Sort out my 300blk

I'm currently sorting out my 300BLK as well (10.5 Rosco Mfg barrel). Similar issues with Hornady 190 Sub-X, 208 A-Max & Fiocchi 220 Range Dynamics Subsonic ammo. I don't have my suppressor yet, but in trying to at least get the rifle working, I have 110gr & 150gr SUPERSONIC ammo working just fine.

The Subs don't lock back with gas wide open AND the 208 A-Max never eject. I've gone through a myriad of buffer & buffer spring configs, to no avail.

Have you measured the actual gas port diameter? If so, what was that measurement? Mine measures 0.0850" (digital calipers). "Might" have to drill out to next size larger using a #43 machinists drill bit, but waiting until my suppressor arrives to further troubleshoot/evaluate.

One suggestion, since the 220SMK subs cycled fine is to acquire some other subsonic ammo in the 190 - 220 grain subsonic range and try that to see what it does. My suspicion is that the Hornady Black 208 A-Max are NOT good since you are the 2nd person I have heard with issues...

You may find the 1680 works a lot better than the lil-gun but if not your gas port may need to be opened up. Subs are hard to get working without having issues with the supers. An adjustable gas block with a larger port gives you a lot more leeway with loadings. Also adding a suppressor will improve operation for the subs and is basically the only reason to shoot subs anyway....

Frank

FUNCTIONAL said:

Built a 300blk and just trying to get it operate how I'd expect.

BA Hansen 10.3" 300blk barrel
Standard brownells melonite bcg
Standard carbine spring/buffer
Sico omega 99% of the time

Cycles supers fine. Hornady black subs do not cycle (dirty as hell by the way), PSA aac 220smk subs cycle fine (1050ish fps).

Handloads with 220 seconds (clear/cloudy poly tip, not smk)
8.5gr lil gun 1080fps does not lock back
9.0 lil gun 1095fps does not lock back

Although my handloads are supersonic and I need to drop the load they already aren't locking back. I've got some A1680 I'm going to try but with lil gun being such a common powder in 300blk I'm wondering if something with my setup is off. Are there weaker springs or buffers for 300blk I'm missing?

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CFE Black for powder. Dirty, but it'll cycle everything. VV N120 is also a good one and a lot cleaner. But CFE is the king of cycling everything. I never had good luck with the A-Max factory loads either, two different barrels. I also use a lightweight BCG and buffer which helps. Lots of guns seem to have issues with the 190 Sub-X rounds, mostly with smooth feeding and bullet setback. My Sig LT runs them flawlessly but that's an expensive answer.

H110 and Lil Gun has worked better for me with both subs and supers. Are you using an adjustible gas block?

What length is the gas system? Pistol or carbine. Sorry, 10.3 should be. Pistol

In my 8.5” pistol gas 300 blackout, 125 gr bthps in front of enough 110 to hit ~1900 fps cycle and lock back. As do 208-220 gr bullets in front of enough 1680 to hit ~950-1050 fps. Some varieties of Hornady factory ammo do not lock back in my gun. I don’t buy them…

I have same barrel as OP, no suppressor. Rolled up some Berry's 220 with Hodgdon's 8.1gr H110 @ 2.24" recipe. Lee autodisk throws 7.9-8.1 reliably. I'm guessing they won't cycle. Cci 400 primers, very first loadings.

Ape_Factory said:

CFE Black for powder. Dirty, but it'll cycle everything. VV N120 is also a good one and a lot cleaner. But CFE is the king of cycling everything. I never had good luck with the A-Max factory loads either, two different barrels. I also use a lightweight BCG and buffer which helps. Lots of guns seem to have issues with the 190 Sub-X rounds, mostly with smooth feeding and bullet setback. My Sig LT runs them flawlessly but that's an expensive answer.

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Cfe black is the perfect for subs albeit I don’t load them anymore, lil gun is a terrible powder for subs but great for super sonic, I personally use enforcer but lil gun is my second choice, but that is for achieving maximum velocity and with lil gun I would normally have to use a magnum primer.

Went through a similar issue with my very first 300BLK build, turned out to be a slightly undersized gas port.

That one was a carbine length gas system on a 16” barrel though, so kind of apples to oranges, but if it’ll cycle supers fine, and some subs, but not all subs, my first guess is the gas system is under sized. 300 BLK subs run very little powder of course and it’s a fine line with buffer, spring, and even BCG weight to get it right; everything has to be perfect if you run a minimally sized gas port.

Do you have a set of pin gauges? If not, invest in a cheap set or try to borrow a set and see what you’re working with on gas port diameter before you run down the rabbit hole of checking every possible combination of spring and buffer weight to see if you can get it to cycle properly.

Ask me how I know! LoL

Also, I typically prefer Accurate 1680 for subs, but 100% success with Hodgden H110 for both supers and subs.

Edit: I typically run 9-10 grains of powder for my 300BLK subs, think the lowest I’ve ever run is 8.5. Curious how y’all are getting yours to cycle at 8gr or less.

I did run H110 for subs and it worked decently but found CFE Black was more accurate for subs. For supers H110 rules. I've never found anything that's as accurate or produces more velocity in several 300 blackout rifles and even with monolithic bullets.

Lil Gun is also LOUD, temp unstable too, and not as good as CFE. I'd try the Vihtavouri N120 powder as well. It seems to work as well as CFE Black but is cleaner by a mile or four. But try CFE Black first and I think you'll find your cycling problems will be gone.

Also check for a misaligned gas block. I had that happen before if the gas block is canted slightly left or right the holes may not line up correctly and cause a under gas situation.

If you're expecting a 1 moa or better result with the subs you're smokin crack. There are so many things that will affect your accuracy that perfection is required on all of them. Neck tension, powder charge, position and place in the firing order and so on. All of those will affect the velocity and surprisingly by a greater amount than you may think. You need to get them all perfect just to stay even. On top of that you have the same issues every cartridge/gun combo has. You need to find the right bullet, powder, cartridge case and primer combo. It is possible of course but trying to figure out which bit has messed up is nearly impossible so blaming it all on the barrel becomes a problem the maker can easily push off on any of those other things. I suppose if they're giving you the guarantee with one particular type of factory ammo you might have a chance to blame the barrel maker but you wouldn't be looking for loading info and such if that was the case.

Buffers and springs can affect things but seldom to a great extent. With a gas operated rifle running 9gr or less of powder the problem normally comes down to insufficient pressure and volume of gas to operate.

In the end, especially with the blackout, most have found that the cartridge isn't what it was hyped up to be and have gone on to others. Remington messed with the chamber spec that made the whisper such a versatile round and accuracy for the subs suffered. Accuracy for the supers did too but not as much....

Frank

Understand fully if you don't want to spend more money. I've found really good success with the Rubber City titanium BCG which is a lighter than a standard BCG but not race gun light. Add a POF roller cam pin and you can get an Odin Works adjustable buffer or the Unrivaled dead blow comp buffer. Very flat shooting and helps reduce felt recoil by a good bit.

I can get sub-MOA with subsonic hand loads at 50 yards no problem and pretty damn close at 100 if I slap a big, ridiculous-looking scope on top. I did a lot of development work and I load for practical accuracy so I do take a lot of steps to equalize cases, charges, etc...but I'm able to bulk load on a Dillon too.

biffj said:

If you're expecting a 1 moa or better result with

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Over how many rounds

biffj said:

If you're expecting a 1 moa or better result with the subs you're smokin crack. There are so many things that will affect your accuracy that perfection is required on all of them. Neck tension, powder charge, position and place in the firing order and so on. All of those will affect the velocity and surprisingly by a greater amount than you may think. You need to get them all perfect just to stay even. On top of that you have the same issues every cartridge/gun combo has. You need to find the right bullet, powder, cartridge case and primer combo. It is possible of course but trying to figure out which bit has messed up is nearly impossible so blaming it all on the barrel becomes a problem the maker can easily push off on any of those other things. I suppose if they're giving you the guarantee with one particular type of factory ammo you might have a chance to blame the barrel maker but you wouldn't be looking for loading info and such if that was the case.

Buffers and springs can affect things but seldom to a great extent. With a gas operated rifle running 9gr or less of powder the problem normally comes down to insufficient pressure and volume of gas to operate.

In the end, especially with the blackout, most have found that the cartridge isn't what it was hyped up to be and have gone on to others. Remington messed with the chamber spec that made the whisper such a versatile round and accuracy for the subs suffered. Accuracy for the supers did too but not as much....

Frank

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this. 100%

I still love the round though, and shoot more of it than all my other calibers combined because it’s so easy to tune for my needs, and shooting subs suppressed is a hoot. Almost all of my shooting is well within 150 yards or so, and this round just flat works for 99% of my needs. And “minute of pig” is accurate enough for my needs.

That said, if I was starting from scratch today, I’d probably go 300 HAMR vs 300 BLK.

And I also have no illusions about how effective it is on game; again, for 99% of my shooting it works great, those 110gr Barnes black tips are awesome inside 100 yards, but if I’m specifically going out on a hunt for feral pig eradication or venison, I’m grabbing one of my 6.8s. Maybe even a .308 depending on where I’m gonna be shooting.