yes it will accept and work just fine with the 20 round mag

Michael, yes Sir. Most manufacturers including Springfield make firearm packages that have 10rd magazines so their firearms can also be sold in states that have magazine capacity restrictions. The firearms are exactly the same but just have 10rd mags included. The firearms will also accept any standard capacity magazines just fine. Cheers and stay safe!

Yes it will accept 20 round M1A magazines.