JWR,
I’ve looked into getting an AR-10 to supplement my AR-15 and considered the SI Defense and CMMG options that accept the widely available and very inexpensive Heckler und Koch G3 20 round magazines. The former requires minor modification of each G3 magazine, the latter requires modification to an upper receiver, and unfortunately those lower receivers are no longer produced.
I know you’ve mentioned your plan to convert to SI Defense receiver AR-10 rifles, but I did not see a mention of the required magazine modification. The instructions to modify the magazines can be found here. The modification involves removing a bit of material from the feed lips, evenly from both sides. I have not yet taken the plunge on this rifle, so I do not know how difficult this modification is or how it affects the use of modified G3 magazines in G3 rifles. I would be interested to see if any of the Survival Blog readers has experience with either of these rifles/conversions. – Andy J.