My friend owns an experimental Garand and has been looking for ammunition for a long time.  Here is one of the rounds we were able to buy recently.

These were made in 1957 by Winchester.  Some held 1 bullet , 2 or three.  This is what they called the duplex.  Project Salvo.  12 guns were made and 10 are in a museum.  Only 2 in private hands.  This one is mine.  He has a single and a duplex.  Now needs to find the triplex.

Edit: to make it clear I do not own one of the guns.  Both were sold at an auction and he was able to get one.  This is the one he has.  He did not buy it from RIA.

https://www.rockislandauction.com/detail/64/3492/springfield-armory-us-m1-rifle-2206-duplex