My baby was army crawling for a month and then just suddenly started proper crawling overnight
This was the same with my baby girl & she started crawling when she learnt how to sit up and then never looked back, all happened within about 24 hours xx
As someone else said, my baby can crawl on all fours but finds it quicker to army crawl, especially on hard floor, so tends to stick with that! Crawling isn’t a milestone that needs to be hit, as babies do it in different ways.
My baby is 10 months and has been army crawling since he was 4 months old - he can crawl the traditional way but he’s faster when he army crawls so he prefers that! Some babies bum scoot and don’t crawl at all - as long as they can get around I don’t think it matters 😊