Just bought a Sig rattler upper in .300 BO. I know I can also convert it to 5.56 with the right barrel, but are those barrels proprietary to the Rattler or can I use Virtus barrels? Also, what’s the threading on the .300 BO rattle barrel? Anything else I should know?

quote:

Originally posted by dwd1985:
Just bought a Sig rattler upper in .300 BO. I know I can also convert it to 5.56 with the right barrel, but are those barrels proprietary to the Rattler or can I use Virtus barrels? Also, what’s the threading on the .300 BO rattle barrel? Anything else I should know?

The muzzle thread is 5/8x24. Standard 30 caliber threading.

Not sure about the standard Virtus barrels. The Rattler handguards seem to be specific to the caliber of the barrel assembly because of gas block size.

I could be mistaken since its been about a year since I went this route. Im fairly certain I went with the 6.5in Virtus in 300blk because I wanted be able to use other Virtus barrels and handguards if need be. The extra 1in barrel was actually preferred so it was no brainer.

To possibly answer your question, I think the Rattler barrel and handguard are specific to the rattler line. The handguards for sure.

Obligatory pic:

Think of it as it's own eco system. The size of the upper is different than any other in the MCX lineup. This is due to the size and length of the piston and gas system.

Your Rattler upper can be used with any of the MCX lowers.

You are only able to use either the 5 inch 300 Blackout or 5 inch 556 barrel on the Rattler. The only handguards that are compatible are the ones specifically made and listed for the Rattler. So if you choose a Midwest Industry handguard, make sure to get the Rattler version.

Also, if you get the SD handguard that comes with the Canbebrake Rattler, it only fits on the Rattler. It cannot be used with either the 6.75, 9, or 11 inch barrels.

Hope that helps, let me know if you have other questions.

Can you use a basic AR Mil spec trigger in a rattler?I dropped my Rattler upper onto a standard AR lower with the Sig lower adapter. However I keep seeing that full Rattler conversion kits come with a trigger as well, and since I haven’t yet fired the rattler upper I’m wondering if it will work with my Mil spec trigger that’s already installed in the lower I dropped it onto. Anyone know?

quote:

Originally posted by dwd1985:
Can you use a basic AR Mil spec trigger in a rattler?I dropped my Rattler upper onto a standard AR lower with the Sig lower adapter. However I keep seeing that full Rattler conversion kits come with a trigger as well, and since I haven’t yet fired the rattler upper I’m wondering if it will work with my Mil spec trigger that’s already installed in the lower I dropped it onto. Anyone know?

I answered my own question at the range today. No, a basic Mil spec trigger will not work with a Rattler upper. All I got were a series of clicks.

quote:

Originally posted by 4.40s1nine:
Think of it as it's own eco system. The size of the upper is different than any other in the MCX lineup.

You are only able to use either the 5 inch 300 Blackout or 5 inch 556 barrel on the Rattler.

The upper is not a different size. The op rod in the Rattler bolt carrier is shorter to pair with the shorter handguard and gas system as you mentioned.

You can use a Virtus barrel if you change out to a Virtus bolt carrier with longer op rod. Obviously longer handguard as well.

quote:

Originally posted by dwd1985:

quote:

Originally posted by dwd1985:
Can you use a basic AR Mil spec trigger in a rattler?I dropped my Rattler upper onto a standard AR lower with the Sig lower adapter. However I keep seeing that full Rattler conversion kits come with a trigger as well, and since I haven’t yet fired the rattler upper I’m wondering if it will work with my Mil spec trigger that’s already installed in the lower I dropped it onto. Anyone know?

I answered my own question at the range today. No, a basic Mil spec trigger will not work with a Rattler upper. All I got were a series of clicks.

Thanks for posting that information DWD1985. I thought I had read that it would not work, but could not find the source. I did not want to post a guess. I like to either have personal experience with something or be able to provide a source for the information that I post here in regards to questions.

It would be interesting to find out what the difference is between the Sig trigger and a mil spec.

quote:

Originally posted by ah3:

quote:

Originally posted by 4.40s1nine:
Think of it as it's own eco system. The size of the upper is different than any other in the MCX lineup.

You are only able to use either the 5 inch 300 Blackout or 5 inch 556 barrel on the Rattler.

The upper is not a different size. The op rod in the Rattler bolt carrier is shorter to pair with the shorter handguard and gas system as you mentioned.

You can use a Virtus barrel if you change out to a Virtus bolt carrier with longer op rod. Obviously longer handguard as well.

Sig has been really great at creating confusion within their product lines. Where a compact barrel for 320's is 3.9 inches in length, but not when there is an "X" in the name. For some reason it means 3.6 inches in length with the X. Yes, that information can be found, but if someone just goes by the Compact name, they can run into issues.

Same thing with the Virtus and Rattler. You can go to Sig's website, and go to the parts section. From there select Conversion Kits. Click on any of the links for the longer MCX Virtus kits, and click on the Questions/Answers.

You will see the question about compatibility with the Rattler multiple times, and with each of the different barrel lengths. Each time Sig's customer service replies that the longer barrels will not work with the Rattlers. They go into varying details for the reasons they are not compatible. Among those is the information about the difference size uppers, as well as differences in op rods, hand guards, and how it handles gas. You can use this link to research further:

https://www.sigsauer.com/mcx-v...change-kit-5-56.html


I wanted to include some pictures as well so that the differences can be a little more clear.

First one is an overview of the Virtus and a Rattler CaneBrake, the handguards, and 3 different barrel lengths.



Next up is a closer view of the uppers. Rattler is on top, Virtus on the bottom. I did not measure the exact difference, but I think it is obvious there is a difference.



Next up is the Virtus handguard for the 9 inch 300 Blackout barrel, next to the Rattler upper.



Now a closer view where you can see that there is a gap between the handguard rail and where is would line up with the upper's rail. The handguard is not an SD variant, but I think you can see what I am referring to.



Next is the side view of both rails and the Virtus handguard top rail terminates quite a bit before the rail of the Rattler.



Final picture is for anyone considering trying to buy a CaneBrake SD handguard in hope that you can use it on Virtus upper.



It can't go far enough back towards the receiver to engage the front take down pin.


I really do hope that this helps anyone considering either firearm to have some information on the compatibility between them.

Hope you have a good night, be safe.

Great info 4.40s. While the info is fresh in my head, I’ll expand on what I learned about Rattler triggers.


As with everything else Sig they’ve made it incredibly complicated. To figure out what trigger will work in your rattler you need to look at the bolt latch. Bolt latches marked with a “G” can use a Mil spec trigger or a Geissele trigger. Bolt latches marked with an S must use the Sig duo trigger. Here’s a picture of the part to check: https://imgur.com/a/qiI2kzS

Reportedly Geissele triggers can also work with S marked bolt latches but may cause damage to the bolt latch. I can confirm for sure that my bolt latch is S marked and a Mil spec trigger did not work in it. I have a duo trigger on the way now.

Good info, thanks!

4.40, I found an arfcom thread where someone successfully converted a rattler to use a 5.56 Virtus barrel using an MCX recoil assembly and MI handguard. However your pics seem to prove regular Virtus Handguards don’t work (not sure why the MI one does). Anyway, can you see if the canebrake handguard on the rattler upper works over a Virtus barrel?

quote:

Originally posted by dwd1985:
4.40, I found an arfcom thread where someone successfully converted a rattler to use a 5.56 Virtus barrel using an MCX recoil assembly and MI handguard. However your pics seem to prove regular Virtus Handguards don’t work (not sure why the MI one does). Anyway, can you see if the canebrake handguard on the rattler upper works over a Virtus barrel?

Happy to help out with that. But first, thanks for the additional information about the markings on the bolt latch. Both of mine are marked with the S.

I would be very curious to see what was done to "convert" the Rattler to use the Virtus barrels. Maybe the person that did the conversion has a different generation Rattler because neither of my Virtus barrels would fit into the Rattler upper assembly.

I tried the 9 inch 300 Blackout barrel first. Right from the start of inserting the barrel, there was a noticeable difference in the tightness of the Rattler's hole. I really tried thinking of a different way of explaining that, but it just fit. It felt the same way with the 16 inch 556 barrel. They also came to a stop at the same point. I did try inserting the Rattler barrel into the Virtus upper, and it did feel looser than the Virtus barrels.



That is a picture of the 9 inch barrel. That was as far as I could insert it. I am pointing at the protrusion that needs to go through a little channel cut on the inside of the black part to the left of it.



That is pointing at where that protrusion needs to go for the barrel to be seated and then locked in with the barrel clamp screws.

Now on to the question about the hand guard. I am fairly sure it will fit. With not being able to screw the barrel clamps screws down to lock the barrel in place, I was not able to fully seat it.



That was as far as I wanted to go with the hand guard. I had compressed the barrel clamp with one hand and then I was able to get it into the position shown in the picture. It stops at the first fully extended clamp screw.

So, if you are able to get the Virtus barrel fully seated using the modification from the post you found, and you are able to get the clamp screwed down, then I did not see anything else preventing the hand guard from getting to the correct position.

Now having said all of that, it brings up a question.

If you have a Virtus barrel, the longer recoil assembly and bolt, and a charging handle (I am assuming that last part), aren't you just missing the correct upper assembly? Why wouldn't you just buy the correct upper assembly for another 450$? Those parts alone are easily over 1000$. Then you actually have a complete factory upper that you can swap out.

Contrast that to modifying the barrel or the upper assembly, or maybe even both. Is it a permanent modification, or can it be restored to factory? Throw in using parts that aren't sized for the Rattler. The longer recoil assembly creates a situation where the end that interacts with the piston is basically out in space. It literally sticks out in front of the Rattler's assembly.

I would be concerned with things like increased wear, possible damage from the way the piston hits that floating end of the longer recoil assembly, and just basic reliability and functioning.

All of those things to save 450$ by not purchasing the correct upper assembly. It just doesn't make any kind of sense to me. Wait, if I already had 10 or more different Rattlers and other Virtus variants, and I had an unlimited budget, and if I already had a healthy stockpile of back up parts for each of them, and if I wanted to see if it could be done, then sure, I could see maybe doing it. But, that is just me.

Final question for you, and it is a serious one, when you said the MI hand guard was being used, did they say whether or not they used the Rattler hand guard from MI, or did they use the other MI variants?

Hope this helps. Be safe.

Thank you for the pictures. I’m quite surprised you’re having an issue fitting a barrel as I have now read multiple accounts of people successfully converting their rattlers. The rattler upper is (should be) identical to an MCX Virtus upper that boy with a shorter top rail to accommodate for the shorter piston system. Locking system should work the same as a Virtus. No clue why your barrels aren’t locking up the same way, my only guess would be something to do with Sigs various generations of the MCX not all playing nicely.

To answer your question as to why, it’s quite simple: complete 5.56 uppers are going for $1800+. I already own a rattler and an extended handguard. This means that all I need is the longer recoil assembly and a barrel, which should cost me $800 or so. Why spend an extra $1000 to get an identical capability?

No problem at all. I would not be surprised at all about it being a generational thing. I wonder if anyone has converted one with the latest gray upper.

I probably did not explain it very well. I am not saying purchase a complete upper. I am saying purchase just the assembly which is only 450$ directly from Sig. So with the 800$ for the barrel and recoil assembly, plus 450$ for the upper assembly, (shipping and tax not included) you are under 1300$ for a complete upper you put together.

I’d be all about that but that’s not possible due to parts availability. Sig parts aren’t in stock anywhere to do a build your own MCX upper.

These were in stock a couple of weeks ago, and they had been for quite awhile. I did the stupid assumption thinking that they were still in stock when I posted, sorry about that.

https://www.sigsauer.com/upper...-gen-2-semi-blk.html

I have never dealt with this company, but they have the black listed as in stock.

https://scorpiustactical.com/s...iver-assembly-black/

I was able to get to the checkout with it still showing in stock.

Thanks for the links but to build yoir isn you need:

Upper

Barrel

BCG

Handguard

Charging handle

Buying all of those parts individually (if you can even find them) would be a drastically higher cost than just buying a complete upper. I’ve never even seen a complete MCX BCG for sake anywhere.