Certain matches have certain weapons requirements and allowed modifications , and you only get so much time to shoot your string. Apparently there is a long range match that allows for highly modified Garands . You insert a round by hand release the bolt shoot, insert a round by hand release the bolt and shoot , faster than single shot bolt when you get used to it.
Back when I was a kid I had a H&R 22lr that worked that way, only you had to manually close the bolt.. very heavy bolt I might add , very large "ejection port" .. you inserted the round by hand and slid the bolt close , took your shot , inserted the next round closed the bolt and took your shot.. it was faster than my JC Higgins single shot 22 bolt.
I later fantasied about adding a spring and some how attaching a magazine... never did , grew up and realized I probably would have got into trouble LOL.
Look, you are probably going to have OAL problems with 190 grain rounds fitting the clip into the magazine .. so it is going to be single loaded anyway.. why make it any harder by making the shooter retract the bolt eject the shell and lock the bolt back. okay it is just one operation .. but still, and hammering the bolt lock up on every shot can't be the best thing for the action, so why not let the action do it's thing once you get the gas port set right.. all the limitation on powders and bullet weights is based on how much gas force hits the op rod.. we know with an adjustable plug you can do things a stock Garand wouldn't like. So tuning the gas to a single consistent powder load and a single bullet weight is not that big of a deal.
Can't deny a heavier bullet is one way to get on paper at 1000 yards.. are there better ways , yes , can you do them in a Garand and retain full semi auto or keep it match legal , don't know don't know what the match rules were way back when or which match it was built for.. I don't know, perhaps some of the other ways weren't available or desired for one reason or another .. lot of new options have opened up in the 20 or 30 years since the rifle was made.. Customer wanted a Garand , Customer thought 190 grain was the best way to do what he wanted to do, Dan McCoy was a certified magician with Garands ,, only 3 folks ever wanted that particular combination .. the Gunsmith made it happen .. Every thing is a trade off , no accounting for taste or what makes a shooter want a rifle that will do something a certain way.