17 years ago there was an interesting thread . It's even more interesting with the advent of the SFAR .
https://www.ar15.com/forums/ar-15/To_keep_it_interesting___the_case_for_7mm_BR_over_all_the_others/121-338844/

What ever became of Marty ?

If that's Marty Weeme last I heard he moved out of the country years ago. 7BR is a good size for large frame guns which I assume is why JDJ used it for his Whispers. Looking at the 8.6 Blackout it's a pretty similar case length. If it ends up sticking around I bet we see some wildcats in various calibers.

The 7mmBR doesn't have enough case capacity to get good velocity out of it's heavy bullet. It was designed to be a handgun cartridge. They need to look at the 6mm bullets. probably the 6mm-6.8 SPC would be my current choice.

He's talking about a 308 sized gun.

Same deal. SFAR is the way to go though.

Not necessarily. He does talk about the 2.26 COAL and the ar15 being his main interest ... but also points to MGI modding a lower that'll house the M14 mag . The only issue at the time was bolts and extensions that would take the pressures and the larger diameter case . If I remember correctly , the shoehorning of the m14 mag also ate into the bolt hold-open space so that was gone.  

As far being considered a "handgun" bullet , any of the intermediate cartridges could be considered handgun cartridges if you go by the silhouette shooting sport with short bbls and fast powders .

For a short bbl of <16" the benchrest case is going to have more volume than a grendel/ppc or SPC ...and higher operating pressures. And , a 7mm bullet is going to have an easier time mustering BC than a 6mm ... as well as more bore area for those faster powders . You can extrapolate that idea out to .50 cal to see what I'm talking about ...it's somewhat similar to altitude , sea level the air is denser ...relative air density also operates on bullet size ...

* the 8.6 blackout is interesting if you're in a ar15 sized envelope. (44.5gr case capacity)

7BR and 6BR were both done on the standard AR-15 platform by American Firearms around 1995.  
They had issues with bolt breakage.  However, using a SFAR extension and bolt, it should work fine in a standard AR-15.
Unfortunately, there's not really a good AR-15 magazine for that case diameter.

Marty came back to the States about a year ago.

Tony

Good to hear about Marty . Shame on the mag situation.  I have seen some ATI 410 mag suggestions , but I'm not convinced .