An Otter Creek OTB brake mount for my OCM5 should be arriving today.

I previously had a barrel assembled with a SE Vortex Ops INC mount and separate shoulder piece. The sound performance was so bad, that I opted to disassemble that upper. Smudge spoke with an AEM5 enthusiast friend, and confirmed that the brake mount is required for the aft portion of the can to perform properly.

Otter Creek makes a hider mount. SE claims their Vortex hider mount was made at the request of Canadian SOF. How did no one understand the performance sacrifices? I understand why the SOF unit wanted it: flash hiders are often preferred when the silencer isn't attached. That's the same reason I opted for it. I am sure they got shelved in the arms room, once the silencer performance degradation was realized.

I guess Otter Creek makes the hider for the same reason: to satisfy customers who don't know any better (yet). The Ops/AE/OC reflex cans have a reputation for superior sound reduction; that's why people want them. It's curious that the fact that the brake is required for maximum performance isn't more talked-about.

My interest in the silencer was rekindled because of recent flash observations at night. I recall the OCM5 having excellent flash reduction, and I want to revisit it. I was not, however, willing to revisit it while the sound performance sucked. There are compromises you need to make, in piecing a gun together for use with the OCM5,and I wasn't OK with making those compromises so long as the sound reduction was crap, even if the flash was a smidge better.

Going to get it together and compare to a RC2 with the same gun/ammo in the next couple days.

Based on the mock-up I have been able to perform so far, my build plan will work out perfectly. A 9" KAK MLOK handguard on an 11.5" barrel will have about 1/8" clearance behind the back of the can. I hope the flash and sound performance is what I hope it'll be.

This message has been edited. Last edited by: KSGM,

I compared the OCM5 to the RC2 last night. 16" mid gas, shooting 55gr XTAC. Three rounds, observed from an offset position, slightly forward of the muzzle, through a PVS14.

The OCM5 was quieter and had less flash. It was quieter from the shooter's perspective as well, according to my wife.

The OCM5 is 1.5oz lighter as well. I weighed them both with their brake mounts.

The OAL of the weapon is very similar; I'd guess it's within 1/2". I'll confirm this and revise this portion.

The drawback to the OCM5 is the fact that the mount interface requires more exposed barrel forward of the handguard. This means it's harder to push your light out, in an effort to reduce shadow and splash. This also means that the user would need to use a shorter handguard than they may otherwise prefer, on a given barrel length. Some may opt to put the silencer beneath a handguard. I would not choose to do this.

I don't know that I consider the mount itself to be inferior to the RC2. There is no redundant latch, but it has a good gas seal. The fact that my particular RC2 barfs gas out the back doesn't help it in this aspect of the comparison. The OCM5 is not as quick to the install or remove, but it is still what I'd define as a QD mount.

My LGS has a cheap-enough 11.5" barrel, that I am going to give it a try for a while. My current setup is a 12" mid length FSB with a 9" rail. The OCM5 setup would be the 11.5" with carbine gas, using the same rail, with a low profile gas block. That being said, the position of my flashlight relative to the end of the can will actually work out to be near-identical, which is cool.

All this to say that the brake mount for the OCM5 seems to have rekindled my interest in it. It's performance (outside of the context of extreme use) is seemingly superior to the RC2.

*3/30/24* I ran into a clearance issue with my light mount, when it came time to mount the silencer on the build. I'll need to have the light mount slightly modified.

I was also reminded why I hate low-pro gas blocks. It's unfortunately non-negotiable on this project, due to the length of the can. The barrel has a dimple opposite the gas hole. The block has two set screws. The rear set screw lines up perfectly (depth-wise) with the dimple. However, the dimple is not at perfect bottom-dead-center, resulting in slightly canted block. That shit drives me bananas.

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I got the gun together; I have not test-fired it yet.

I ended up getting a TREX Arms MLOK Pro Lightbar, in hopes that it wouldn't need modifying, but knowing it was a 50/50 shot. It still needed modifying, but I ended up with a light that was pushed out more, which is a good thing. The light bezel is just under three inches from the end of the can.

OAL of the upper is unchanged, when compared to the RC2 on the previous barrel. However, this new barrel is ~3/8" shorter. So, the OCM5 with the Otter Creek OTB brake adds 3/8", when compared to an RC2.

I hope everything works. I decided to try a YHM suppressor gas block. It's a low-pro set screw block with a reduced aperture. So, a tuning approach similar to a BRT tube. I am hoping for proper function without having to change the spring and/or buffer. If it runs good, I'll likely send it out for pinning.

I know the RC2 is louder and flashier, when compared on the same gun. This build has the OCM5 on an 11.5" carbine gas; the previous setup played host to the RC2 on a ~12" mid gas. It might be wash, but I am pretty sure it's going to be quieter and less flashy. Hell, I could half-ass build that former upper, and shoot them side-by-side. I may do that.

How much barrel forward of the gas block do you need for traditional mounting?

Depends what you mean by traditional. Each collar has different dimensional requirements, as far as I can tell. There's no less than three different collars for a two-piece mount setup, and then there's the OTB.

https://ottercreeklabs.com/pro...zzle-devices-mounts/

I did test fire mine a bit ago. Works good. Recoil impulse seems normal, fed reliably, and locked back on empty. I didn't pay any attention to ejection pattern, and I didn't have a full mag to start off. So, I'll do another test run, to make sure it can power through feeding off a full mag, while paying more attention to ejection.

I did notice it was gassier than the previous RC2 setup. So, the OCM5 has higher backpressure characteristics, but I suppose that comes with the better sound and flash performance that I am after. Oh well.

I didn't think to measure the port in the YHM block, before I assembled the upper. I should have. Internet research puts it at .055", for what that's worth, when considering this project.

Zeroed my optics to the rifle yesterday, fed from fully-loaded 20rd and 30rd mags, and noted ejection patterns. All is well.

I am very pleased that I didn't have to play with the spring or buffer in my lower. I have an H3 and Sprinco Red, and would like to keep it that way.

It ejects at the 3-4 O'clock position.

So, I reckon I lucked out, when it came to all the stuff working well together.

This is my first-ever rifle that doesn't have iron sights installed. I do, however, have a zeroed set in my equipment. I also have no shortage of aiming redundancy: The Elcan Specter has obtuse irons on top of it, I have the Eotech zeroed to the rifle, and a Leupold scope is zeroed to it too.

For those interested, page four of the Surefire silencer thread has a good bit of OCM5 vs RC2 comparative discussion, and gets into the idiosyncrasies of the OCM5.

https://sigforum.com/eve/forum...858/m/9550067994/p/4

I think the OCM5 is a bust. My attempts at further integration proved unsuccessful. I suspect the high back pressure nature of the silencer is a big part of it's troubles. It's the quietest silencer I have; it's the least flashy silencer I have; but it's cons seemingly outweigh those pros.

So what are the cons that make it a fail? Are you getting gassed out? Is it the recoil impulse?

The meat of it is here:

https://sigforum.com/eve/forum...0105858/m/9280040905

Bottom line is the high backpressure and handguard length compromise. I am confident that the high backpressure is causing significantly more heat buildup and lube burn-off.

I'll put the OCM5 aside for when I inevitably get a MK12 rifle of some kind, years down the line.

It is gassy, but that's not too problematic.

Doesn’t the collar flush up against a sight base on a 14.5” barrel? Maybe a carbine gas 14.5 with a rifle speed gas block?

The first build I ever used the OCM5 on was actually a mid gas 14.5". I am thinking about re-assembling something similar. The adjustable gas block wouldn't help; this last build was properly gassed, and the high backpressure still causes problems. A longer gas system combined with a regular port and vented carrier might help to mitigate the backpressure problem.