I have too many 30-06 rifles I have collected over the last 30 years.
Itching to build something different. Like all old farts, recoil is not fun anymore.
I always wanted a 25-06, and I have never owned any 6mm of any kind.
From recent research it seems building a 6mm on a long action is a good thing.

It could be a good thing, especially if you go with the right twist. A 25-06 does not suck either.

6mm/06 if going long action and you want a 6mm.

I have a nice .25-06 with a custom barrel... It is one of my most used rifles, but now mainly a coyote rifle... I picked up a clean older 700 in 6mm... I would like it on a long action, but that is not what I bought... Always thought a 6mm on a long action would be just too cool...

I would go 25-06

The biggest advantage of the 6 mm rem over the .25-06 is it's ability to shoot 55 grain bullets and give very serious competition to the .22-250. It can also kill deer at extended ranges with 100 grain bullets. Go 1-9" twist or faster....you will still have a few .30-06 rifles for other larger game so go the distance and rebarrel toi the 6mm Rem....it's a lot more than anyone thinks.

I think maybe the barrel life of the 25 06 is probably mo better by a good margin over a 6mm 06.
A 6mm rem better even still I guess.

I've never felt a 25-06 ever did anything better than a 243, especially if the 243 was set up to handle bullets heavier than 100 gr. With the best loads a 105 out of a 243 isn't that much slower than a 115 out of a 25-06 at the muzzle; and the better BC's of the 243 will close that gap even more down range. I just can't see any big game animal ever noticing the difference in 10 gr bullet weight and .014" bullet diameter at similar impact speeds. And the 243 is the better choice with light bullets.

But a 6mm/06 might be too much of a good thing.

I've got a 25-06 that is my main deer rifle. It likes the heavier bullets in the 110 to 120gr range. On whitetails as long as I have done my part on shot placement tracking has been 40 yards or less. Its lightweight, minimal recoil and easy to maneuver with in thick cover. That said, I picked up a 243 last fall and shot a decent sized buck at 50 yards with 95gr Nosler BTs. Never took a step, just fell over DRT. I might prefer the quarter bore, but you wouldn't be wrong with a 6mm either. There is a better selection of bullets for 6mm, but both will shoot light bullets flat and hot and the heavier bullets in their class at respectable velocities. Whichever you chose just make sure the twist rate is what you need.

Robert,

If you want to rebarrel a .30-06 rifle to a lighter-kicking cartridge I would go with the the .25-06 over the 6mm Remington AI, mostly because its a lot easier to find .25-06 brass than 6mm Remington cases.

If you want to make it a 6mm-06 there's also no reason not to, if you think that wildcatting would be fun. If you're just going to use it for big game hunting I doubt you'd shoot the barrel out much sooner than with a .25-06.

Personally, I've never been able to tell any difference in "killing power" between any of the commercial 6mm's and .25's, and between my wife and I, we've used the .243 Winchester, 6mm Remington, .240 Weatherby, .250 Savage, .257 Roberts, .257 Roberts Ackley Improved, .25-06 and .257 Weatherby to put dozens of animals in the freezer.

I just did both and like them very much. It will depend on what the use is for and how much you like to tinker with loading. For typical hunting not much difference on targets the 6AI will out shoot the 25-06 at distance with typical bullets. If I had it to do over I would go faster twist on both rifles, the 25 is 1-9 and the 6mm 1-8 on each I would go half to a full inch faster. A fast twist 25-06 will lose nothing to the 6mm with specialty bullets like the Hammer, Matrix and Blackjack bullets over 120 grains. For most hunting the 25-06 is simpler and easier and may have better re-sale if that is a factor.

For each there are better designed cases like the 6mm Crusader and the 25 SAUM or PRS or the .284 cased versions of each caliber but these are even more obscure. In Texas the 25-06 has an almost cult following. I have hunted on some leases where every member shot a 25-06 except for one .257 Weatherby. As much as I like the 6mm AI I think I would still go 25-06 or do what I did and do both. For some reason it seems a couple of companies have made their 6mm brass runs this year. At one point I think 4-5 companies had it. I just stocked up on Winchester brass at a 20% discount from Natchez. I did this so I would stop looking a 7x57 Lapua and Norma brass to convert. But if you decide to go 6mm stock up on brass now as at other times it can be almost completely unavailable. Ideally I get enough of the same lot to shoot out the barrel when possible.

A 75 from a 25-06 is no slouch when compared to a 55 from a 6mm.

They are tops.

I have never understood why they are not more common and ditto for the 22/243.

A 6-06 would be awesome, but I can almost get there with the 6mm AI. But the no case forming would be nice, I will have to consol myself that the no or rare case trimming off sets. But a 1-7.5 twist 6-06 would be stellar. The 6mm AI will beat it on case life but not by that much.

If that competition is colony varmint shooting neither gives the 22-250 a run for its money. Both simply have way too much horse power for prarie dogs and the like.
I have a 25-06AI and typically fire form cases by shooting prarie dogs. After a a dozen or so shots it becomes apparent it's way too much.