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MCX Rattler options

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I recently purchased a Sig MCX Rattler in 556. It wasn’t my first choice, but with the recent developments in Oregon I didn’t have much in the way of options.

From what I’ve been able to discern, other than doing a the 300 blackout conversion, there aren’t really any options for a longer barrel. Is that correct? The other MCX variants are not compatible? If that’s the case, then 300 blackout is really the only option if I wanted to suppress it.

What about other parts? How much of the Rattler is compatible with aftermarket Ar-15 parts? I know the charging handle is different. What about the bolt release, safety selector, mag release etc? Other than proprietary replacement parts, what are my options?

Any recommendations, tips, or resources would be greatly appreciated.
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I use a Radian lower with a Radian mag catch & bolt release with a Sig trigger, and it works flawlessly with my Virtus upper. Dedicated AR triggers will work the the Spear LT (which I understand you don't have, just saying) but not with previous iterations of the MCX as far as I know.
 

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The rattler has a specific op rod. If you can get a standard op rod from sig you can use any of the other barrels without getting a whole new upper. There's a couple of threads that covered it but I'll have to search.

Other issue is it's got the short upper so you're tied to "rattler" length handguards unless you're ok with a gap in your top rail.

The mcx lower hasn't changed much (minus some different buffers and the swap from the plunger style upper tensioner and the bolt catch on the LT) between mcx gens. I've run legacy, virtus and rattler uppers on the same mcx lower and a hybrid conversion lowers with no problems.

If you're willing to remove the firing pin block on the carrier, you'll have more options on which triggers will work. Your choice.

I've used a few different AR safeties on mcx lowers without an issue. Haven't personally tried swapping the bho and mag release but have used the mcx versions on other AR lowers without issue.

New upper is easy to do but not the cheapest route. I've got a 16" 556 and a 9" 300 virtus I'd let go. Osage has them pop up occasionally. GB too.
 

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@littlejacks is spot on! Just remember if you build off what you have you may save but it will look re-tar-ded with Rattler handguards and the Rattlers gas cutout in the handguard for quick gas adjustment won't line up. Your better off getting the upper and setting it up how you like and dropping it in.

I was just about to do the OP Rod / MCX Caliber X-Change kit for my Rattler but after researching, I would've spent alot and not got the look or functionality I wanted. I said fukit and bought a SIG WRMCX-16B-TAP-P Friday. I will now search for the 9" .300BO and 11.5" 5.56 X-Change kits and have a few options. I would like another upper but having a Rattler and Virtus makes it hard to justify.... Maybe that MCX SPEAR LT 7.62x39 upper would be the perfect in-between.
Here's the math that forced my hand....
https://osagecountyguns.com/sig-sauer-mcx-recoil-assembly.html
$309 + tax
Sig Sauer - MCX Virtus 11.5" Caliber Exchange Kit - 5.56 #SIG WCALX-MCX-11B-556-TAP
$599 + tax
Then add the $150-$200 for a new handguard ... GTFO.... $1200ish bucks....
Or you can search and be patient for the upper brother!
SIG SAUER MCX 5.56 Nato Upper Receiver WUAMCX-11B-556-BLK-TAP Color: Black, Caliber: 5.56x45mm NATO, w/ Free Shipping
Good luck whichever way you decide.
 

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Discussion Starter · #10 ·
I'm kind of stuck, and can't come to a decision. I know I don't really want a 5.5" 556. I'm not familiar enough with the 300 BO to have an educated opinion on the 5.5" barrel. I wouldn't mind a Canebrake upper, but I don't even know if they sell those separately. Even if they did, the cost of the uppers is almost enough to get a complete new gun.

I want to keep it as compact as possible, so I guess I either go with a 300 BO in 5.5" or 8"ish barrel or jump to 11"+ for a 556. For now, I've had to slow down with Xmas coming up but my wife has a full list of parts I "need", so fingers are crossed.
 

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One benefit of the Rattler is it's a pistol... Later on when you buy different uppers they can go on the Rattler lower if there below 16" inches and you can take them wherever you'd normally take a pistol. The Rattler was my first MCX and I bought it knowing I couldn't afford .300BO rt off the bat but it's such an important gun IMO. What it's capable of is so niche it's almost priceless to me. You did good bro!
 

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Discussion Starter · #13 ·
One benefit of the Rattler is it's a pistol... Later on when you buy different uppers they can go on the Rattler lower if there below 16" inches and you can take them wherever you'd normally take a pistol. The Rattler was my first MCX and I bought it knowing I couldn't afford .300BO rt off the bat but it's such an important gun IMO. What it's capable of is so niche it's almost priceless to me. You did good bro!
I feel a bit better about where things are at. I appreciate the input. 556 is more affordable to plink with, and gives me something to work with while I work up to the 300 BO, and suppressor. It definitely checks all of the boxes with regards to what I was after.
 

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Bro I have a 5.5" 5.56 and I was stumped too but M855A1 does just fine at lower velocity it's just hard to find. There's probably a few rounds to choose from but the selection is thin. Once I start buying and shooting more .300BO I'm sure I'll want that 5.56bbl Conversion kit for my Rattler.
 
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