What is te goldilocks cart for the AR15 type rifle?
300 Blackout on one end and 30 RAR on the other.
If you use the 6.8 SPC case and have a case length of 1.41"
Caliber length neck
Same Taper as the SPC will give you about a 35 thou shoulder per side.
These dimensions will allow use of unaltered 6.8 SPC mags.
About 20% more capacity over Blackout for better supersonic performance.
Any thoughts?

Makes sense, from a case capacity view. That taper will feed well

The downside:

1. 6.8 magazines are a thousand times as hard to come by, and a lot more expensive than the standard 223. Brass is very hard to find, once-fired military harder, and expensive to purchase commercial, if you can find it.

2. The 7.62x40mm Wilson is very close, and uses the .223 mags.

3. An improved 7.62x39 duplicates, and is a rechamber away in std 7.62x39mm chambers.

I built a pistol AR in 300 BLK last summer, before Delta Company Arms blew up. You know me and cast bullets; I have an NOE mold that makes a really neat spitzer flatnose 252gr (w/GC and Lube) that I shoot about 1080fps. Same basic ballistics as a factory lead bullet 44 Magnum, and thirty in a standard AR magazine. Other end,125-135gr over 2200fps.

My opinion, which worth about as much as you paid me for it; if you had a 6.8 and about 1000+ pieces of brass, your thirty would be a neat companion cartridge as a home wrecker.

regards,

Rich

Thanks Rich
Hope you are well.
Mags are not something you purchase often but the concern was using stock mags.
The 308 bullet shaft and 6.8 SPC neck diameter are virtually identical so a nominal 308 protrusion would just replace the neck position and the neck would start where the normal 6.8 shoulder would start.
Pretty much just up the dimensions all around of the 300BLK a bit but more taper as you say to give better supersonic performance.
Loaded out to 2.3" you could see some decent performance shooting those 115 and 120 grain bullets.

I've got a reamer and gages for a 30 AR Turbo 40 Improved. This is based off the 6mm AR Turbo which is a round designed for Highpower XTC competition and is essentially a necked down and slightly improved 6.5 Grendel. This design is blown out with a 40 degree shoulder and seems to get about all the available capacity out of the Grendel case for a 30 cal AR round. There are several similar rounds based on the Grendel, one being the 30 Major which was developed for BR score shooting. I haven't chambered a barrel with it, but I bought it to build a mini-mauser for my daughter and will probably put together an AR upper with it. Probably will build a 30 blackout too.

There are a several reasonably priced mag options these days for the 6.8 and Grendel cases. Yeah, they might be a little more expensive than a .223 mag but if you are dealing with a wildcat, you are already incurring more cost for brass and dies. It's a small penalty to pay for the additional performance.

I like the Grendel and variants. Heck look at my sigline with the 358 Gremlin.
I like the 30 Major but was thinking for a 6.8 specific cart. You hear about some issues with the 300 BLK like shoulder bumping forward in a ar15 or anemic supersonic performance so this would be a better cart if you wanted something similar and had a 6.8SPC already. Part of my thinking was have a cart that you could use 30-30 bullets for hunting within the 30-30 functional impact velocity envelope to 150 yards. The extra capacity and loading out should make that happen. I think the cheap 30-30 bullets could make this a good plinker/hunter/home defense cart. It just needs a name lol. Well a name and reamer and dies and 2k for development ect. I need someone with the right program to help me estimate case capacity for estimated performance. It is funny the parentage starting with the 30-30 case to the 30 Remington to this 30 ____? Hmmm what to name this.

30 Badger!
AKA 30 Honey Badger AKA 30 H.B.
How does that sound?
Sounds better than 7.62x36 I think.

I also shoot the 6mm PDK which is 6.8 SPC necked down, shoulder moved forward about .050", and 30 degree shoulder. It would give decent capacity in 30 cal and may have already been done. The 6mm Hagar, for which Hornady now supplies factory brass would give the most volume on the 6.8 case head (I think it is the same/similar), moving the shoulder even further out, but there won't be much mag length left for a bullet. That is the biggest issue with the SPC case and where the Grendel platform has an advantage being shorter and fatter to get the volume, but still leaving enough length that the bullet base isn't intruding into the powder space as much.

I am in the process of building a 6.5 GM ( Grendel Max) which is simply the Grendel with the shoulder moved forward by .05 and gains about 10 % case capacity although that is with the 123gn A Max loaded to 2.4.There are a few other mods like neck dimensions and throat length to suit the new loading requirment etc but Grendel dies are used with a .05 spacer fixed onto the top of the shell holder. A simple neck up and a run through the FLS die with a .006 shim to give a crush fit on Grendel brass and fireforming is all that is required. I prefer to have my die set against the shell holder which is why the spacer is on the shell holder to get consistent sizing. If you had a Grendel die opened up in the neck to suit your 30 cal, and changed the 6.5 expander for a 30 cal you would be away laughing. A reamer of course and hopefully you could get it sooner than the wait we are having from Mansons. GRRRR, a 6 week eta has turned into over 15 weeks with paitence wearing thin about now.