Racing Muzzles

[RealMadYid]

I started racing my dog with a size 5 plastic coated muzzle, but I noticed when he is free running he really opens his mouth wide. So, recently I bought I size 6, but this too does not allow the dog to fully open it's mouth. I have noticed all greyhound muzzles are really deep set, but are of the wire variety, do you think these are better if bought in whippet sizes? Just worried about potional injuries, as they don't look too strong...

Paul