I’m a little over 30 days on my Eform’d 4 for a SRD762TI and I want to know if there’s a consensus on if there’s a “Goldilocks” length that gives the best balance of reliability, compactness, and sound suppression when paired with an SRD762TI (or similar) suppressor. I currently have the 9” 300BLK barrel. Is the 6.75” better? Does the 9” have any benefits over the 6.75” is it personal preference? This suppressor will live on this MCX 99.9% of the time. Basically a dedicated rig.

Honestly I've tried shorter barrels, 7.5 and 8.5" and while they ran ok they weren't 100%. They ran great with supers but had the occasional hiccup with subs, which is what I mainly shoot. Of course I could have tweaked the gas and springs but instead I went to a 10" and it runs like a sewing machine.  Yes it added a couple inches but I'm willing to live with that.

In your case I believe that 8.5" is optimal for 300AAC in terms of stabilization and velocity.  In other words the longer barrels don't have much of advantage. I'd stick with the 9".

8.3" is the perfect balance.

My opinion. It can be 100% reliable. Just requires a bit of tuning, but not much at all.

I think you can get perfect reliability down to about 6.5" if you're willing to put the work in AND you hand load. Powder selection gets critical below about 7-7.5".

I’m talking about factory barrel lengths for the MCX. The options are 5.5”, 6.75”, 9”, or 16” for 300 BLK and I have zero interest in the 16” barrel length. I’m basically asking if there’s any reliability difference between the 9” and 6.75” barrel lengths. I don’t reload, so I’d only be using factory subsonic loads.

Factory loads, 9".

No question.

6.75" will get a little finicky with certain subsonic ammo.   If you want to not worry about it and shoot almost anything off the shelf, then go with the 9 incher.

Awesome. I’ll stick with my 9” barrel then. It’s tempting to get my overall length down, but not at the expense of reliability/ammo flexibility.

In my opinion the 9” will be more reliable and also quieter.

it's kind of odd to hear of all these issues with barrel's under 8 or 9 inches

i have a 6" barreled 300blk that runs great with any factory ammo that i've fed it , no special "tuning" was done

i'm just getting setup now to start rolling my own for it

Non MCX I take it?

yup , forgot i was in the sig forum and not ar

it's still odd to hear the mcx with the short barrel has issues

The factory 9" MCX barrel has more dwell time compared to the factory 6.75" MCX barrel since they have the same gas port position, just like a 12.5" AR15 barrel with a carbine length gas tube compared to a 10.5" AR15 barrel with the same gas port position.

With the SRD762TI on it, the 6.75" barrel won't have any issues with subsonics.  Without the suppressor, some subsonic ammunition may have problems.  Ironically, the 5.5" barrel is less picky than the 6.75" barrel, but you have to find a short op-rod if you have a Virtus.  I like subsonics with the 5.5" barrel, and the SRD762TI suppressor.  While the windows for flipping the gas setting don't line up ideal, if you get the Sig Virtus 12" handguard that goes over the suppressor, the length of the 5.5" barrel and SRD762TI work well together.  The primary disadvantage of the 5.5" or 6.75" barrels is with supersonic ammunition, you shed a lot of velocity going under the 9" length.  A lot of people emphasize the difference in noise, but I'm not sure how many have really tried the various combinations.  My hearing is already damaged, so it may just be me, but I don't notice much difference between the 5.5" and the 9" when shooting subsonic ammunition with the SRD762TI on it.

There are indeed some over the counter subsonic ammo that won’t cycle in the 6.75 MCX suppressed.  For example:  Sig even makes a subsonic hunting round that is recommended to run in 9”+ barrels.

The LVAW has a 6.75 barrel and suppressor and the high speed guys don't seem to have issues.

Very true, but the high speed guys have very specific ammunition selection which the system is probably tuned for.

Civilian guns tend to have to work with a much wider variety of ammo from great to crap.

A lot of folks are implying that the 6.75" MCX Virtus barrel suppressed has problems with subsonic factory ammo.  I personally, have never had an issue, so it isn't hard to find factory ammo that works.  It might be helpful for others, if you list the specific ammo that you have found doesn't work.  Not what you have read about, but what you have experienced personally.

I have several of the 6.75 and 9s and they have all run flawlessly with subs and supers of all types but I only run supressed.  The only problem I have is a custom length gun cut from 16 down to something like 11 or 12 and it won't run subs.