‘Buckle Crunch’. So firstly let's try and describe buckle crunch.
Let's pretend you had a pencil and you wanted to break it. If you pulled it from one end and a second person pulled from the other the chances are the pencil would not break. Now if you placed the pencil in both your hands and "snapped it" the pencil would of course break immediately.
In theory we have described "buckle crunch", except you need to replace in real life the pencil with the buckle located on the cars seat belt.
In normal passenger usage the force in a accident is applied down a seat belts length but when placed around a car seat sometimes the seat belts buckle lies across the frame of the child cars seat. In event of an accident the seat is then thrown against the seat belt buckle and it can burst open as the force is going "across" as opposed to "down".
In practice however many car seat fitters definition of buckle crunch varies.
Some people consider that even if the buckle touches the frame of the car seat that is enough to consider the child's car seat as being unsafe in the car.
Other people are more tolerant and define buckle crunch when the buckle lies across (or nearly across) the car seats frame.
In short this is a matter of opinion but everyone would agree if the buckle is laying across (or nearly across) the car seats frame it is without question unsafe to be used in the car you are trying to get it to fit.
We started out with a group 0+ baby carrier which was part of a travel system, and then moved on to a group 1 seat when our daughter was about 13 months old. The new seat has a nice recline feature, so that when she falls asleep in the car, which is quite regular, we can recline her so she is nice and comfy and isnt slumped forward.]
DAD TIP: Try and go for a baby carrier that has a base that you fix permanently in the car. This means that everytime you take your baby in and out of the car (which will be a lot), all you need do is click the carrier into its base, rather than mess about with a seatbelt everytime.