The 'elbow' measurement is so common because it gets an
average person close very quickly without needing an actual firearm for the measurement.
"Close" is good enough for most people.
Well, just shows what I know. Thanks for the info.
It might be cool to have a gun built for you specifically for your dimensions but I'd bet a lot of money that I (personally) would consider it waste of money.
My info about 'try' guns came, not from shooting sources but fictional stuff written about 'the English' of about 60 to 100 years ago which sometimes included their shooting habits. I bet I'm just as misinformed about their horse riding skills too.
Once you have a rifle that fits and balances with you like an extension of your body (rather than just a tool in your hands), you'll understand.
Having a rifle or shotgun
stock made for you isn't all that expensive, either. The entire rifle/shotgun? Yes, very expensive. But just a new stock for an existing production firearm -- not that bad, unless you want high-end wood.
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Just getting the correct length of pull can make a huge difference in how a rifle/shotgun points and feels.
Then add the correct comb, cast-off, camber, wrist angle and thickness, and it's another world.
I've been shooting factory LOP rifles and shotguns most of my life; but I've been working a few custom stocks the last few years. In the process, I discovered that I prefer a LOP of 14" to 15", depending upon the firearm design. That, alone, made a huge difference once applied to a few rifles (even if just adding a spacer under a butt pad/plate).
The biggest proof for me was a custom 444 Marlin that I built from a Marlin 336. Part of that project included rough-shaped blanks for stocks. I couldn't find any (affordable) sources for learning how to measure and then apply those measurements to shaping a stock. So, I just started test-fitting the butt stock and removing material that didn't feel like it belonged.
By the time I was done, the stock had cast-off, camber, comb, toe, and LOP dimensions that were as good as I could get them for fitting my body.
Aside from fitting the stock to my shoulder, I essentially inletted
my face into a piece of wood.
Now, I grab that rifle and shoulder it, and it feels like an extension of my body, with my eye perfectly aligned with the iron sights.
(Oh... LOP on that straight-grip lever-gun is 15", for the record.)