Sorry to split hairs here, but can you simply swap the upper from a Rattler LT with any of the Spear LT complete uppers?

The description says barrels and handguards are compatible with all other MCX ... "is now fully compatible with all MCX barrels and handguards making it the ultimate modular package", which I'm thinking might not mean all uppers.

But I just watched this video and it seems like they're swapping complete uppers. https://www.youtube.com/watch?v=0YLxgLnnHuM

I am after a 300blk 6.75" Rattler LT, but I'd like a complete upper for swapping in 556 11.5"

this upper: (UAMCX-556N-11B-LT-BLK) MCX-SPEAR LT 11.5 UPPER AR CONVERSION KIT 5.56 NATO-BLACK
for use on this lower: (PMCX-300B-6B-LT) MCX-RATTLER LT 300BLK 6.75"

All parts on the Spear LT and Rattler LT are interchangeable.

thanks. that's simple and exactly what I was hoping to hear.

looks like I'm going all in on Sig's gas piston rifles.  ordering a Rattler LT, MPX and the Spear very soon then.

I realize I'm in the Sig forums, but is this a good longterm setup or am I being stupid and buying trendy gimmicks?  

My state is about to ban all the guns they can, so I'm having to catch up and stock up quickly. It's been about 8-10 years since I last looked at ARs. Lots of good info in these forums. Ty all

The MCX platform is excellent and I think the Spear LT is one of the best piston systems out there and with the quick change barrel/ caliber system, it’s hard to beat. The MPX is ok, but there are better 9mm carbines out there like the B&T APC9.

Likely you are from the same state as me. I recently acquired a MPX , SIG LT , Sig Rattler and rattler LT in 7.62x39. I also bought a virtus upper conversion kit. So I'm pretty muched all set.  All are sbr'd because why not.

I’m from the republic of Wa state and they passed an AWB last year and a magazine ban the year before. Keeps on coming.

Is there going to be a Rattler LT conversion kit? Or will I need to build one from scratch for my AR lower?

n/m: ordered a complete Rattler LT pistol.