How do you index wood screws ?

[Tom]

Here is where I get lost, if the screw requires a full or almost full turn due to head configuration, how do you handle that without washers ?
If the trigger guard is against the wood, and the screw head is against the trigger guard tight yet an almost full turn will draw that screw down quite far. It does seem you would almost have to re inlet the guard, but that only helps the first screw you would still have one to go.
I'm not trying to be a pest, just trying to learn to do it right. If you take apart a nice factory gun all lined up your probrably not going to find washers, so there must be a simple way.
Thanks for all the replys, Tom