the best solution would be for SB Tactical to go back to their original design.  It's more secure and won't fuck up your receiver.  Thoughts?

I didn’t like the SB mounting option either. Took the original Sig Sauer version and the SB tactical arm brace itself and replaced the Sig Sauer rod and butt stock. Same thing Sig Sauer is putting on their pistols now, the telescoping arm brace.

https://www.sigsauer.com/store/sig-mpx-k-psb.html

Does it wobble at all?

I was thinking about that.  So based on what you said, the MPX brace rods will work fine with the Sig attachment block?  Do you know if they sell that block separately?

How much are you tightening the set screws?

That's what I did as well.  Also used blue loctite.  Stock still works itself loose after a while.  

Does your stock have a lot of play in it with the setscrews removed?  In my case, if the setscrews aren't tightened down, the stock can slide up and down until the cross bolt contacts a rail wall.

OK.  Thanks for letting me know.  I guess I'll clean up the threads, add some more loctite and let it set for a while.  If that doesn't work, then I guess I'll call Sig and see if they'll sell just the mounting block.

I added more blue loctite... some of it actually came out and coated the inside of the block and picatinny rail.  Grub screws were also hand tightened.  Not much... just enough equal pressure on both screws to preven the block from wobbling.  Finally added some blue loctite to cross bolt.  Let it set for at least 24 hours.  Hasn't budged a bit.  Ran about 100 rounds through it last Sunday.  Was still tight.  Also whacked the stock a bunch of times with my palm from the sides, top, and bottom... still very solid and did not loosen up.

I still don't like the mount design though so I called Sig today to see if they sell just the block that they're using on the MPX K PSB.  Turns out that they don't.  Rep said that I could buy the regular telescoping stock and use that block instead as he says they're the same.  He also suggested waiting a bit since the parts are still new, that they may sell them separately over time.

We'll see I guess.

Well whadya know?  SIG has started selling the brace, but with their 1913 mounting block instead of the POS SBT block...

https://www.sigsauer.com/store/collapsible-stabilzing-brace-mcx-mpx.html

Should've waited...

Anyone want to buy my SBT brace?

I got a good deal on an a genuine SIG collapsible stock.  My plan is to swap the rails and brace end from the SBT block over to the SIG 1913 block.  Question is, how do the rails come out of the Sig collapsible stock?

And when I get my tax stamp back, I'll have the regular collapsible butt pad ready to go.

Just get the stamp and SBR the thing save yourself the headache. I did the arm brace thing it was a waste of time. the wobble The screw coming loose what a pain!