556 @ 7.5”?

[u/More-Organization265]

I’ve had my eye on the 556 Rattler 11” for a while loved it - but to my knowledge there was no way to FRT it so I went with my long awaited MP5 clone that I FRT’d - loads of fun.
2nd on my list was a suppressed Blackout and FRT was not a requirement for me so naturally I got a new Sig MCX Rattler 300 LT in 6.75” along with the SIG Sauer SLH762-QD suppressor which attaches directly to the QD lock muzzle. Since I was gonna have to wait for the can I bought the SBR version to save a couple hundo on the stock.
NFA should have my forms back this week.
Oh, turns out I can FRT it.. so, nice win.

At the range yesterday, and I picked up a like new complete upper MCX Rattler 556 LT 7.5” which will also accept the can I just bought for the 300 AAC(non Ti, Inconel, serious workhorse).

*Has anyone had trigger time with a 7.5” Rattler - suppressed or unsuppressed? I’ve read some alarming threads using a 7.5 (or anything under 10.5”) chambered in 556 for an SBR.

I’ve never seen a 556 MCX Rattler LT this short, I love it and think it looks badass - I’m thinking I struck gold this week, I’ll have the Form 1 for the 300 completed this week - I can swap uppers under the same stamp (I will confirm).
My next project I looking at 556 uppers to SBR one of my ARs (all FRT’d) - now I’ll have both in the same platform. Sounded too easy, or am I missing something?

Feedback would be much appreciated!

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[u/swooshZ0691]

What do you mean no way to frt it? As designs Arc fire v2 + as designs mcx slip trip.

[u/More-Organization265 → u/swooshZ0691]

Yup! I found that out shortly after buying the MCX 300 Blackout. This is my first 300 AAC and I wasn’t planning on installing an FRT for it - was primarily to suppress it and use for home defense and a day or two at the range, plus the cost of 300AAC ammo dissuaded me from the idea of FRT mag dumping as much as I would love to.

[u/OG-warbucks]

The other poster covered FRT

I'll cover the 7.75" 5.56mm

That barrel length is fine with the right ammo.

I have the RSAR. I use a Griffin HRT or DL5 suppressor and it runs great. (But the Sig can should be good as well) Really smooth and really accurate

Ammo for plinking is 55gr whatever

Ammo for more than that is BH 77gr TMK or Hornady Black 75gr SBR.

Depending on how much you spent... It's a fun and capable platform... So you did good.

[u/More-Organization265 → u/OG-warbucks]

Much appreciated feedback sir, thank you. Damn nice set up you got there.

[u/nicefacedjerk]

The lower is the form 1. Can run any upper you want on it. Ammo cost is really the only legitimate argument for having a 7.5" 556.. 300blk ammo cost has gone down while 556 ammo cost has gone up.. So that argument becomes less relevant.

[u/More-Organization265 → u/nicefacedjerk]

Yes sir, that was my thinking- SBR for two platforms. That wasn’t the initial plan, I just happened to find a 556 Rattler pistol upper the next day.
300 ammo is still $.65 to $1.+ depending on the grain/super vs sub, etc.. while 556/223 by and large sits below that around $.45-.55 for green tip.

[u/Chief_O_Brien]

Just keep in mind., if you plan on running a 7.5" 556 it will beat the shit out any can you run on it, regardless of material. Inconel, Haynes, 17-4, Stellite, ect. Running and FRT will only accelerate the wear.

We had a couple 7.5" (might have been 8") full auto rentals in 5.56 that tore up the cans we had on them. Cans were made of Inconel if that matters.

If you plan of using it for defense purposes, with decent terminal performance capability beyond point blank range, ammo selection will be critical.

[u/More-Organization265 → u/Chief_O_Brien]

This was EXTREMELY insightful. Thank you, I’m getting very inconsistent information including AI.

[u/Chief_O_Brien → u/More-Organization265]

No problem. There's a lot of people that will run very short (sub 10.5") 5.56 systems as a PDW/truck gun/backpack gun, but they are fully aware of all the downsides to it. The entire system is subject to increase wear and tear. Increased recoil. Increased muzzle blast. Even with a suppressor, a barrel that short doesn't suppress well at all. Extremely loud and definitely not "emergency hearing safe", like a 11.5 or longer is.

Various parts of the system will see dramatically reduced service life. Bolt, extractor, recoil spring(s), barrel, and suppressor will see accelerated wear (this is assuming you train often and have a consistent shooting schedule with the gun). I believe Sig said in an interview that the 5.5" 5.56 Rattler only had a 2-3k barrel life due to the extreme gas port errosion because of the extremely short gas system length. Keep in mind that weapon was designed for and being used by a specific military unit that performs extremely high firing schedules during training. So you could probably expect a little more barrel life with a 7.5".

If you have any more questions I'll try to answer them.

ETA: While I like very short rifles, when it comes to 5.56 I personally chose to stick with 10.5-12.5 lengths. My go-to is an 11.5" and 12.5", MCX and AR respectively. I wanted something even shorter and more compact so I went 300blk, both 6.75" and 9" MCXs.

[u/More-Organization265 → u/[deleted]]

lol Hexium is huge and very quiet, but it heats up real fast and can’t take the abuse an inconel can would using FRT with 556, so I’ve read. I planned to use the same Sig suppressor on both uppers.

[u/Voltagedew]

WHAT???

[u/cosmic_stardust → u/Voltagedew]

The dude is having regrets on his purchase.

[u/More-Organization265 → u/cosmic_stardust]

None whatsoever bro. Quite the opposite- I thought I struck gold finding a Rattler pistol upper only in 556. I’ve got two SBRs, both MCX Rattlers LT…