AR10 6.5CM 147gr Load Issue with Suppressor

[MNtadpole]

So long story short, last weekend I shot my established load through my LMT AR10 and everything seemed normal until I attached my buddy's Surefire SPS-300 suppressor. Once the suppressor was attached, my loads started blowing primers out of the pockets. I didn't notice until one of the loose primers caused a malfunction.

Prior to ever attaching a suppressor, my established load has never showed excessive pressure signs (heavy ejector smearing and primer cratering/flattening).

Firearm:
LMT AR10 6.5CM 20" SS barrel.



My load:
-Hornady 147gr bullet
-Hornady brass (likely 5th or 6th loading)
-CCI No. 34 primers.
-40.9 grains H4350
-Velocity 2635fps (per Magneto Speed)

Un-Suppressed Brass Fired:


Suppressed Brass Fired


Environmentals: 65 degrees F.

I'm wondering if this is a known issue when using suppressors? Was my brass at the end of it's usable life at 5-6 firings? I would really like to not go back to the drawing board and develop a new load. I will upload photo of the brass shot un-suppressed and suppressed when I get home.

Any help would be greatly appreciated!

 

[Mouseater]

I realize this is an old post, but only just saw it today. I don't know if you have solved the issue or not, but the exact same thing with my gun happened. AR-10, 6.5 creedmoor.
To answer your question though, yes this is a known issue. When you add a suppressor onto your gun you increase pressure even more so if it's a standard baffle design suppressor.

 

[MNtadpole]

I realize this is an old post, but only just saw it today. I don't know if you have solved the issue or not, but the exact same thing with my gun happened. AR-10, 6.5 creedmoor.
To answer your question though, yes this is a known issue. When you add a suppressor onto your gun you increase pressure even more so if it's a standard baffle design suppressor.

Yeah, I just chalked it up to a variable that I didn't take into account. With a factory LMT barrel I was forced to shoot 147s at a higher velocity for accuracy. I believe it was just too much pressure for a traditional SF can.

I haven't done too much more testing on this issue. I'm going to be doing some testing with 130gr ELDMs and back to 140gr ELDMs. Without a tunable gas block I think it is the way it is.

I'm hoping that I can go head a find two accuracy nodes without the can and maybe use the lower velocity accuracy node as a suitable suppressor load. I do realize that it would make more sense to tune a load with the suppressor on, but I'm not a owner of any cans, so...yea limitations.

 

[Mouseater]

Yeah, I just chalked it up to a variable that I didn't take into account. With a factory LMT barrel I was forced to shoot 147s at a higher velocity for accuracy. I believe it was just too much pressure for a traditional SF can.

I haven't done too much more testing on this issue. I'm going to be doing some testing with 130gr ELDMs and back to 140gr ELDMs. Without a tunable gas block I think it is the way it is.

I'm hoping that I can go head a find two accuracy nodes without the can and maybe use the lower velocity accuracy node as a suitable suppressor load. I do realize that it would make more sense to tune a load with the suppressor on, but I'm not a owner of any cans, so...yea limitations.

This is exactly what I did, found an accuracy node that was 1 gr below max and another just above max. Neither showed any signs of pressure without a suppressor. Once I put on on though I started to get ejector swipes on the hotter load, so I moved down the the lighter load and was good.

 

[MNtadpole]

Yup that is my plan! I have noticed that the SF can didn't seem to really effect the groupings...maybe a slight POI shift, though minimal, I can work with that.

 

[MNtadpole]

This is exactly what I did, found an accuracy node that was 1 gr below max and another just above max. Neither showed any signs of pressure without a suppressor. Once I put on on though I started to get ejector swipes on the hotter load, so I moved down the the lighter load and was good.

I think down the road I would like to experiment with a Proof CF D.Wilson conversion with the adjustable gas block. At this point in time though, I'm still waiting on a .308 CL LMT 16" barrel, and supplies aren't abundant enough yet to do excessive testing.

 

[Snuby642]

Maybe adjustable gas block ?

 

[Mouseater]

Maybe adjustable gas block ?

Well with an AGB the pressure is still in the system, unless you get one that bleeds excess pressure which I didn't want to do. The ones that bleed do work to a degree, but even then I don't want to have to adjust my gun every time I want to shoot suppressed and then unsuppressed. I ended up getting a flow through suppressor and just dialing back the load a bit. Still shoots 1k yards, which is all I wanted it to do anyway and saves me the hassle of fiddling with the block.

 

[Snuby642]

I went for a very long time without using an agb suppressed and not.

I wish I had not. There are some that adjust from the front that make it easy and the standard side adjusting brands some of which can end up needing relief cut in the rail to adjust.

I had false over pressure signs on a 6.5g not long ago that an agb took care of but your primers popping looks like definitely presure.

I hate it when you get an accurate load and figure out it's too hot.

 

[Snuby642]

So I looked up that suppressor.

It's tuned for 300blk .
A low presure round in a small case.
Just because its 30 cal does not mean it was ment to eat 308 well.

 

[Evintos]

If you don't want to mess with an AGB and want to shoot that hotter load with the suppressor on, increase your buffer mass.

 

[Mike_in_FL]

Without a tunable gas block I think it is the way it is.

Have you considered an adjustable gas key for the BCG?

 

[MNtadpole]

Have you considered an adjustable gas key for the BCG?

I had to look that up. Interesting thought, but I don't think I would ever consider it. I would add an AGB way before ever modifying the LMT BCG, just for reliability reasons. At least if an ABG went down, I could barrel swap in a few minutes. If a BCG went down, due to the AGK, I would be hosed.

 

[TheOE800]

That suppressor is optimized for 300BLK subsonic, meaning it will have more backpressure for a high-pressure cartridge as compared to its brother the SOCOM 7.62 RC2. That's not helping.

 

[MNtadpole]

That suppressor is optimized for 300BLK subsonic, meaning it will have more backpressure for a high-pressure cartridge as compared to its brother the SOCOM 7.62 RC2. That's not helping.

Makes sense. Like I said before, the can wasn't mine, so I was just trying to see if I could make it work. Obviously, it didn't work with the particular load.

 

[Gtscotty]

Well with an AGB the pressure is still in the system, unless you get one that bleeds excess pressure which I didn't want to do. The ones that bleed do work to a degree, but even then I don't want to have to adjust my gun every time I want to shoot suppressed and then unsuppressed. I ended up getting a flow through suppressor and just dialing back the load a bit. Still shoots 1k yards, which is all I wanted it to do anyway and saves me the hassle of fiddling with the block.

From this and your first post it seems like you think that adding a suppressor to the rifle increased the chamber pressure, that is not the case. Peak chamber pressure occurs in the first few inches of bullet travel and the suppressor is not materially in play at that point. Adding a suppressor can't make a 60ksi load into a 65ksi load or whatever.

Adding a suppressor to a gas operated semi absolutely can change the shape and duration of the positive pressure pulse in the gas system. This often causes early unlocking and can look like over pressure.... But it's not (unless you started out over pressure, in which case it still will be). Everyone is suggesting AGBs and adjustable carriers because this is a common problem, and those are the common mitigation approaches.

 

[Mouseater]

From this and your first post it seems like you think that adding a suppressor to the rifle increased the chamber pressure, that is not the case. Peak chamber pressure occurs in the first few inches of bullet travel and the suppressor is not materially in play at that point. Adding a suppressor can't make a 60ksi load into a 65ksi load or whatever.

Adding a suppressor to a gas operated semi absolutely can change the shape and duration of the positive pressure pulse in the gas system. This often causes early unlocking and can look like over pressure.... But it's not (unless you started out over pressure, in which case it still will be). Everyone is suggesting AGBs and adjustable carriers because this is a common problem, and those are the common mitigation approaches.

yes, it was my understanding that adding a suppressor increased chamber pressure. If I understand what you are saying above, what your saying is the chamber pressure is always the same it just changes dwell time and lock time and since it's unlocking sooner than expected due to the suppressor slowing the release of the hot gasses it looks like increased pressure but what needs to happen is the excess gas just needs to be bled from the system somewhere else to restore the proper dwell/lock time. Am I understanding that all correctly?

 

[Snuby642]

I run several suppressed / unsuppresd calibers in multiple uppers.

All but the sbr's have agb's.
Why exactly the short barrels get away with it is something I can't explain.

The long guns also run h2 buffers and extra strength springs on some.

Getting something that runs well both ways off the shelf is a search for the holy grail.

 

[Mouseater]

I run several suppressed / unsuppresd calibers in multiple uppers.

All but the sbr's have agb's.
Why exactly the short barrels get away with it is something I can't explain.

The long guns also run h2 buffers and extra strength springs on some.

Getting something that runs well both ways off the shelf is a search for the holy grail.

Running my OSS cans on my AR-10s and 15s I've never had to mess with anything adjustable. I use the same load for suppressed and un-suppressed.

EDIT: One correction to make, my AR-10 is chambered in 6.5 and I do get a light ejector swipe from time to time with the suppressor on, but that's it. Running over max load for H4350 so it's not a light load.

 

[Mike_in_FL]

I had to look that up. Interesting thought, but I don't think I would ever consider it. I would add an AGB way before ever modifying the LMT BCG, just for reliability reasons. At least if an ABG went down, I could barrel swap in a few minutes. If a BCG went down, due to the AGK, I would be hosed.

I was guessing that your LMT was an MWS which I've never owned. I have however heard that when released the gas ports were drilled at an angle so adding an AGB may or may not be an easy task. Carriers rarely fail and keys don't generally leak if installed and staked properly with the correct grade bolts. Gas rings, cam pins, and bolts are the failure points on a BCG.

If your rifle is an MWS, I'd suggest you read through the thread on them that's pinned to the top of this forum. There's a lot of good information in it.

 

[Snuby642]

Running my OSS cans on my AR-10s and 15s I've never had to mess with anything adjustable. I use the same load for suppressed and un-suppressed.

EDIT: One correction to make, my AR-10 is chambered in 6.5 and I do get a light ejector swipe from time to time with the suppressor on, but that's it. Running over max load for H4350 so it's not a light load.

Maybe I should add some loads are also subsonic.

Luckily all your bases are covered.

 

[Gtscotty]

yes, it was my understanding that adding a suppressor increased chamber pressure. If I understand what you are saying above, what your saying is the chamber pressure is always the same it just changes dwell time and lock time and since it's unlocking sooner than expected due to the suppressor slowing the release of the hot gasses it looks like increased pressure but what needs to happen is the excess gas just needs to be bled from the system somewhere else to restore the proper dwell/lock time. Am I understanding that all correctly?

Pretty much, you can reduce the area under the pressure curve in the gas system by restricting flow into the gas system, or allowing bleed off venting from the gas system to slow down unlocking some. In my opinion, longer gas systems for a given barrel length tend to be easier to tune for suppressors, but of course you can't do anything about that after the fact. You can also slow down unlocking with increased inertia (buffer weight) and buffer spring strength, but then you have more weight being moved around faster later in the cycling process, so I prefer to dial the gas down first and minimize the extra buffet weight I have to use to keep it relatively soft shooting.

If you're using OSS cans you may not have to do any of this tuning, that's pretty much their sales pitch.

 

[Kocher]

good information guys I do think there’s some things that could be done to these semiautomatic rifles to make our lives easier when we do suppressors I think adding another length to gas tubes in general would not be such a bad idea . Ex +3 instead of +2 gas tube length on our 24” barrels.

Has anybody done that experiment what happens when you have too long of a gas tube? Talk in real world experience not guesses?

Failure to feed, eject? seems like those could be remedied with an adjustable gas block?

Perhaps there would be a way to have two gas blocks and one and you would open one up or close the other depending on the system you’re using?

Maybe develop some sort of ring that you would turn, close one port and then open the other port and then lock it in place?

 

[Snuby642]

Like an agb and on some turds an ajustable buffer was not enough for the avreage dumbass to screw up. Lol

The post always starts, I have an adjustable buffer and can't make shit work with my agb and shitty ammo bla bla bla .

Every damn time.

Dual adjustable progressive auto sensing spin drift compensated gyroscope enabled organic grass fed WTF.

Eugene Stoner just had a rollover moment.

The dipshits that adjust both ends of one device in a never ending tailspin amaze me.

 

[MNtadpole]

I was guessing that your LMT was an MWS which I've never owned. I have however heard that when released the gas ports were drilled at an angle so adding an AGB may or may not be an easy task. Carriers rarely fail and keys don't generally leak if installed and staked properly with the correct grade bolts. Gas rings, cam pins, and bolts are the failure points on a BCG.

If your rifle is an MWS, I'd suggest you read through the thread on them that's pinned to the top of this forum. There's a lot of good information in it.

Yes, I'm running a MWS. From my understanding the gas port is angled. I have referenced some other posts concerning suggested reloading for the MWS and similar platforms. I haven't referenced too much in regards to suppressed fire of the MWS though. I'm currently living in a state that doesn't allow suppressor ownership, so being deeply invested in the topic doesn't make too much sense for me. The events the spurred the original posting was when a buddy allowed me to shoot his can (SF SPS300) on my MWS.

Since this posting was made, months ago, I have kind of gone into a different direction/mindset. 1. If a can is to be used on my MWS, it should be one that is optimized for full power rifle calibers (not SF SPS300). 2. Try to identify a suitable mid-power reload charge for the MWS, which shoots well unsuppressed and won't be too hotrodded for any increased pressures.

 

[smoothy8500]

Unsuppressed Brass Fired:

Is the cratered primers on the unsuppressed brass from high pressure or just a loose tolerance firing pin hole?

 

[bobke]

These little gauges are essential bench tools for measuring state of primer pocket tolerance.

 

[Snuby642]

Ya liberal use of the primer pocket gauges is a good thing.

Started that with range brass 223 but now used on everything.

Pass or fail plain and simple, definitive.