The Garand En Bloc system is retarded. I'm sorry. There I said it. I'm not a fan boy of either design here either I have no vested interest lol It's just goofy. That's why it was never used again anywhere in any design that mattered half a shred. Take a step back and think about it for a second.
Anyhow as for using an AIA mag in a 7.62mm NATO Garand I can't speak to that as I have no experience with a 7.62 NATO Garand but my Norc M14 runs a 10 round detachable AIA magazine perfect "friction fit" or not. I can top it up with stripper clips or single rounds from the top without even removing the mag very quick or I can just take the mag off and put a fresh one on very fast.
It's not just 5/20 mags or 5/5 mags
There's also 10 round mags that actually hold 10 rounds like the AIA
Sorry for the necro bump, but saw this and almost spit coffee all over my monitor.
The en-bloc system is NOT retarded. It is, in fact, a VERY effective way to maximize ammunition carrying capacity. The en-bloc went away for two important reasons:
1) After WW2 the US military went to a very tiny standing army and relied heavily on the conscription model - so much so that they seriously struggled to field a fighting force for the Korean war. Rifles were in such poor state of availability and good repair that they had to start building new M1's again and slap-dash refurb the ones they had not scrapped or given away as war aid to european and arab allies. They actually sent supply ships to pacific islands to recover combat-damaged and abandoned M4 shermans, brought them stateside for refurbishment and hurriedly shipped them to korea because they had bought so much into disarmament and the peace dividend post-war.
What this meant was that in Korea they had almost no time to train conscripts after depleting the Japanese garrison for trained fighting men. The M1 is most effective when the manual of arms is trained to the point it becomes muscle memory. A well-trained M1 rifleman can reload the M1 in less than half the time to reload the M14 with its rock-lock magazines. But such training could no longer be afforded from a time perspective.
The USA went into vietnam with the same conscript army mindset. They had just trained the south vietnamese army for almost a decade, and the suoth vietnamese gave the M14 a pass and stayed with the M1 rifle and carbine until very late in the war. Their well trained infantrymen liked the M1 over the M14. The court-ordered and drafted US conscripts with 5 weeks training did better with the easier to learn M14, despite its foibles.
2) The loss of marksmanship value in training. The M1 relied on aimed deliberate fire to be effective. The doctrine around the M14 was on volume of fire over an area of effect. "covering fire" over aimed fire. The idea behind the M1 was to get as low to cover as possible with no protruding magazine to raise up your profile and to deliver sustained aimed fire. In a squad of riflemen, the 2 second reload times would not affect overall rate of fire. The M14 was based around the idea that poorly trained soldiers could deliver the same volume of fire as well trained M1 riflement, but they did so at the expense of less cover in some cases.
In terms of advantages, the weight per volume of ammo is lower with an M1. The three en-block clips weight less than 20% the mass of an empty M14 magazine. Despite the longer rounds, a .30-06 round weights only a few grams more than a 7.62x51 round. An M1 rifleman could carry more ammunition in en-blocs than an M14 rifleman could carry in magazines, the weight of which diminished the overall carry capacity. En-blocks could also be carried in lightweight disposable cloth bandoliers. and en-blocs could be made very very cheaply compared to magazines.
The advantages of en-blocs have long been known. The French preferred the 3 round Berthier en-bloc to any other system in WW1 because it could be reloaded so quickly by a trained soldier. The same was true of the M1.
I would submit that if anyone thinks the 8 round en-bloc is a bad system, they need more time with the system and train to use it properly. It says more about your skill level than anything else.