Welcome to TFL Pettitbuilt!
While I love good sporterized rifles, and have done it myself to several over the years, I too must cast my vote with those who say DON"T DO IT!!!!
When it comes to market value, today's market is 100% upside down from the market that existed from post WW I through the 1970s, particularly in the decades after WW II.
In those days, unaltered surplus military rifles were nearly as common as, and nearly as cheap as dirt. Today, they are pearls of great price.
Back in the 60s, sporterizing a milsurp meant taking a $20 or$30 rifle, spending maybe $100 (often including the price of the scope), and getting a $200-300 dollar rifle, with all the features, you wanted, and in any possible caliber, as fine, or finer than anything coming out of the factories, for less cost.
Today, sporterizing a milsurp means taking an $800 rifle, spending $400 (or more?) and winding up with a $300ish rifle. Maybe.
This is my 1903. .25-06. BUBBA did NOT do THIS. While I do agree that "Bubbas" did butcher a lot of guns, I find it sad that so many people assume, and refer to, ALL sporterizing as "Bubba'd" today.
Bubba has always been with us, but back in the day, there were also a LOT of skilled craftsmen, good people with their hands and their tools. Today, those skilled artisans are retired or passed away, and while we still have skilled folk today, their numbers are few and their work commands a premium.
That .25-06 I bought at a gun show for $350, and I bought it for 2 reasons, the main one being I wanted the scope it had on it for another rifle, and #2) I didn't have a .25-06. Current scope and bipod I added to it. The work done on that rifle is absolutely top shelf, its scary accurate and has a fantastic 28oz trigger (scaled). But it isn't worth crap on today's market.
A beat up GI condition "clunker" bring 2-3x as much money. Here's another example,
somebody used a lot of love to turn one of these
Into this
Which I also picked up at a show for $350, just as it is, scope included.
This 1909 Argentine Mauser cost me $450, but its a .458 Win Mag!
If the market value of your 03A3 is important to you, DO NOT SPORTERIZE IT!
Returning it to full GI issue condition will make it worth the most it can be, today, and is entirely possible, as long as no permanent metal work has been done to the rifle. Correct stocks & handguard are available, still, and while you can't get GI replacement wood for $3.50 like we once could, you can still get it, until the supplies of originals run out.
If you are looking for an "upgraded" rifle to fit your needs, I strongly suggest you start the upgrade with a different rifle, one commercially produced today.
You can be just as happy, probably get off cheaper, and you won't have legions of beginner milsurp collector fanatics berating you for "destroying" a (holy) piece of history.
