I have a left handed 788 Remington that by my count has over 12,000 rounds through it. With the 165 g bullets loaded to 2.875" it has almost .200" inches of jump to the lands and it's finally unable to group less than an inch.

I want to rebarrel it into something new and was wondering if the the 2.9" magazine is long enough to chamber the longer 6 mm or 6.5mm bullets?

The .284 case is about 2.2" in round numbers. That means that a bullet will protrude about .7" of an inch from the neck. That seems ok.

It would be much much easier to just barrel it for the 243 Win and buy another rifle that takes a larger case. In 6mm the 243 or a 6mm Rem are pretty good as is.

the .260 Rem makes more sense to me sir....the .284 cases will not likely feed thru the clips......unless of course you want a single shot.

I thought about the magazine thickness after I asked this last night, thanks. A 243 is a great idea but I have two already. Converting it to 6mm is interesting because that and the 308 were the original left handed chamberings for the 788.
The 260 is a good choice also as it solves the clip feeding problem. I guess a 22-243, 25 Souper or 270-308 would also do. Decisions, decisions.

It is much easier to set the .284 case up as a single-shot, anyway, so if you are satisfied with a single-shot a 6mm or 6.5mm-284 would do fine. That way you can disregard bullet seating depth and LOA (provided you don't run into problems unchambering a live round through the 788's loading port. You might have to unchamber a too-long live round by removing the bolt.

If you want/need a repeater, the 6mm Remington would be a much better choice. If you have need of a close-range, heavy game rifle, .358 Winchester is a nice old round.

Frank, I have several Remington 700 SA rifles chambered for cartridges like you mentioned. On all of them the bullets protrude into the powder space but this hasn't affected accuracy or my desired velocities. Among these are the 6mm-284, 6.5-284, 284 Win, 6mm Ackley Imp, 7x57 Ackley Imp and 6.5 Swede.