I started a previous thread on building a “short mag” AR but I cannot find the thread.

Basically, I stumbled on the KAK magnum bolt carrier group late last year and decide to build something I had been wanting for years. An “AR10” in a short magnum. Unfortunately KAKs 300WSM uppers wouldn’t fit my mega lower. So the parts sourcing began. The upper parts list is as follows.

- Mega MaTen upper + enlarged ejection port, .125” removed from the bottom lip
- Xcaliber barrel - 7mmSAUM, 1/8 twist, 18”, rifle + 2” gas length, threaded muzzle
- rifle speed gas block & white oak 14” straight gas tube
- KAK magnum bolt carrier group(barrel was headspaced to the bolt)
- SLR Handguard
- Xeno mount
- Nightforce mount
- Bushnell DMR3

My first loads were 150 ELD-X over H4831sc in ADG brass. The initial shots went well, I had the rifle speed set at six and single fed rounds over an empty mag. The gas was enough to lock the bolt back and the brass would just clear the port. Grouping was “good” for a load testing. Recoil was more than a 6.5 Grendel less than a 308 both from mid length gas systems.

Overall I’m ecstatic, the rifle comes in at 12lbs empty, it’s on the heavy side but it’s well balanced. Other than load development the last problem to solve is the mag. There are no available magazines that will feed more than 3 or so of the rounds reliably. I’ve tried a new follower to no real positive result, a fully 3D printed prototype single stack has some material issues that aren’t worth fighting, but I have a hybrid approach in the works that may solve it.

Does anyone have experience with the 7mm SAUM?

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Armalite was able to make SAUMs work with four or five rounds in a magazine back in the day.

Not sure what they did to the mags as I’ve never seen one. Maybe worth the research.

I built one in 300 SAUM. For magazines, use the Turkish 1919 shotgun mags. They work great. I can't say I've used every variation of the mags, but  what I bought off GB was fine.

I did a .358 HDH cat based on a .325 WSM.  It will only feed 5-6 rounds with tweaked DPMS mags.  Lancer and Magpul mags (any polymer) swell too much to even insert after 3-4 rounds and any metal mag with the feed lips closed at the rear are too difficult to tweak to make them work.  The follower of the DPMS mags don’t even need any tweaking, just the feed lips.  But… after 5-6 rounds, not going to happen.  What’s needed is a .308 based mag on the outside dimensions but inside using a .308 length sled follower that allows single stacking.  If only one had a good 3D printer.