Suppressors One for the think-tank

[NukeMMC]

I have 3 rifles made to accept my .30 Ops Inc 3rd model:
- Rem 700 in 260 with a match chamber, Heavy Palma, 25" Bartlein 1:8" barrel.
- GAP10 Gen 2 in 260 with a 20" Bartlein 1:8.2" barrel
- Armalite AR10 in 308 with a 20" WOP 1:10" barrel

Both gas guns use adjustable gas blocks and are adjusted so that it reliably cycles to an open bolt with a magazine inserted.

Shooting the GAP10 today doing load development with 140SMKs and H4350 from 42.0-43.0 in 0.2gr increments. Without the can on, I had NO ejector marks on the brass ... at ANY load level. I had an extra 5rds each in 42.4, 42.6 and 42.8 and wanted to see POI shift and accuracy difference with the can on. EVERY round fired with the can had ejector marks, some even swiped enough to raise a pretty good burr. Since I FL sized all the brass to the same headspace dimension (1.629-1.631) and I load powder to +/-0.1gr, sort bullets by ogive dimensions so that my Base-to-ogive dimension is +/- 0.002" I can discount the ammo being the cause.

Since the un-suppressed rounds show no signs of ejector marking, I assume the bolt is locked for the duration of the pressure in the barrel. I have to wonder, knowing a very little about the Ops Inc can design, if the first baffle and the large chamber that sits over the barrel are creating a secondary pressure wave that comes back down the bore either after the bolt has started to unlock or more likely when the brass case has "sprung" back to a somewhat smaller size, and thrusts the brass back into the bolt face.

I have been less-than directly behind enough muzzle brakes to be very familiar with the pressure wave from them. This is the only thing I can think of to explain the difference between unsuppressed and suppressed rounds. The raw pressure difference within the barrel cannot be significant enough of a difference as there is only a 15-30fps increase in velocity with the suppressed ammo.

Top-to-bottom rows:
42.6gr H4350 - avg. vel = 2685fps - no suppressor
42.4gr H4350 - avg. vel = 2688fps - suppressed (I rotated the raised ejector marks that I filed down to the 6 o'clock)
42.8gr H4350 - avg. vel = 2701fps - no suppressor [IMG2=JSON]{"data-align":"none","data-size":"full","src":"https:\/\/farm5.staticflickr.com\/4313\/35303029984_ffa7a378fa_b.jpg"}[/IMG2]




OP Seberger patent drawing for Ops Inc can: [IMG2=JSON]{"data-align":"none","data-size":"full","src":"https:\/\/farm5.staticflickr.com\/4329\/35751345890_004a42b16e_b.jpg"}[/IMG2]




What says you folks???

 

[FatBoy]

That's a good one. I'd reach out to Ron Allen and see what he thinks. Ron has always been very responsive to my questions.

 

[Zak Smith]

There's more backpressure from the can so a higher volume and a higher pressure flow comes through the tube. Makes some sense that it happens on the more overbore cartridge. Adjustable gas block should fix

 

[NukeMMC]

As above, both gas guns have AGBs. The GAP10 has an SLR and the AR10 has a Superlative.

The point that there is back pressure from the can is undeniable, as every can I have been around (and I have been around a bunch of different ones) causes some degree of backpressure in any/every semiauto or full auto. The question here is the timing of an apparent pressure impulse to be occurring when the brass is not solid against the face of the bolt as during the initial pressure pulse that expands the case against the chamber walls and bolt face.

 

[Zak Smith]

As an experiment, can you adjust the gas down far enough to make that not happen on the .260, regardless of the bolt cycling enough?

 

[NukeMMC]

As an experiment, can you adjust the gas down far enough to make that not happen on the .260, regardless of the bolt cycling enough?

Great minds think alike. My next step is to throttle down the AGB on the GAP10 until it doesn't show ejector marks. That should determine if it is excess pressure dwell time during the firing impulse or a secondary impulse. I believe the AGB is 6 "clicks" (1.5 turns) open, so I will fire 3 shots each after closing the AGB 1-click at a time.

 

[bab029]

How old is the brass? May be a little work hardened if more than a few firings old. A quick test could be to shoot a couple pieces of fresh brass.

 

[NukeMMC]

How old is the brass? May be a little work hardened if more than a few firings old. A quick test could be to shoot a couple pieces of fresh brass.

That was 1x fired brass. Now it's 2x fired.

 

[Dirty D]

...That should determine if it is excess pressure dwell time during the firing impulse or a secondary impulse...

Pressure always flows from high to low just like concentration gradients and thermal energy so unless you somehow introduce a 2nd volume of gas it is not possible to have a secondary impulse. Suppressors by their very nature and intent slow down the venting of the gas expansion that creates all the noise so your chamber pressure is elevated for a longer period of time, you are probably on the right track by restricting your AGB.

Please report back your results, I'm curious to see if restricting the AGB will remedy the issue.

 

[NukeMMC]

Here is my testing.
Shot pairs of rounds with different gas settings. From L-to-R in order of firing: 6-clicks open/4-clicks open/2-clicks open/FULL SHUT/5-clicks open/5-clicks open & mag-fed
The 4-clicks open ejected brass but failed to lock back on the empty mag. The 2-clicks open never ejected and had to be jack-hammered (pogo-stick) open. The Full shut left no marks. The 5-clicks open showed a definite mark on 1 piece of brass but the bottom piece only has a very faint mark where ".260" is. Oddly, the 1st shot, the lower of the 6-click pieces, shows no mark.

[IMG2=JSON]{"data-align":"none","data-size":"full","src":"https:\/\/farm5.staticflickr.com\/4321\/36181896416_6deb66d98f_b.jpg"}[/IMG2]

 

[Smokyred]

Pressure always flows from high to low just like concentration gradients and thermal energy so unless you somehow introduce a 2nd volume of gas it is not possible to have a secondary impulse. Suppressors by their very nature and intent slow down the venting of the gas expansion that creates all the noise so your chamber pressure is elevated for a longer period of time, you are probably on the right track by restricting your AGB.

Please report back your results, I'm curious to see if restricting the AGB will remedy the issue.

^This^ The pressure is not subsiding as quickly with the suppressor attached, and the bolt is unlocking before the pressure has dropped enough. This is why guys have suppressed and unsuppressed settings on their adjustable gas blocks.

Most likely what is happening is the bolt tries to unlock, the pressure drops off enough for the case to start to free up in the chamber, and then the case slams back into the bolt face.

Dave

 

[agbm]

Like Smokyred said, you got to slow down the bolt from opening so quickly. It's going to be trial and error, it's just got to be fine tuned for the Ops Inc can, it's going to be different by their can design . Your gas block is doing it's part, move to the opposite end and try a stiffer buffer spring and heavier buffer. You could try a different .30 cal can, but my guess is it would work fine. Do you have another .30 cal can or know anyone that would allow you to test their can on your rifle to see if you get the same outcome? Also in which position is brass ejecting on a clock face? The OTB(rear) portion appears to hold pressure longer than the front portion, look at the area between G to K on the drawing. K appears to have just a center hole drilled so the rear end bleeds of slower than the front. They are quiet can's and a great design.

 

[NukeMMC]

Cannot really use other cans as the Ops Inc profile uses 11/16-24 threads instead of 5/8-24.

I am unsure, and hesitant to go adding mass or added spring weight to the reciprocating mass (either bolt carrier or buffer) as the bolt stop has to stop either the added mass or added bolt velocity. This has been known to break bolt stops.

I have a query in to GA Precision and an ongoing conversation with Ron. If need be, I will just live with it. Grumbling and bitching as us old fart CPOs do, but it is what it is. Damn thing is pretty impressively accurate.

 

[Dirty D]

...Grumbling and bitching as us old fart CPOs do...

Keep your mug full of shitty coffee, your forearm square to the deck while holding said mug and go rip some non rate a new asshole for no reason at all and you will cheer right up.

 

[chorizo]

Keep your mug full of shitty coffee, your forearm square to the deck while holding said mug and go rip some non rate a new asshole for no reason at all and you will cheer right up.

Or jack up an ensign.......

 

[NukeMMC]

Or jack up an ensign.......

Too easy ... LTs and the occasional LCDR are more fun

 

[NukeMMC]

Keep your mug full of shitty coffee, your forearm square to the deck while holding said mug and go rip some non rate a new asshole for no reason at all and you will cheer right up.

Actually had some poor new MM3 in the division got steered toward cleaning my well "seasoned" mug by a couple of the more senior MM1s. Poor kid got my attention for that. Little did he know my attention then turned to the MM1s. Having new guys screw with the Chief is not the smart thing for guys to do when they are trying to get selected for CPO.

 

[chorizo]

Based upon my experience with my 21 years in the USMC and by having an Uncle for a Master Chief, I can sum it up more crisply: It is not a smart thing to screw with a Chief.

 

[-Time-]

As previously suggested, look at a heavier buffer (then a stiffer spring if you're so inclined.

The brass marks are from the increased pressure coming back from the suppressor. While adjustable gas blocks slow down the the carrier speed, and thus cut down on the gas to the face/hard ejection, since the bolt unlocking is such a short period of carrier movement, it doesn't effect it a whole lot. (Adjustable blocks cut down on how much gas, not how fast the gas travels.) IMO the reason you run an adjustable gas block on a suppressed gun is not really to be easier on the brass, but to cut down on excess gas to the face, and be able to tune the rifle to run with a heavier buffer.

A heavier buffer (and stiffer spring) will keep the bolt from actually rotating longer, so pressure will settle before the bolt turns, giving you less marked up case heads. I personally run an h2 on my suppressed 5.56, and I get just a little swipe over, if anything at all, compared to it trashing my brass with a carbine buffer.

hope this helps!

 

[Dirty D]

Based upon my experience with my 21 years in the USMC and by having an Uncle for a Master Chief, I can sum it up more crisply: It is not a smart thing to screw with a Chief.

When I got off active duty I took a job at a local lumber yard just a few miles from my old base because they worked around my college class schedule. One day my boss gave me a sheetrock delivery on base so I loaded up the truck and headed on over, when I got to the base there was a detail to unload the truck and heading the detail was a CWO3 who was a notorious asshole and got off on shitting on his guys. My truck was a dump bed to slide lumber off, if I tilted the bed about 30 degrees the top sheet of Sheetrock would slide off with an easy push but the stack stayed put. I started to tilt the bed and CWO3 Asshole starts yelling at me like I'm one of his guys, I looked at the CWO in front of his guys and said "Is that how they teach you to talk to civilians?" and he about had a stroke, turned very red with anger and I just stood there and stared at him. CWO was an asshole but he was smart enough to know that I would drive right over to the admin office and run his ass up a flagpole. One of my old friends was on the detail, he told me that story was retold many times on base.

Its nice to have a magical piece of paper called a DD-214 that lets me tell any chief or officer to piss off.

 

[chorizo]

When I got off active duty I took a job at a local lumber yard just a few miles from my old base because they worked around my college class schedule. One day my boss gave me a sheetrock delivery on base so I loaded up the truck and headed on over, when I got to the base there was a detail to unload the truck and heading the detail was a CWO3 who was a notorious asshole and got off on shitting on his guys. My truck was a dump bed to slide lumber off, if I tilted the bed about 30 degrees the top sheet of Sheetrock would slide off with an easy push but the stack stayed put. I started to tilt the bed and CWO3 Asshole starts yelling at me like I'm one of his guys, I looked at the CWO in front of his guys and said "Is that how they teach you to talk to civilians?" and he about had a stroke, turned very red with anger and I just stood there and stared at him. CWO was an asshole but he was smart enough to know that I would drive right over to the admin office and run his ass up a flagpole. One of my old friends was on the detail, he told me that story was retold many times on base.

Its nice to have a magical piece of paper called a DD-214 that lets me tell any chief or officer to piss off.

Everyone has their cross to bear. Looks like you have yours. It appears to me that you and the service you were in are better off that you are a civilian.

 

[NukeMMC]

I was re-reading a bunch of info and something else came to mind and it really was a forehead-slapping moment. I remember when I first started loading for the NM M1 Garand I had just started competing with that folks discouraged the use of slow powders like 4350 in a Garand as the pressure in the barrel was still too high as the bullet passed the gas port and caused excessive operating forces on the oprod whereas I was running a .30-06 bolt rifle with a 28" barrel that would shoot 190s with a big dose of 4350 without pressure signs.

Seems to me that with a 20" barrel the 4350 is still building pressure even after the bullet has passed the gas port. So much so that by adding the suppressor, I have effectively extended the containment of the pressure well enough that the bolt has certainly unlocked and pressure moves the case back into the bolt face.

I have loaded some 130gr ELDMs with 38.4-39.2gr of 4064 to see if running a faster powder with the lighter bullet will alleviate some of the issue. I will surely not get the velocity I was with 4350 but I am hoping to get enough to be useful up to 1000. May have to experiment with RE15 and Varget that I have.

 

[NukeMMC]

Well, a big no-go with the faster powder/lighter bullets. They also showed ejector marks at every powder load.

I guess next step is to add some weight to the buffer, then increase spring rate.