300 BLK advice for a noob?

One thing I will say is never bring a 300and a 223/556 to the range in the same day.

Oh boy. My list of lessons learned on this cartridge is long enough to write a book on. I load Blackout for my rifle and a friend's. Both have carbine length gas systems.

I found I needed more power than I thought I would to cycle the action of my gun. Then I found out my friend's gun wouldn't cycle until we upped the charge even more. This had me pushing into the supersonic range. That's still a work in progress. We are exploring alternate gas tubes and buffers to try and make something work.

I found out the blunt nose heavy projectiles don't get along with standard feed ramps - most of the time. They also have a tendency to get stuck in the rifling when loaded to the recommended OAL. I ended up seating them much deeper and the rifling wasn't a problem anymore. Seating depth was an issue with PMAGs also. I settled on metal 20 rounders and Hexmags and have been happy.

I have had trouble seating bullets into cases without tearing them up. I fixed this by using a .32 ACP powder die to slightly flare the cases. I assume this will cause my brass to fail earlier than usual but I don't care.

You can make your own cases from .223, it's really not that hard. There are jigs out there that make it easy to do with a small chop saw. Last I checked Amazon sells both.

If you aren't running a can, standard .308 projectiles will run your gun with authority. I usually have a box or two loaded up but you could certainly get by with just subsonics. It would definitely help logistically speaking.

I think that depends on who is loading and what for. The reason the service rifles have standard loads today is because there WAS in fact a military standard load for many years that bled over into civilian reloading. After all, if it works, why mess with it? I do think there is something to your comment about variety within the platforms available. Service rifles were available in far fewer configurations for sure.

I have seen powders and loads on the internet that range across the spectrum for this caliber. I personally use 4227 exclusively now, but I used to also use Lil Gun, mostly for supersonic loads. Many people report success with other powders, so YMMV.

I just shot my first BO reloads on Sunday. It took some experimenting to find a bullet that would work and it took some time getting the dies setup correctly. I was using cast bullets for most of the reloads. I had good luck with some 180 grain bullets meant for a Swiss K31. The nose on some of my other bullets were too wide to chamber because they'd hit the rifling. I had to experiment some with the die setup to get good neck tension. I used Win 296 powder with good results

There's quite a bit of variation in the load data out there. One of the benefits of the cartridge is that it's very versatile. I'm not aware of any standard bullet or powder for the round. Honestly I'd wait until you've got the rifle in hand that way you can verify the rounds will chamber, feed, fit the magazine before cranking out a bunch of ammo.

I purchased my brass from a vendor on Castboolits forum - $69 for 500 pieces. I didn't want to go through the hassle of converting brass.

Subsonic won't cycle either of my 300BOs, and since I don't have a suppressor that size anyway, I don't reload them anymore. But I was loading either 10.1 grains of IMR4227 or 11.6 grains of CFEBLK with a Hornady 208 BTHP or XTP, OAL 2.250 for subsonic. Supersonics will cycle well, and my favorite load is 17.0 grains of CFEBLK behind a 150 Hornady XTP or SST, OAL 2.070 which goes 1731.4 fps at an SD of 11.8.

I've got the stuff to convert 223 and it's easy to do, just don't need it very often.

1KPerDay said:

1. Is it worth the trouble to convert .223/5.56 brass to 300 BO?

2. I assume the real value lies in subs but is there any reason to load "standard" .308 projectiles?

Any gotchas to be aware of?

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Was for me, I really had no intentions on getting one but I was commissioned to build a machine for cutting 223 cases for 300 blk. This was the “proof of concept” machine I put together to test blades with and a rudimentary version of the final machine for the facility.



I had so much brass ready after R&D I threw one together. Never played with subs, just what ammunition they manufactured and asked my opinion on. That said I already had a 9mm AR that does anything I’d use sun 300’s for.

I have used lighter .308 supers on a couple hogs, it works but isn’t my favorite.

Main “gotcha” would be that you can chamber and fire a 300 blk round in a .223 barrel, with less than ideal results.

Converting the brass didn't make sense for me. It could IF you had a lot of the same maker 5.56 on hand and free time. I simply bought brass from Starline so I had a start point.

Each gun, especially short ones are unique and so need to be loaded for one at a time. I have been working the other way to find subs for a bolt gun, takes time.

I use 30 carbine bullets, and load for around 19c/round, they do about 2100fps, and are reasonably accurate. They feed fine out of Pmags, and GI mags loaded to 10 rounds. I use a pipe cutter and drill, size with wax, and can convert cases in about1 minute each. The only issue I have had is neck tension using converted brass. I sanded my expander to 0.306 and have not had an issue since.

I have converted some 223 brass and bought some 300 AAC brass. I found some reasonably priced 300 so pretty much just use it.

I realize you have a carbine length system but I can only share my experience. I have a 10.5" AR pistol and run an Omega 300 suppressor. It functions well with sub and super sonic ammo. I easily achieved quiet, soft recoil, 100% cycling subsonic loads with reasonable accuracy (for a 10.5") using the combinations below with published data.

I focus my loading on sub-sonic and have had particularly good luck with A1680 and CFE Black with Sierra 220 Match King, 220 gr Pro Hunter and Berry's 220 SP. You can spend more for projectiles but for my plinking purposes these work well.

Hope this helps

Wellll, glad to see Im not the only one in the same boat, thanks for posting 1K. In the same boat, buddy of mine built a 300 but is wanting to reload. So we have the cases and the 300 ACC die, FL, RCBS. The first issue we ran into I had a question about is that he is using 200gr plated from Everglades and there is no taper to the base of the bullet. So it is it common to have to hold the bullet on the case until you run the ram up?
Since I cannot locate data in the printed manuals I have, just pre Blackout by a year, went to Hodgdon's site and found a max of 11.4 for 208gr for CFE Blackout. Figured we start at 10.4 (-10%) also have to seat it further down because a dummy bullet jammed up in the mag (2.250 COL). That issue gets worked on Friday.
Edited- read label, actually plated bullet, loading for subsonic anyway.

bbqreloader said:

Wellll, glad to see Im not the only one in the same boat, thanks for posting 1K. In the same boat, buddy of mine built a 300 but is wanting to reload. So we have the cases and the 300 ACC die, FL, RCBS. The first issue we ran into I had a question about is that he is using 200gr FMJ from Everglades and there is no taper to the base of the bullet. So it is it common to have to hold the bullet on the case until you run the ram up?
Since I cannot locate data in the printed manuals I have, just pre Blackout by a year, went to Hodgdon's site and found a max of 11.4 for 208gr for CFE Blackout. Figured we start at 10.4 (-10%) also have to seat it further down because a dummy bullet jammed up in the mag (2.250 COL). That issue gets worked on Friday.

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As I mentioned above, I am actually flaring cases to avoid problems, particularly with flat bottomed bullets. My OAL tends to be around 2.10-2.15 so I can fit the bullets into the magazine without issue.

This is my current curse. The 300 AAC Blackout is what got me building ARs. My wife wishes I’d never heard of 300 Blackout because I’ve developed an addiction. I started with an assembled 8.5” upper and a lower build kit which I put on an Anderson stripped lower. I had issues with factory subs keyholing, so I started a new 10.5” build. I picked up plenty of 223 cases and proceeded to load subs and supers using A1680 and CFE-BLK with some success...but couldn’t get the subs to cycle reliably in either pistol. Sidetracked and built a couple ARs in .458 SOCOM, then built a 223 Wylde upper (to get my own once-fired brass to cut down for 300 BLK. Found out my range won’t let me shoot .458 SOCOM, so I bought a steel hanging target for my DIL’s uncle who just bought property one town over so he can shoot in his backyard (and me too, I hope).
Last week I tested some 150gr Hornady and some 220gr Lapua Scenar L rounds I made up over IMR 4227. Nothing cycled. Neither supers (150s) nor subs (220s). Back to the drawing board, as single firing rounds from an AR really sucks.
Admittedly, I did make the error of building the 10.5” with Carbine gas, and no suppressor. I have a new 10.5” barrel with pistol gas...I’m debating whether to start a new build or just do a barrel swap. Meanwhile, my addiction has me considering putting a suppressor in jail, now that Gander Outdoors has all suppressors on 50% off sale...plus my $15 off on $75 coupon. Makes the price ridiculous vs outrageous! Still trying to be nice to wifey to gain approval for such an unreasonable purchase. But then it was my stated intention, prior to my original build, to build a suppressed .300 AAC Blackout AR pistol for home defense.
Okay...saga over. I’ve been off the board for quite a while so had to do a brain dump, and this was the thread I needed.

troy fairweather said:

One thing I will say is never bring a 300and a 223/556 to the range in the same day.

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Actually, I’ve done this...But...I had my 300 BLK AR pistol, and a separate.223 Wylde upper. Transitioning from one to the other definitely took conscious thought and deliberate action. We shot all the Blackout ammo first, then swapped uppers and I had a 120 rd box of American Eagle 223 ammo. For me it’s a bit easier to keep things separated, as I don’t reload 223/5.56, and the purpose for the 223 Wylde was to have fun shooting cheap ammo, which feeds empties to my chop saw and 300 BLK jig.

I do like the 300 Blackout. Started back in the day when I shot a 30/221 AKA 300 Whisper in metallic silhouette. Today I have (had) 4 300 BLK Rifles and 1 300 BLK pistol. The H&R Single Shot and Ruger American Ranch Rifle along with the AR pistol and 2 AR rifles, all three Frankenbuilds. One of the Rifles and the pistol are built around a pistol length gas system, the other is carbine length. Carbine length was built as a dedicated supersonic rifle. I sold the pistol length rifle barrel because it did not suit my needs.

I don’t think I have ever shot but 20 factory rounds. Almost all are handloads formed from 223/5.56 brass. It is an easy conversion. The 30/221 has to be formed from 221 brass or the necks have to be turned, the 300 BLK I have never had to turn a case neck. Brass is free, just pick it up off the range floor. My favorite bullets are the Sierra 125 grain PH, the Hornady 208 grain and for plinking .308 150 grain FMJ that I buy in bulk. I also use cast bullets, the Lee 230 grain and the Lee 155 grain designed for the 7.62x39. Favorite powders are 4227, 296, 4198, and CFE Black.

I have learned to not much like the subsonics for hunting, they were much better on the 200 meter ram when silhouette shooting. I will only use supers when hunting.

Found out my range won’t let me shoot .458 SOCOM

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Why is that?

1KPerDay said:

Can someone go over the process for converting brass? Chop, lube, size, trim/deburr? Or is it more or less involved than that?

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pretty much.

Is it really worth converting 556/223 brass anymore? Buy a couple Freedom buckets of Remington 120 gr 300 BO when its on sale (seen it as low at $0.50/rd) and you will have some decent cheap Super sonic plinking ammo and plenty of brass for use later.

troy fairweather said:

pretty much.

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That is about it.


My 300BO is a carbine and I've only shot supers to date. Other than a handful of factory rounds, I load pulled 147 grain bullets for plinking. I haven't really done much target shooting with it so I haven't loaded any premium bullets yet, but I have a few boxes of SMKs on the shelf that I may use someday. propelled by H-110.

For me, it is just a fun gun. Down the road, I might suppress it. Maybe.

1KPerDay said:

Can someone go over the process for converting brass? Chop, lube, size, trim/deburr? Or is it more or less involved than that?

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My process. Run through FL sizing die with expander removed. Chop. Deburr. Run through FL sizing die with expander installed. Trim, chamfer and deburr. Done

mcb said:

Is it really worth converting 556/223 brass anymore? Buy a couple Freedom buckets of Remington 120 gr 300 BO when its on sale (seen it as low at $0.50/rd) and you will have some decent cheap Super sonic plinking ammo and plenty of brass for use later.

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It depends on you. For me? Absolutely. My 300 BLK brass is free. Yes, I have my time invested, but I look at it as time invested at the range. I enjoy handloading and I enjoy shooting. The time is a bonus to me, not a deterrent.

So you still like and use the cartridge even supersonic and without a suppressor? May I ask what you like about it?

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308 size holes at a 223 recoil and rifle weight...or thereabouts. It's a nice medium between 223 and 450Bushmaster in recoil, has a nice thump when it hits. Relatively inexpensive to load given the powder charge and availability of 223 brass.

Can someone go over the process for converting brass? Chop, lube, size, trim/deburr? Or is it more or less involved than that?

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Yeah, as said above, pretty much. EXCEPT my primary initial 300BO dies were Lee but I had to buy an RCBS Small Base 300BO sizer for the cases I converted from 223. The Lee dies will resize used 300BO brass just fine but the first conversion needs to be done with small base dies or they won't fit the cartridge gauge. Or at least that's how it was for mine.