Groups open up significantly with suppressor

TSloper

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LWRCI M6A3 (16.1”)
AAC M4-2000 Suppressor

I’m able to achieve consistent groups (1 MOA to just under 1 MOA) when shooting without the suppressor at 200m. At 100 yds with the suppressor the groups are worse than my 200m groups.

I’m not sure why this is happening. Charge weight off for suppressor? Thermals? Harmonics?

Any guidance on diagnosing would be appreciated. The alignment is perfect when mounted. I have a Geisselie rod to check. No indications of a baffle strike.

My first inclination to sweep the charge weights and see if that is the culprit.

Thanks,
Tim
 

nn8734

nn8734
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What direction is your brass ejecting when suppressed vs unsuppressed? Should be 3-4 o’clock..

What ammo are you running?

Any pressure signs on the brass?

After firing a round or two, is the suppressor loose on its mount? 51 tooth mount, right?

AAC cans add a lot of back pressure. My SDN 6 can’t be run on any of my ARs without an adj gas block...additionally, those mounts suck as they often dont allow the locking tab on the can fully lock up (teeth are mistimed).
 
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Cascade Hemi

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LWRCI M6A3 (16.1”)
AAC M4-2000 Suppressor

I’m able to achieve consistent groups (1 MOA to just under 1 MOA) when shooting without the suppressor at 200m. At 100 yds with the suppressor the groups are worse than my 200m groups.

I’m not sure why this is happening. Charge weight off for suppressor? Thermals? Harmonics?

Any guidance on diagnosing would be appreciated. The alignment is perfect when mounted. I have a Geisselie rod to check. No indications of a baffle strike.

My first inclination to sweep the charge weights and see if that is the culprit.

Thanks,
Tim
The M42K mount allows movement of the silencer on the end of the barrel.
 
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TSloper

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What direction is your brass ejecting when suppressed vs unsuppressed? Should be 3-4 o’clock..

What ammo are you running?

Any pressure signs on the brass?

After firing a round or two, is the suppressor loose on its mount? 51 tooth mount, right?

AAC cans add a lot of back pressure. My SDN 6 can’t be run on any of my ARs without an adj gas block...
I have a brass catcher mounted so wasn’t able to witness the brass direction.

Reload... I’ve never seen a smidge
What direction is your brass ejecting when suppressed vs unsuppressed? Should be 3-4 o’clock..

What ammo are you running?

Any pressure signs on the brass?

After firing a round or two, is the suppressor loose on its mount? 51 tooth mount, right?

AAC cans add a lot of back pressure. My SDN 6 can’t be run on any of my ARs without an adj gas block...additionally, those mounts suck as they often dont allow the locking tab on the can fully lock up (teeth are mistimed).
Great questions. Rather than potentially ending up on the wrong track I’m going to spend some time this Saturday at the range making sure I answer the questions as accurately as possible.
 
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nn8734

nn8734
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Sounds good @TSloper . Also note any dramatic increase in bolt carrier group speed when suppressed vs unsuppressed.

A priori, your can may be coming loose on its mount creating irregularities in barrel harmonics which would explain the group sizes when shooting with the can. Confirm it is on tight after each round fired for at least 5 rounds...

Let us know what happens...
 

nn8734

nn8734
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@TSloper One other thing....since you hand load, if you have a dial caliper or one-inch micrometer, take it to the range with you. Measure a few unfired cases before firing the rifle with the suppressor then measure those same cases after firing. If you don’t they are usually available and inexpensive at places like Harbor Freight

You are measuring the case body diameter just above the case head. Record the first and second measurement and compare them. If the second measurement shows expansion greater than .002” that load has too much pressure. Excessive case body expansion is considered a leading pressure indicator and will usually show up before things like cratered primers or extractor marks on the brass.

One more thing to rule in or out in diagnosing the enlarged group size
 

Snuby642

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For center fire rifles I have 2 cans from AAC 7.62 and 5.56.
Mixed hosts and cals, bolt and gas.

Have mixed results on accuracy as 2, 5.56 hosts and 1, 7.62 host
does not like can on and the rest of my hosts shoot better with them on.

Powder charges have failed to fix them, so on the gas guns seems AGB's
are the way to go. Have run h2 bufffers and going to get flat wire springs and h3
to test asap.

I was hoping it was just harmonics but think the nasty brass is a clue.
 

nn8734

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@Snuby642 - I’m in the same boat...all my AR’s have adj gas blocks except my SR25 which doesn’t seem to need it as long as the load is not over pressurized.

Also use Slash’s heavy buffers to soften recoil and manage dwell time. I also reload for all my platforms, semi and bolt. I’ve recently moved away from putting my AAC on my gas guns and gone with surefire Socoms for my 5.56 platforms and a KAC QDC 7.62 for my SR25 as those cans are optimized for use on semi autos...POI is non existent with the surefires; I’m still waiting on the stamp for the KAC.
 
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Snuby642

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I have avoided AGB's but guess it's time to throw in the towel.
In olden days I can't remember over gassing being as prevalent as it is now.

May have been there and gone un noticed but as expectations on performance increase
no detail escapes scrutiny these days.

I have been looking at superlative arms agb but see many others nearly half the price.
I have at least 3 uppers that I know are over gassed.

Any suggestions or recommendations would be welcome.
 
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nn8734

nn8734
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I use and love Syrac adjustable gas blocks. They fit under most (if not all) rail designs and are easy to use (just need a long Allen wrench).
 
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Alarka

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I have avoided AGB's but guess it's time to throw in the towel.
In olden days I can't remember over gassing being as prevalent as it is now.

May have been there and gone un noticed but as expectations on performance increase
no detail escapes scrutiny these days.

I have been looking at superlative arms agb but see many others nearly half the price.
I have at least 3 uppers that I know are over gassed.

Any suggestions or recommendations would be welcome.
@Snuby642 - I’m in the same boat...all my AR’s have adj gas blocks except my SR25 which doesn’t seem to need it as long as the load is not over pressurized.

Also use Slash’s heavy buffers to soften recoil and manage dwell time. I also reload for all my platforms, semi and bolt. I’ve recently moved away from putting my AAC on my gas guns and gone with surefire Socoms for my 5.56 platforms and a KAC QDC 7.62 for my SR25 as those cans are optimized for use on semi autos...POI is non existent with the surefires; I’m still waiting on the stamp for the KAC.
Just got the SLR myself for my LaRue 6.5CM and going to test it out this weekend a friend of mine recommended them as he has a Seekins AR 10 in .308. The lever adjustment along with the bleed off capability were what tilted the choice to SLR.
 

TSloper

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I will measure some cases tonight. Attached is my QuickLoad output for the load I'm running. Ba is dialed in to match the non-suppressed velocity. All other metrics are measured as accurately as possible (case volume, etc). I'm definitely not riding the line at 23.8 gr per QuickLoad. Magnetospeed V3 MVs are as follows...

15 rnds un-suppressed @ 23.8 gr once fired LC 18... 2609 fps

10 rnds suppressed @ 23.8 gr new LC 18... 2624 fps

As for gas control... in full disclosure I forgot to switch to the suppressed setting on the M6A3 before going suppressed. Unfortunately I've seen the large groups suppressed before with the gas block set correctly. I WISH that was it.

Tim


@TSloper One other thing....since you hand load, if you have a dial caliper or one-inch micrometer, take it to the range with you. Measure a few unfired cases before firing the rifle with the suppressor then measure those same cases after firing. If you don’t they are usually available and inexpensive at places like Harbor Freight

You are measuring the case body diameter just above the case head. Record the first and second measurement and compare them. If the second measurement shows expansion greater than .002” that load has too much pressure. Excessive case body expansion is considered a leading pressure indicator and will usually show up before things like cratered primers or extractor marks on the brass.

One more thing to rule in or out in diagnosing the enlarged group size
 

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nn8734

nn8734
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Quick load can help with establishing a baseline for estimating/predicting pressure dynamics but unless you have the equipment to take direct pressure measurements when firing the rifle for comparison, it’s purely theoretical. Most of us don’t have a need for such equipment anyway, given the other ways to detect pressure signs.

Your MVs are not substantially higher suppressed vs unsupressed which is a good thing.

Any significant changes in standard deviation and extreme spread when shooting suppressed vs unsuppressed?

Also were those measurements taken when the bore was “cold” (first 20 rounds through the rifle in that session) or warm?

I like to take data over 10 rounds from a cold barrel then continue to shoot then 50-100+ rounds later, gather data for another 10 rounds on a warm, heat soaked bore to measure a given load’s sensitivity to chamber and barrel temp. I only do this sort of test after a given load has shown promise after initial testing.
 
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TSloper

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I didn’t have the luxury of measuring the same cases before and after firing tonight but I did compare a small base resized twice fired case against a non resized once fired case from the same lot. I used two samples for each. The difference measured at the exact same spot was 0.001”. Compared against never fired (new) samples it was 0.002”.

Here’s my plan for the weekend...

- suppressed with my BH MK262 Mod 1 rounds. These rounds are on average 12-15 fps faster than my rounds. Historically I don’t recall them shooting poorly even suppressed but it’s been a while and I’m focusing much more on accuracy and grouping than I did before
reloading.

- suppressed with a sweep in 0.3 gr increments to see if there is charge that hits a sweet spot with the suppressor. I have enough 8208 XBR left of this 1 lb canister to accomplish this. I have a new 8 lb canister but will wait until I get through this exercise as I don’t know how far off the burn rate will be.

- verify suppressor isn’t loosening every group (I don’t think this is it)

If nothing conclusive shows up I will send the can back to AAC to be evaluated.

On the case prep front I annealed and SB resized 25 twice fired cases. I recorded neck runout (Sinclair tool) at each step starting with just fired. After pre-trim prep all cases are showing no more than 0.001” to 0.0015” of TIR at the neck (LC 18 brass) with most being 0.001” or less. Moving to a mandrel vs expander ball was the differentiator. Lube is critical and expanding slowly with the mandrel does make a difference vs doing a quick pull.

One other tidbit... annealed brass should see SS tumbling before the resizing die. My RCBS die has never left a mark on my brass necks. Well in getting my homemade induction annealer setup tweaked I ran some annealed pieces through resizing with dry lube. Big mistake. The oxide layer on the brass was messing up the die and causing longitudinal marks on the brass necks. I polished the die with Flitz and all was perfect again. With over 100 annealed and SS tumbled pieces I haven’t seen a hint of a problem return.

Anyways... thanks for the help!

Tim
 
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TSloper

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So I made it to the range yesterday. Here's the issue... the damn M4-2000 wobbles horribly once a round or two has been fired. The teeth don't engage properly when fully tight so it backs off to the closest full engagement of the teeth and at that point there is a tremendous amount of play. When fully tight I can't change the alignment relative to the Geissele checking rod. When the can backs off there is enough wobble for the Geissele rod to touch the can. I did fully clean the mating surfaces yesterday before going to the range so there is no carbon buildup to take up some of the slop.

So options as far as I can tell...

1. Call AAC and let them deal with it completely. The can went back to them a few months ago for refinishing and it looks like they changed the latch while it was there. This may have been the point at which this problem started I just didn't notice it until I started focusing on reload accuracy/grouping. Not to keen on sending in the upper but may have no choice.

2. Try a Viton o-ring to take up the slack with full teeth engagement. I have the Geissele rod so I can ensure the can isn't out of alignment when installed. This honestly would be my preferred path if it works. O-rings are cheap.

3. Take off the flash hider and carefully remove material on the 45 degree mating surface of the flash hider to allow it to engage the next pair of teeth. This would achieve a perfect fit but what a hassle.

4. Let the carbon build up but given the amount of play I'm seeing I don't view this as having a high chance of completely eliminating the play.

On a positive note I spent some time un-suppressed dialing in my new Vortex Viper PST Gen II 3-15x FFP... I had 3 touching at 100 yds. It is by no means enough of a sample statistically but I've never accomplished that before with my ACOG. I'll get to the 200m range next week and see how things are looking. This lot of 8208 XBR seems to have a sweet spot at 23.8 gr on my rifle without the suppressor. MV average is 2609 with this load (77gr SMK, CCI 41).

Tim
 

nn8734

nn8734
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Yep, figured...fucking AAC mounts are worthless, excepting maybe the 90 tooth version.

Edit: I have 5 different mounts for my SDN-6 and only one of them allows the suppressor to fully and properly lock up so that it doesn’t come loose.

Possible course of action for you:
 
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TSloper

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Wow... I really like that system referenced in the other thread. I believe that is the way I will go. Thank you for the link!

Tim
 
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TSloper

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ECCO wrote back. I'm going to send the suppressor out tomorrow and have it converted to an Omega mount pattern. It sucks having to pay for something like this but it beats the cost and time of getting a new can.
 

diving dave

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I have the same can, and on a few rifles it locks up tight. But on others I also get that rotation on the FH. For the most part, I run it on my SBR's and a scar 16s, so tiny groups arent much of a concern. I also have an 18 inch BCM build, and with 75 grn PRVI loads it shoots3/4 MOA with the can. Something to think about too, instead of an adjustable gas block I went with a Bootlegcarrier . adjusts easily and perfect for use with a gassy can like AAC
 

Strykervet

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When I read the title, I knew it was an AAC can with a 51T mount. Sure enough, it was. Didn't bother reading the other posts, BTDT and I know pretty much everything there is to know about these goddamn things with respect to making them work. Also been in enough of these threads to have a good idea what's being said anyway.

I own four of these motherfuckers and have been through several mounts, don't ask, and can help you out (well, two ea. 556SD's and SDN6's, basically the same thing and definitely the same problem). First let me say AAC did make a good can. They're solid and well made and they do the job. The SDN6 works phenomenally well with the .300BLK and it's quiet. However, their mounts SUCK and their engineers just would NOT listen and their RnD guy was online constantly defending the shitty design instead of fixing it. They just wouldn't let go of that damn ratchet design and STILL use it today. They claim it's just "axial rotation" but that's bullshit and you don't want that either anyway! The movement can range from nothing to some axial rotation to absolute wobble such that it eclipses the bore. I had an FAL mount just like that, took YEARS to get them to give me a replacement!

You have a few options and I'll list 'em from best to worst:

1: Swap it out: If it's for a precision rifle, get a TBAC and don't look back. If you need an abuse can (mag dumps, full auto, etc.) consider a Sico Saker --you can find 'em on sale and they're solid cans that addressed ALL the problems AAC has including the tuning fork flash hider. Got two this summer for less than $500 ea., mounts plus brakes were $150 making both cans $1300, about how much they cost new when they came out. This is the easiest fix and then you can put those AAC cans on something like a rack grade M4(s). There are of course other great mfg.'s out there that make good cans, but I know these, own these, and furthermore, with respect to the Saker, I don't think you can find a better can in it's class for the price when they go on sale, and when they do the BOGO in the summer, well, it gets even better. TBAC will handle more abuse than you think but due to the nature of those cans it's wise to call them and get the exact specs for your application. They make a 5.56 hard use can now as well, I don't have one but knowing TBAC it's probably amazing. Elite Iron is another good one, precision, hard use, high performance, low cost, all rolled into one. KAC is great but it's gonna be costly and I'd go that route if I had a KAC rifle or upper with the mount already.

2: Keep 'em but fit 'em properly: My favorite 51T method, basically order the right one. In order to do this, you're gonna have to buy about ten or so mounts. What you do then is carefully test YOUR cans to those mounts. You're looking for one that locks up tight like it should, like you'd expect from the factory. But their QC and tolerances are such that this is about the only way to mate a can with it's proper mount. Once you find the ones that fit, send the rest back for the refund. Unless you have a shop or dealer that'll just let you fit it right there on the spot, which would be nice.

3: Fix the existing mounts you have. There are numerous ways but they all involve mating the bevel portion of the mount to the can such that it'll latch properly. Light grinding and stoning can reduce that part to the point the ratchet will grab the next tooth, giving a solid mount. But if the bevel isn't perfect, it'll leak or cause other issues. It can be done by hand or by dremel, hand is better, but ideally the way to do this is with the proper tools and either a lathe or a mill, or even a drill press and certain fixtures, tools. This is the most haphazard way of doing it, the hardest and most time consuming, the most prone for error. But I've done it before using a machine blue marker and a fine whetstone of all things, and six hours later I had a perfect mount. SIX HOURS of tedious stoning and bluing. Some folks buy Viton seals, they're like rubber but MUCH higher heat, and they put that between the bevel and the can and it will squish down enough when you tighten it on that it'll cause it to latch and seat without leaking and without any grinding. But you have a Viton ring to keep track of when you take it on and off, sorta negating that QD feature IMO. Note any grinding or modification will void warranty but good luck getting them to honor a warranty in the first place. If you DO have a strike in a can, do not show or send them the modified mount!

As for the mounts, IME the brakes seem to be made to a closer tolerance? Anyway, they don't "TWANG!" when you fire so get those instead. Brakeout doesn't bring anything to the table really but it doesn't twang. The old ones worked okay I guess, better than I'd have given 'em credit for. Not so sure about the non-timed new ones though. Stick with the brakes. There are ways to take the twang out of the flash hiders, generally by removing material from the tines with a mill. Or you can fit and weld a ring to the end of the tines (which also keeps it from snagging on vines if that's a problem). Better off with the brake and it looks cooler to boot.

Ideally you'll order ten brakes or so and find the right one (or ones if it works out that way) and send the rest back and never look back. They'll fit snug and there won't be any wobble and the group should go back to a group with a somewhat repeatable POI. However, I felt it better to just relegate these AAC cans to SBR's and rack rifles and go with something more solid for precision anyway.

Good luck and hope this helps anyone else with an AAC can.
 
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nn8734

nn8734
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@Strykervet - see post 16...strongly thinking about having this done to my SDN-6 as well. This will permanently remove the piss-poor mounting system from the equation.
 

TSloper

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Ordered a Dead Air Key-Mo Omega mount and flash hider. Can’t wait to have this thing performing as it should.
 
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TSloper

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This endeavor was worth every penny. 100 yards / 77gr SMK / 23.8gr 8208 XBR. This was off of a brand new Atlas bipod on the bench. That’s 4 touching with the 5th being my screwup. My groups before the ECCO work were 2-3 MOA. I repeated this on another group of 5.
 

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nn8734
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This endeavor was worth every penny. 100 yards / 77gr SMK / 23.8gr 8208 XBR. This was off of a brand new Atlas bipod on the bench. That’s 4 touching with the 5th being my screwup. My groups before the ECCO work were 2-3 MOA. I repeated this on another group of 5.
That’s fucking awesome, man!
 

gunrunner2019

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I have an M42K on one of my M4's... The first time out with it, it was tumbling rounds from the first round fired. Key holes from 25 yards (I didnt fire it any closer than that). Tried it at 100 and had rounds slapping the ground in front of the target... Double checked the mounting, and it was tight, no wobble to speak of. Took it off and tried the rifle with no issues what so ever, looked at the can, baffle strikes from front to back with it. I measured my barrel when I got home, and it was concentric, the mount was back against the shoulder of the barrel, and tight... It was sent back to AAC for repairs, and they rebuilt the can, refinished it, and sent it back at no cost to me. I have not used it again since. I would double check and make sure you werent getting any baffle strikes.
 

TSloper

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My M4-2000 is clean as far as any baffle strikes. My issues were directly caused by the mount. When they refinished the can at AAC a few months ago they put a new latch in. I believe the new latch introduced play that didn't exist in the original. AAC may say rotational play is acceptable but I don't believe that it is after seeing how tight my groups are with the Dead Air mount. The can is an old design so it is what it is but on any future cans the mounting system will be my top priority.
 

Crang

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My older Gemtech bi-lugmounted can trashes the accuracy o some rifles and doesnt on others. Its about 50/50. My newer designed mounted ones dont near as often but do throw harmonics occasionally. Nice to have options and try a few different ones on a rifle before deciding which works best.