M1 Garand

[mg34]

The ammo it isn't. This a Springfield Armoury Inc Copy? Or an issue rifle? Are you getting the trigger group in properly? It's not just closing the trigger guard. Original stock, a snythetc or other aftermarket stock? The trigger group is held in mostly by pressure on the stock. It may be too small or mushy. 'It' being the stock. A bit of bedding material on the stock will fix it.
"...it was originally designed for 147-150gr..." Nope. 174.5 calibre .30 M1 ammo adopted in 1924.

You are right, it wasnt the ammo. An reference book on ammo shows that the M.1 ball round used a 173gr bullet, while the M.2 ball150gr and that was the standard for World War 2. The 173gr M.1 ball round was used in 1906 and that was used for the 30-06 bolt action rifle.