300 Blackout subsonic loads- observations. Weird or normal?

[hlee]

Ok, so I have a newish 300 blackout build that I loaded up some subsonic loads for. But before that, I took several factory offerings (subs and supers) and ran them through the gun just for function check. Everything but the Hornady black 208gr Amax subsonic cycled fine (2 of 5 short-stroked and did not pick up the next round). Hornady black and the remington 220gr OTM subsonic both failed to lock on an empty mag.

Fast forward a few weeks and I have a 5x5 load test for hornady 208 and 220 gr BTHPs using Accurate 1680 powder. I still had the hornady black so I used that as a baseline for speed testing. The max was 1061, min was 1029, SD was 14 with an ES of 32 fps. The last shot of this string clocked at 1060 on my magnetospeed and did not lock the action back on the empty magazine.

Ok, this is what I find odd, and maybe those more knowledgeable can 'splain me this...

The 208 gr BTPHs locked the action back on an empty mag with measured velocities of 801, 872, 971, 1006, and 1065 fps. The 220 gr BTHPs locked the action on an empty mag with measured velocities of 772, 797, 866, 933, and 976 fps.

Don't get me wrong, I'm glad that the firearm is cycling with the anemic loads, but it seems odd to me that the factory subs consistently cause malfunctions at higher velocity (and I would assume pressure) than the hand loaded ammunition. Anyway, I'd love to hear your musings on this.

In case you are wondering why I have not listed powder weights for the velocities; in my excitement to make up the loads, I did not record the charges in my reloading book. The scrap of paper I used is lost to time, though the entry where I was going to enter the charges in my book remains. Oh well, back to square 1...

 

[Snuby642]

220 gr rnfb 12.1 cfeblk coal 2.1 puts it righ in the cannelur slight crimp.
Hodgen published round. It sits right on top of the powder. 1060fps

It is my understanding that accurate 1680 is very similar to cfeblk.

I think the crimp lets it build some initial presure.

Also have a friend that does not crimp it but seats compressed, in a gass gun probably
not the best idea?

Try a crimp on it if you can, that should help especially with an ar.

Imho

Edit: This was with suppresor on and may make a differance as to the locking bolt back.

 

[Snuby642]

You may be gassed for use with a suppresor.

 

[kindabitey]

A little more info would be helpful, as in: barrel length, gas length, buffer weight, etc. I've switched to VV N120 for most of my semiauto Blackout loads.
It's cleaner and quieter than 1680. In my bolt gun, mostly VV N32C and 150 fmj.

 

[Snuby642]

10.5 inch aac pistol length gass upper
Stock lower with standard carbine buffer, magpul stock enhanced trigger.

This upper is not finicky runs subs and super suppressed or not.
I normally run suppressed with subs.

There is no agb or fancy springs.
The bolt group is not special.

I have thought of adding 1 or 2 tungsten weights.

Sounds like you may be under gassed?
Just a small amount.

I don't think addons will fix that.
Are you shooting suppressed?

 

[kindabitey]

10.5 inch aac pistol length gass upper
Stock lower with standard carbine buffer, magpul stock enhanced trigger.

This upper is not finicky runs subs and super suppressed or not.
I normally run suppressed with subs.

There is no agb or fancy springs.
The bolt group is not special.

I have thought of adding 1 or 2 tungsten weights.

Sounds like you may be under gassed?
Just a small amount.

I don't think addons will fix that.
Are you shooting suppressed?

Sorry, I meant for the OP.
That being said, you've got almost the same setup I have.

 

[hlee]

8.5" ballistic advantage barrel
Anderson upper/lower
pistol gas
non-adjustable gas block
pistol receiver extension
carbine spring
spikes T2 buffer
full mass carrier
flash hider- not suppressed as of yet. Function checking sub loads for speed and stability before my can arrives.

I'm actually quite happy with how the firearm functions- no malfs or stoppages with my hand loads. I'll be running hand loads through it pretty much exclusively, and only had the factory loads for some benchmarking. I just found it odd that the factory sub loads short stroked the gun while the hand loads did not- even though the bullet weights were similar and the speed on the factory ammo was greater (as much as 37% greater) than the handholds.

Has anyone tried trail boss in an AR? I did. That data is odd... But, the pop (not loud enough to be a bang) the rounds makes is quite satisfying.

 

[bfoosh006]

God only knows what non-canister grade propellant is used in the factory ammo.

You'd like to think the OEM ammo would function properly, just as a function of "duh".

But your proof is in your results.

 

[Snuby642]

I will bet your barrel and gas setup will work fine when suppressed, on the subs.

Sounds like you are on the very edge of that.


The factory loads worked mostly in my rig but accuracy was bad.

Try some round nose flat base bullets, while in development mode.
In subs they will out preform any of the boat tails with the points.

I will bet factory or your loads in subsonic will not out preform them no matter what you try in any gun you have.

This is not about my shooting or reloading skills at all.

It is about subs at 100yd range.

You will need a 1 in 7 twist to stabilize the 220.

 

[Hellbender]

The reason for the odd behavior is the different burning rates of the powders used. The slower the powder, the further down the barrel the pressure curve moves.
The CHAMBER pressure or velocity does not really matter, what matters is the PORT pressure and the DWELL time after the port, these two things are what affects the operation of the system.
PORT size affects the immediate quantity and velocity of the gases allowed to pass into the gas tube (then the BCG), the DWELL TIME affects HOW LONG the gases can push on it before the bullet "uncorks" and releases the pressure. The longer the barrel length AFTER the port makes more dwell time (more dwell time equals more VOLUME of gas, all other things being equal).
A suppressor effectively increases the dwell time.
The factory loads are optimized for a specific barrel length and port location and size, your combo does not like their formula.

 

[Snuby642]

Any speed you run it rnfb will best any thing you try, for subs.

Your loads your shooting your gun.
Imho