I posted on GD a little while back that I put together what could very well be the worlds biggest LR-308/AR-10. Just to recap the 547 Ronin is a .55 caliber wildcat based on a cut down 505 Gibbs case. Despite trying to get it approved for a sporting exemption I did have to DD this. All in all a bit of a hassle but its up and running....sort of.  I will say sorry ahead of time for the lack of mag dump vidoes



Build list:
Doublestar Star 10 receiver set
23" PacNor barrel with 1:16 twist- carbine gas length
Kak Magnum Bolt carrier group
Wojtec Adjustable Gas block .936
ACE Rifle Stock
Apex Gator grip handguard
Blackwood trading grip
KAK Heavy Rifle buffer
Safety I had in parts bin
Stock trigger I had in my parts bin
       - Final rifle weight without scope/mag is 9.1 lbs

Really the only problem I had was getting it to feed properly. My first outing I tried DPMS steel mags I found at the local gun store. I could only get one round to feed and the steel lips gouged up the sides of my very expensive rifle brass. Moving on I went to tried and true magpul. Now, the only problem with these is that they have a ridge going down the length of the mag to help guid the follower. Only problem is that it blocks the rounds from single stacking. I also found out pretty quickly that the mag lips flex open if I load more then two down. I managed to fix both issues with some properly applied epoxy and quite a bit of file work. Its definitely not pretty but at least now  it feeds reliably.

Performance: I was hoping to hit 1800ft/sec with these loads so real world performance is a bit lacking. H335 is kind of my go-to big bore powder and even that turned out to be lack luster.  Which now puts me at a bit of impas with powders. This round definitely needs something a bit quicker then H335 but the only power that is available  to me right now is CFEBLK, which  i'm a bit hesitant to use.  For the time being I might just try a compressed load of H335 and see where that gets me.

67 grains Accurate 2460 (approx 95% fill)
600 Grain CEB bullet
2.81 OAL
Remington 9 1/2 Magnum primer
     -1584 ft/sec average

Same set up with 70gr. of 2460
       -1601 ft/sec

70gr of of H335
     -1638  ft/sec

Is the case head rebated to .473" or did you open up the bolt a bit for something a bit less rebated?

As for feeding, you might want to try a steel ProMag.  The feed lips aren't closed out at the back, making them easy to bend up and outward to allow your case head to sit higher.  ProMag on Left.

Tony

He has the KAK magnum bcg so I'd suspect his rim is .535ish .
Awesome rifle and round OP

Yep, has a standard magnum bolt face and I turned the rims down to .532

I have the LR-308 in .500 Phantom and .470 Rhino, both done on the .500 Jeffrey case and they use the "magnum" bolt face.  I have had perfect success in feeding using DPMS steel mags, open at the back so the feed lips can be adjusted all the way from rear to front if need be.  The bottom of the feed lips did need to be polished nicely to keep them from scratching the case.  They single stack just fine with the Jeffrey case so I would think they would single stack just fine with the Gibbs case too.  I'd be a bit surprised if you have had any feeding problems with DPMS mags.  Polymer mags, such as P-Mags and even Lancer, swell too much and will not insert bast two or three rounds.  I have some old CProducts (before CP became CPD) steel mags as well as Brownells mags and they sort of kind of want to try and work but the closed back on the feed lips causes tweaking problems.  D&H mags are dead ringers for the DPMS mags with the open rear feed lips and I am certain they could be made to work, but I have so many DPMS mags tweaked to work I have not felt the need to muck with the D&H mags.

I have had to grind out a little on the follower to keep it from pushing the round to the side, which seems to have caused a problem but I won't swear to it, but if it did, a few seconds with a Dremel 1/2" sanding disk takes care of it if it was an issue.  I'm still not real sure why the follower could have caused any feeding issues with such a fat case single stacked.  On the .500 Phantom, I have had to put spacers in the magazine on the front side if I want to use short hunting bullets in it, but I did that rifle with the intent of using 647 gr. BMG bullets loaded out to full magazine length so no spacer is needed with them.  The .500 P is a very short case, really designed to play in the AR platform, not the LR, but since I wanted to use BMG bullets, I had to have it done on the LR-308 frame.

You might other powders and if you don't have a program like Quick Load, I'd highly recommend getting it.  My .470 Rhino (case cap 100 gr. H2O) was Quick Load projected to throw a 500 gr. bullet at 2250 fps, cloning the .470 Nitro Express, but I could only get 1850 fps with a 500 gr. Woodleigh and IMR 3031 powder and have any kind of case life.  As you know, parent brass is outrageous and parent brass cost is what killed both the .500 P and the .470 R.  

My .500 P is a 16 inch with CAR gas and the .470 R is 24 inch with Rifle gas.  Both function perfectly.  The .500 P started life as a midlength gas and I had a horrible time getting it to function properly with BMG bullets until I redid the barrel to CAR gas.  Then it functioned perfectly.

https://www.grafs.com/retail/catalog/product/productId/76160

That's a heck of an investment.  I had looked at the 416 Thumper and thought it'd be nice necked up for some .452 high bc muzzle loader bullets . But I'm poor so I decided no .

I have another question for you.  Did you modify the barrel extension to big bore configuration like Tony does with the .458 SOCOM?  Marty (actually Paladin since he did all Marty's barrels) did the barrel extension mods so that it looks like the .458 SOCOM barrel extension, of course, without removing the bottom center bolt lug on the BE.  If your BE is not modified then that could have contributed to your feeding problems, especially with the DPMS mag?  DPMS mags-and probably D&H-are the only ones I can modify by tweaking the lips just a bit and have them feed perfectly with full mag length rounds, even when loaded up fully.

I actually was able to purchase the barrel extension with the big bore modification already done.
https://triuneprecision.com/product/ar10-big-bore/
I should have mentioned that Paladin did the work on my barrel. His work is top notch and I couldn't be happier with how it turned out.

This week I should have time to work on tweaking the DPMS mag that I have. I'm sure I will get them to work, time and effort.

Glad to see this posted and interested in further testing. The last time I went to see Thomas he said check this out and google wasn't bringing up anything.

I haven't used the Triune AR10 big bore extension, but the 15 big bore works great as well as their standard offerings.

Wow, those BE look niiiiice.  Well worth the price IMO.  
Another little tidbit of info that might be useful, might not though too.  I did a .358 cat on the LR-308 based on the .325 WSM case shortened to 1.800 inches and that has been a nightmare.  I cannot load more than 6 rounds in it without problems but one of the things I found out while fooling with it was that if I bent the feed lips up too far, they dragged on the bottom of the bolt carrier (that turned out NOT to be the cause of my problems though).  If that happens with yours since your case is so fat, you may have to pull out the Dremel and widen the gap between the feed lips by removing metal instead of just bending them up.  I am assuming (uh-oh) that your feeding problems might be caused by the case not coming up far enough out of the magazine for the bolt to pick it up and feed into the BE.  IF that is correct, then you can only bend out the feed lips so far before they rub the bolt carrier, then the only way to widen the gap between the feed lips is to remove metal.  And that's a situation where once removed, you cannot put it back on again.  Good luck with it. Too bad the NFA would not give you the sporting cartridge exemption from DD, but we all must lick the boot of our overlords.