I just bought myself a Browning 71 chambered in the 450 Alaskan,the chamber has some issues & will not chamber some makes of brass,I have it at the gunsmith's now to check the specs on the chamber,I am wondering whether this chamber can be cleaned out to the improved version?
Is anybody loading for the standard & the improved version? maybe can help out with some info? Thanks.

Could be as simple as tight head space, if it shoot well you may just want to feed it what it likes.

Sounds more like a brass issue rather than a chamber issue. i.e. rim thickness variation.

Thanks gents,
am waiting on the smith to get back to me & if he does not give me a suitable answer ? I will take some new brass & fireform it in the gun & she how she does.

I have had a 450 Alaskan in the Browning. The bbl threads
are different, and the original 348 chamber is tighter
than the Winchester. Its a good idea to study a fireformed case. That is where you start with a new wildcat rifle.

Brass brush & clean the chamber thoroughly-then fireform.
RCBS makes some really good form dies for the 450. Currently, I have two 450s on early Model 71 Winchesters that I have had for 20 years. They work very reliably.

If you plan to carry it hunting moose or bears, make sure it feeds and chambers consistently. You may also want to
experiment with various 458 bullets for accuracy and reliability. Good luck.

Somewhat related but I just bought a Win 71 in "450 Alaskan" on Gunbroker. The 450 Alaskan has a ghost shoulder (no shoulder) and just a long taper. I fired a true 450 Alaskan round in it and if fire formed to a 348-450 Ackley with the 40 degree shoulder and more of a straight case with slight taper--will measure with calipers to confirm later this week. Funny because I am in process of buying a 348-450 from a guy in Florida so now I will have two of them. I have been in contact with a guy in Alaska (who works the North Slope up there) and he has a 450 Alaskan so maybe this will be the right caliber--we shall see. I have a Win 71 in 50 Alaskan--no trouble distinguishing that caliber. Love the big 50! The 348-450 is very close to the 450 Fuller--I think the Fuller is more rare (450Fuller would probably know). I love the true big bore levers wildcatted in Win 71. With that said, I think the 348 by itself is a great round--shot my new (to me) Win 71, .348 on the 4th--- 1939, long tang, deluxe, carbine and had 3/4" group at 50 yards, 2.5" at 100 and oddly, 1.75" at 200. All using solid Caldwell lead sled and bolt peep. Anyway, great round and, due to powder capacity, an even better wildcat platform (with the Win 71 being a great donor rifle). mt

I've got two sets of RCBS 450 Alaskan dies. My rifle is marked 450 Alaskan. There is a pronounced shoulder on the cases. The die set I bought at a gunshow size the base two thousands more than the dies that came with the rifle. I do not know who the builder was, but it is an interesting and wonderful conversion.

I like a nicely tapered case on rounds that could be used in anger. I don't see the added powder capacity and slight increase in velocity worth the trade off of getting rid of a case that will feed slicker-n-snot to replace it with a herky jerky improved case.

True enough 458 Lott---5% increase is the improved case capacity--compressed load probably could get you there anyway--that did not stop me from buying a couple of the improved versions--it is a sickness I tell you

I hear you, I've had a few improved chamberings over the years, and I'm sure will have a few more in the future. Have a real hankering for a 22 k-hornet even though I have a 221 fireball which is really better all the way around.

I have a Winchester '71 in 450-348AI. This a beautiful gun! It has engraved Redfield rings, 4X Redfield scout scope, and Redfield sights. The barrel is 18". I don't believe it has ever been fired since built. I would be willing to sell to an interested party. On the same hand, I may keep it and fire 450 Alaskan through it. As nice as it is, not much call for it in central Texas. If anyone is interested, I can post pics.

Sounds nice!. I am interested and sending you a pm sir. Thanks, Steve