I just picked up a 9" Virtus Pistol. NIB its a PMCX-300B-9B-TAP .

I would like to get a 9" 5.56 barrel for it -HOWEVER  I also want to get a rattler length 5.5" barrel for it 300 or 5.56 and see SIG has a rattler 5.56 conversion kit on the website.  But says it won't work with an MCX VIRTUS "you need a rattler" Rattler 5.56 barrel conversion kit

Whats the scoop - It won't work?  I understand they have some gas mitigation for 5.56.  Curious why not?  Can the gun I acquired take the 300 AAC Rattler barrel conversion okay? or are their specific other rattler dimensional considerations.

I like the 9" length fine but certainly wouldn't mind having a short conversion to 5.5"

PS where are any 5.56 conversions in stock?

The Sig Rattler upper will not be compatible with the Virtus because if you compare the semi-monolothic upper lengths, they're different, in addition to the piston rod lengths not being long enough and therefore incompatible. Because of the top rail length differences, I don't think there is clearance with the gas block. In terms of the 5.5, it's actually about 1.75" longer (with the muzzle device). The muzzle device is actually threaded into the gas block and pinned, so the extra length is a result of that. From what I can see on mine, it looks like the barrel ends inside of the gas block. I've never seen this design before. You'll also have to get the 5.56 compatible handguard because the 300 BLK Rattler handguard will be too short and have a cutout within the handguard that will still work, but look awkward. You have to change the 5.56 Rattler gas from the front and not the side.

Osage County Guns had the 5.56 5.5" conversion I just snagged from them a few days ago. If you're willing to wait, till they get them back in stock, you're looking at paying about $100 less than every other website (including Sig).

Your best bet in this situation is to just get a Rattler 5.56 complete upper.

Do believe @coregon mentioned you actually can use a Rattler barrel in the Virtus but you'll be required to get the Rattler carrier op rod connecting point to work with it. You'd also need to use a suppressor use handguard with a can attached, as it will be too short for any MCX Virtus handguard.

If I were you I'd get the 6.75" MCX Virtus barrel and the Midwest industry Handguard for the same and call it good enough. several folks have done what you're suggesting and it seems like it's more of a pain than it's worth. Alternatively, I'd recommend getting a Rattler Upper assembly.

This is correct.  A Rattler barrel (300BO or 556) will work in a Virtus, but you’ll need the correct op rod.




This is good advice.

Thank you all for your help.  I really appreciate it.

I'm doing the 6.75" path myself. Just need the handguard but that will be a long wait.