You can use Ruger RPR 223 mags if they are still available.
Any gun store should be able to order them.
They seem to work well... At least in an RPR.
Pay attention to the actual internal round OAL for whatever you get. Some mags are shorter than others.
I got a couple actual AI mags from Jerry Teo at Mystic, but they were used and didn't feed reliably any more, (he warned me about it first BTW... not like he screwed me over or anything) so keep it in mind... the feed lips on plastic mags do wear out.... So order extra.
There was a thread a while back (Around December 2016) that discussed internal OAL length and someone uploaded good pics of the plastic insert inside the steel Accurate Mag 223.
If you go with those, it looks like you can modify the insert to extend the round OAL, but youlll need to fill the ends with something like JB weld before you do or you'll over shoot the mark.
MDT Mags didn't feed worth a SH!!T in my RPR. I have 2 of them and they both end up with the bullets pointing straight up in the mag and obvious jam and cursing to follow. I don't know if they work any better in a Rem 700 foot print though.
I'm barreling up a Defiance action for 223 so I'll know soon enough.