Desperately seeking subsonic cycling - new guy.

[Westerly]

I’m new to the 300 AAC Blackout. After painstakingly developing a subsonic load for .223 and asking myself “why bother?”, I finally got the right caliber. I received my cheapo entry-level AR (S&W M&P15) the night before last. Made various loads with once fired Remington brass (w/300 Blackout headstamps). I wished to use A1680 powder, but I’m stuck using the pistol powders on hand due to temporary lack of availability (of any component whatsoever). The recipes found for powder I have included: IMR 4227, AA #9. Nothing cycled the action (questions below). Almost none of them even stripped a cartridge from the mag. None would come close to locking the bolt back on empty mag.

Details:
220 gr. Sierra Match kings, all rounds OAL 2.260” (just fitting into the magazine), CCI #400 primers. Shot from a bone stock weapon with no silencer. Surprisingly, all somewhat stablised….at least no tumbling.

Powder Grains Average FPS Accuracy Function
AA #9 8.6 804 best ??? No cycle, some stovepipes
AA #9 8.7 832 mediocre No cycle, ejection ok.
AA #9 8.8 1039 mediocre No cycle, ejection ok.
AA #9 9.4 1142 worst No cycle, ejection ok.
IMR 4227 10.5 1125 mediocre No cycle, ejection ok.

Barrel length = 16”
Gas Port = unknown diameter @ 8” (is that a carbine length?)

Questions:
How could the 804 FPS give the best accuracy? Weird…
I have read on this forum that some rifles need the silencer installed to cycle the action. Can I assume that a proper 308 silencer will add some muzzle back pressure to contribute to proper cycling?

BTW - I just shot some above rounds through my SilencerCo Osprey-45 (w/bushing and fixed barrel spacer), but no difference in function was observed. Pretty big hole through that can, though.

What next? Enlarge the gas port? Would an adjustable gas block be advised for switching to supersonic, yet not abuse the action?
If Noveske Switchblocks were available, would one of these be an option? http://noveskerifleworks.com/cgi-bin/im ... us=&title=

Thanks for any response.

[darr3239]

First off, don't do anything to change the gas port. Once you do that there's no going back. Switch out some ammo components and work with it.

Ammo and reloading components are hard to find, but it does sound like you are aware of A1680 as being the preferred powder for subsonics. Here is AAC's recommended load, and they are the ones who developed the round:

Suggested subsonic load:
• R-P 300 AAC BLACKOUT brass
• Remington 7.5 primers
• 11.2 grains of A1680 powder (Adjust powder charge down as needed. Factory ammo is 1010 fps for 16" barrel)
• Case length is 1.368 +0.000 -0.020
• Sierra 220 MK loaded to 2.120 OAL (this length is optimal for reliable feeding from USGI magazines)
• Chamber pressure 21,100 psi.

Generally, reliable functioning with subsonic 300 BLK is only achieved with a suppressor. You need a .30 cal. one.

There could be other issues, but you didn't mention what barrel length and buffer you are using.

[Westerly]

Thanks for the astute input. Sounds like I’ll need to scrounge up some A1680 by hook or by crook. I’ve got pounds of Bulleye, Unique, Clays, TiteGroup & Trailboss, etc. to trade. I’ll start with a bulletin board post at the local range. Actually I did mention the barrel length at 16”. I wouldn’t start drilling on the gas port unless I was sure that I could limit it back down with some kind of adjustable gas block. I was kinda fishing for recommendations along those lines too. A switch (rather than some funky hex screw), would be nice....hence the gesture towards the Noveske Switchblock (incase anybody had experience with those???).

Thanks again!

[tfbit]

Someone who used to work at Noveske has said many times on this forum that you cannot over gas a 16" carbine gas Blackout AR. Most of the 16" barrels that run well with subs (no suppressor) have very large gas ports, close to the inside diameter of the gas tube. My 16" Daniel Defense barrel has a large gas port (sorry, didn't measure it before I built my upper but I'm pretty sure it's near 0.125") runs very well with subs using A1680 and supers using H110. It's also been suggested many times that the rifle should be run wet.

See -k- reply here:

viewtopic.php?f=136&t=81459&p=764202#p764202

[darr3239]

Yep, you got me there on the barrel length. I missed it on the first go round.

Noveske saw no need for a switchblock on the 300 BLK, since it doesn't have the pressures of the 5.56, and thus would be of no real value if the gun is set up right.

Why don't you just walk down the block and ask the experts at Noveske about your situation?

[Westerly]

It must be a crappy handload issue. With no factory ammo available, the current time is less than optimal to get familiar with a new gun in a new caliber and try to equal or better a factory loads for accuracy and function. I just checked out the gas port. I didn’t have an exact drill bit to gauge how it shipped, but when I took a #31 (.120) to it practically no metal was removed. I assume that S&W provided a generous hole there. Despite being a brand new gun w/ only 70 rounds through it from yesterdays trials, the BCG was a mess. Must have had some protective coating on it or something. It almost looked like rust and rough as a cob. I cleaned it all up with some Kroil and ScotchBrite. After the drill job (changing almost nothing), tried it out again with the Osprey (temporary can). No joy, all the same as expected. Sounds like I’ll have to heat up the search for some A1680.

When I was messing with .223, I had a pleasant email exchange with Joel at Noveske. Great guy. John met his/our maker about 4 miles from my house on the very road I live on. The same nasty corner that my daughter crashed off of when she was 16 and the very one my 22 year old son departed the roadway through a guardrail about a week ago. No injuries either time and both cars (ancient Beemers) drove away on their own power. Regretfully, I never had the pleasure to meet John Noveske, but by all accounts he was quite a character, a credit to the human race and a gleaming example of our American spirit.

[Quiet Riot]

Your OAL is longer than what Remington loads the220gr SMK, and I'm sure they have their reasons. Try a shorter OAL as previously mentioned before getting too drill happy on your barrel.

[Westerly]

Since I can't actually buy and measure the Remington ammo during this shortage, what would that OAL actually be? I'm pretty sure they are using something better suited than AA #9 or IMR 4227 for powder, but yes my OAL is probably way off (particularly with all that empty space in the case with both powders).

[Retooferab]

2.12 is recommended oal by AAC. I've loaded mine as short as 2.09. The 2.09 to 2.15 has given me the best accuracy with smk and amax bullets.

[Dolomite_Supafly]

Good luck. There are plenty of members here who gave up on subs with a carbine length gas system. I know I have helped a number of friends with carbine length gas systems and unless we swapped out parts they would never work. And after swapping out the parts a standard 5.56 upper could not be used on the lower.

I have said it before, and I will say it again, if you want reliable subsonic function you need to have a pistol length gas system or a suppressor.

In my pistol length gun I can use a very wide variety of powders when shooting both supersonics and subsonics. I have used Lil'Gun, N110, H4227, A1680, H4198, A2015, SMP842 and A1680 to shoot both subs and supers in my gun. The gun cycles perfectly and the only variable is that some powders have lower velocities than others but they ALL cycle reliably. I can shoot 180's at subsonic velocities as well as 245 grain cast bullets at under 800 fps. And I do not need special buffers or springs, just what any 5.56 lower comes with.