HobbyFix did a very limited number of M4A1 models (150 and 70 pieces) in 1999/200, and M16A4 (160 pieces) in 2001. These models are very realistic, but dummy only models. This means you can only cycle dummy rounds manually by pulling the charging handle. They are very expensive today if available at all. I have not seen one cheaper than 2500 – 3000 USD over the last years in Japan.
In 2022 Marushin did a new production run of their XM177E2, M16A1 and M655 (Model 651 would be correct for a 14.5 inch barreled carbine with rifle stock) series. It has some minor modifications and a gas tube added. Also the black finish is glossier, thus more realistic. The greatest improvement is a new cartridge design. These models were sold out fast in Japan, but there is a chance that Marushin may do another production run. It is made of zinc.
If you like to “fire” the modelgun often and if you want a reliable model, the TanioKoba model is the way to go. It is made of plastic, but it is the most durable and reliable “AR-15” model out there. You will have to spend at least 1000 USD tough to get a good one. It is also the most modern copy of an AR-15 out there, featuring a modern charging handle and more modern stock.
We have to bear in mind that the Marushin series was designed in the late 1970s, the HobbyFix 199-2001, the MGC M4A1 around 2003 and the TanioKoba around 2010.