or buy a gun with some built in adjustability:
a. some guns have shims between the reciever and stock that you can adjust or change....
b. some guns - like Benelli's have options with their Comfort Tech system - with comb inserts that can be changed to raise the comb ...and they have different snap in recoil pads that increase or decrease the length of pull / while the additional pads are about $ 90 each..../ that system -- along with the shims ....on some Benelli models - not all models....make it pretty adjustable...
c. buy a gun with a parallel adjustable comb ....so the comb is parallel with the rib ....and then you can adjust it up or down - to change the point of impact up or down...or left and right for cast../ and fine tune your fit ...guns like the Browning Citori XS Skeet with the adjustable comb or the Beretta DT-10 ....
parallel combs mean your point of impact won't change if you shoot in a T shirt in the summer ....and a heavy coat and vest in winter .../ parallel comb guns will fit 99.9% of shooters in my opinion.
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custom stock work ...custom stock fitters.../ are typically done when you have a custom stock made ( average cost about $ 2,000 )....
There are comb pads that you can adhesively stick on an angled comb gun...to raise the comb for adjustability ...( pads are about $ 35 )...and you can change the recoil pad, put in spacers, put on an adjustable plate ...to change the length of pull, change the cast, etc...as well.... ( $ 150 - $200 usually )....
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Shooting a gun that doesn't "Fit" gets really frustrating....and I wouldn't recommend it ....but sometimes young shooters grow so much / some guys put weight on in their faces, etc....that you need some adjustability ....and if you can't afford a parallel comb gun ....then do the best you can with shims, etc..../ but "custom fitting" a stock ...is more for shooters that are into high level competition ...that can afford the $2,000 to get it done......so at this point, I would not suggest you go down that path....
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Some retailers understand the "Fit" concept.....but 99% of them don't .../ so don't expect the typical dealer to help you much .../ unless you're into using some really high end dealers, on high end guns - like DuPont, Pacific Sporting....
http://www.pacificsportingarms.com/
http://www.halkguns.com/
and most of us cannot afford that level of custom service....