What am i doing wrong with 300 blackout subs????? Help please.

[7magsavage]

Ok, so Ive been tinkering with my shiny new 300 BO upper from AAC. I can't for the life of me get it to function with subsonic rounds and suppressed. I have hand loaded all the rounds that have been through it thus far.
Specs on the rifle:
AAC Upper 16"
Colt m4 lower

Ive tried a H buffer as well as a T2 (heavy) by Spikes Tactical. The supers work fine both suppressed and unsuppressed. The subs will fire and eject spent round, reset the trigger, but will not strip of the supsequent round and chamber it. So essentially i have a single shot. Even when I was working up a subsonic load the rounds that were going around 1100+ FPS were not cycling. I have used the heaviest bullets I had on hand which were 208gr Amax. I ordered some 225gr bullets to see if that helped anything.
I also tried using the BCG from my M4 (223), and I get the exact same results.

Of note, all rounds were fired with a 10 round Pmag. It dont think the magazine is the issue, but it's a variable that I haven't eliminated.
Any wisdom would be greatly appreciated.

Steve

 

[FF05]

Try adjusting your COAL, if they are too far off the lands they won't cycle. I had the exact same failure to feed with hand loads.
File/remove the rib out of the Pmag, they tilt the 300BO rounds abnormally and cause feeding issues. Or just try a steel mag to rule it out.

 

[Gunfighter14e2]

Ok, so Ive been tinkering with my shiny new 300 BO upper from AAC. I can't for the life of me get it to function with subsonic rounds and suppressed. I have hand loaded all the rounds that have been through it thus far.
Specs on the rifle:
AAC Upper 16"
Colt m4 lower

Ive tried a H buffer as well as a T2 (heavy) by Spikes Tactical. The supers work fine both suppressed and unsuppressed. The subs will fire and eject spent round, reset the trigger, but will not strip of the supsequent round and chamber it. So essentially i have a single shot. Even when I was working up a subsonic load the rounds that were going around 1100+ FPS were not cycling. I have used the heaviest bullets I had on hand which were 208gr Amax. I ordered some 225gr bullets to see if that helped anything.
I also tried using the BCG from my M4 (223), and I get the exact same results.

Of note, all rounds were fired with a 10 round Pmag. It dont think the magazine is the issue, but it's a variable that I haven't eliminated.
Any wisdom would be greatly appreciated.

Steve

You are under gassed via or by the combo your are using or have. Could be the burn rate of your powder vs bullet weight to the wrong gas hole size in the barrel. Before doing anything try some factory subs for it, if that works the weapon is fine an it's your ammo. If factory ammo does the same, a lighter buffer an spring combo, or increase the gas to the bolt carrier.

 

[Mullhead]

What powder are you using?

 

[Spicerack]

Agree on checking powder type... Using AA1680 I get my 9" Franken-upper with NON- adjustable gas block to run everything subsonic (208gr Hornady, 220 SMK, 220 Leatherheads). Yup, it's dirty but gets the job done with ease and no fuss. Super I run H110 and 110gr Barnes black tips just fine.
ETA standard carbine buffer, spring, whatever came with the Aeroprecision lower, standard BCG, Ballistic Advantage barrel gas Port measured 0.093, GI mags for everything.

 

[craigos]

I run 1680 on lehighs 174s and sierras 220 without an issue ... i am running JP LMOS bolt and JP silent spring however (no gas block adjustment). I can shoot factory subs and factory super without changing a thing, Barrel lengths of 14.5 down to 8

 

[10ring1]

Had the same issue. I wanted mine to run supers to suppressed subs. I tried everything you did but I then ended up trimming a few coils off the buffer spring that worked well. I heard you shouldn't do that but working well for me

 

[10ring1]

Also, you may have mentioned or not, are you using a pistol length gas system? Pistol length is more reliable with subs. The one i mention above though is a carbine length

 

[Yondering]

What powder are you using?

This should have been the first question asked. Nothing else matters until we know what powder you're using for those subs; there are lots of powders that won't work at all. If you're using the right powder, only then should we look at the other potential issues.

The next would be - why are you using heavy buffers if your rifle won't strip the next round from the magazine? Go back to a standard carbine buffer and spring and start from there until you have information that tells you to change something.

 

[7magsavage]

I'm using h110 for powder and it's carbine length gas system.
Also I have switched back to the lighter buffer. Just put the heavy buffer in there for shits and giggles.

 

[ZY100]

H110 is usually not a good combo with subs and a fully functioning AR without an adjustable gas block. It's common to use H110 for your supers, AA1680 or even trail boss for subs. It's all about gas volume....

ZY

 

[powderBurnz]

adjustable gas block solved it for me.

 

[Mullhead]

AA 1680 AA 5744 lil gun all should easily . Change your powder before you go changing springs or buffers. I think I load 10.2 gr of n110 for 208 amax never had a problem. Remember start high and work low.

 

[7magsavage]

H110 is usually not a good combo with subs and a fully functioning AR without an adjustable gas block. It's common to use H110 for your supers, AA1680 or even trail boss for subs. It's all about gas volume....

ZY

I have some trail boss. Was going to try it but didn't figure it would work. How much trailboss are you using?
I filled a case completely full with trailboss and weighed it and it was something like 5gr.

 

[redneckbmxer24]

I'm using h110 for powder and it's carbine length gas system.
Also I have switched back to the lighter buffer. Just put the heavy buffer in there for shits and giggles.

Are you certain it's carbine length? All the AAC uppers I've seen were pistol.

If it is carbine that's your issue. There's some things that may fix it that have been mentioned here already like adjustable gas block set wide open, and light buffer. You can also open up the gas port. These still may not work though, what does work is a pistol length gas. I'd offload the barrel or entire upper and swap it for something pistol length.

 

[7magsavage]

https://www.midwayusa.com/product/9...-receiver-assembly-300-aac-blackout-16-barrel


This is what I bought.

 

[Belisarius]

I'm using h110 for powder and it's carbine length gas system.
Also I have switched back to the lighter buffer. Just put the heavy buffer in there for shits and giggles.

I had a similar issue. My 208s were at 1200 fps with AA#9 and it wouldn't load the next round from the mag. Switched to AA1680, velocity dropped to 1050 fps, problem solved.

 

[10ring1]

Yes. Exactly. A1680 with pistol length gas or lil gun is just the ticket. But for a cheap fix instead of selling the barrel, I trimmed coils. If u want to do it right, change the barrel for pistol length and don't look back.

 

[7magsavage]

UPDATE

So today I went out and bought two boxes of factory subsonic rounds and ran them. They all ran flawlessly. I didn't have any short stroking, no failure to eject or failure to feed. However, they were still supersonic. The Hornady TAP ammo was quieter. They were loaded with 208 amax and the box said they were 1020fps at the muzzle. Still had the crack though. The Remington ammo was loaded with 220 smks (I think they were smks) and advertised 1050fps per their ad in Midway. (The velocity was not printed on the box, so I googled it.) The Remingtons were, IMO as loud as my 125gr supersonic hand loads. I ordered a couple hundred 220gr smks and I am going to pick up some 1680 and give it a whirl. I will report back when I get them loaded up.

 

[10ring1]

Can try this too..
http://www.brownells.com/rifle-part...=Avantlink&utm_content=NA&utm_campaign=Itwine

 

[10ring1]

The Remington subs are known for a lot of supers. I have noticed about 4/20 per box out of a carbine barrel. Did u have the heavy buffer in? Increased dwell time may be causing that as well

 

[Mullhead]

Trailboss and heavy bullets do not mix well. 5.7 gr and 125 TNT hollow will give you 1050 fps. That is a compressed load for bolt guns. You can not get enough powder in the case to push a 208 over 1000 fps. Trailboss will not work the bolt on an AR it does not make enough gas.

 

[7magsavage]

The Remington subs are known for a lot of supers. I have noticed about 4/20 per box out of a carbine barrel. Did u have the heavy buffer in? Increased dwell time may be causing that as well

No I had the standard buffer in.

 

[7magsavage]

The Remington subs are known for a lot of supers. I have noticed about 4/20 per box out of a carbine barrel. Did u have the heavy buffer in? Increased dwell time may be causing that as well

No I had the standard buffer in.

 

[10ring1]

I did as well when I was having the supersonic 4/20 per box. I don't think you could go wrong with the new powder and bullets you're going to try. I have learned a tremendous amount from the members at 300blktalk.com. Those guys are gurus over there. They know everything 300 Blackout. They're all cool too. No attitudes. They're always eager to help a new guy with problems. I have learned a tremendous amount from these guys and girls over there. Both of my 300s run like a top now and haven't had a problem since. It sounds like you were on the right track.

 

[ZY100]

I have some trail boss. Was going to try it but didn't figure it would work. How much trailboss are you using?
I filled a case completely full with trailboss and weighed it and it was something like 5gr.

Just getting back to this....Mulhead already said it, but trailboss is only good with lighter bullets, and a 125gn in the 5.4-6.0 range. You might be able to compress 6.0+ a little depending on brass, but not much more. AA1680 is what you want for the heavies. I should have been more clear, was just trying to point out powder choice is probably a better way to solve this.

You have some AA1680 on the way, you are on the right path. I was using an adjustable gas block at first, but found that after i got my loads dialed in, didn't really need it.

ZY

 

[7magsavage]

UPDATE

Got my shipment of 225gr Hornadys yesterday and loaded up six rounds just to see how the function was. All six functioned flawlessly. The 1680 was the trick.
Those are some BIG bullets.
Thanks to everyone for the input and assistance.
Steve