I don't take heed of the haters that poo-poo on sporterizing a mosin 91/30...it's my rifle and I'll do what I want to it. If they wanted to preserve every one of them, then they should have bought them all...lol. That said...I hope I didn't sporterize that one in a million last surviving example...etc...hahaha.
If you're moderately handy and wanted to sporterize a rifle then you'd be hard pressed to find a cheaper rifle to give it a try on. Be conscious of altering head space and creating a bore/bullet obstruction and you'd be hard pressed to go wrong assuming it was ok to being with. From what I've seen/read many barrels had bores that weren't centered...so if the exit hole of your brake isn't inline with things you could be creating a big problem.
I took the worst 91/30 I had and cut the barrel down to 22". Figured I'd have a couple of more inches to do a retry. I did it by hand and the results were ok. I then redid it on my hobby lathe and was much happier with the results. I turned the end of the barrel down to fit a slip on brake I had laying around...milled a flat spot and pinned/silver soldered the front sight back on after figuring/guessing the front sight height. I checked the bore alignment/bullet clearance and red loctited and pinned the brake. I cut the stock, free floated and added a forend tip. Stripped, stained, finished and bedded the topside of the front and rear screw areas. Cleaned up the sear surfaces, cut the bolt handles, bent a bolt, drilled tapped the bolt body, fitted a stained wooden bolt handle. All in all a really fun tinker project. It may be hideous to some, others may think otherwise...doesn't matter to me though.
It shoots a bit better...surplus ammo pre-sporterizing vs surplus ammo post...but still nothing to write home about. It was in bad shape to being with...barrel was dark and mildly pitted, fouled like crazy, stock was soft with oil...and it not surprisingly shot large groups. Post mucking about it's lighter, handles/balances better, groups are a bit better and better shaped. Cleanup is still horrible...you can't fix a rough bore. I was considering lapping it but I suspect I'd be casting bullets for it then and that would defeat the purpose of shooting surplus for me. I really like shooting it for fun...and the gas exhaust cloud it puts out straight up is funny to see.
Still more to do...maybe. I might bed the bottom metal area and put in pillars, recontour the stock...I might chop it again and put a different brake on it. That's the fun of sporterizing something like this. Inexpensive platform to try things out on. I bought a Zastava Z5 .22LR for the same purpose. Cheap gun with a decent trigger...shoots ok out of the box. I'll mess with the bedding and stock first...then maybe the chamber and head space.