I'm in the process of building a new varmint rifle and have decided on 22-250 but need advice on whether or not to ream the chamber for "improved" version.

Advice/comments/experiences please.

I would run the standard 22-250 first, it's a great round all by itself.
Then save a few pieces of brass for fire forming later.
Later being when you shoot the barrel out, and need to rebarrel, then go to 22-250AI

I had a 22-250 AI and worked up several rounds with it.I really never was that satisfied with it.I shot with a buddy that had a .220 Swift and I didn't shoot any faster than he could.About the same fps. Fireforming brass and brass prep is a real time consuming pain in the rear.I removed the barrel and had Hart rifle barrels re-barrel it and chamber it for .220 Swift. I couldn't be happier.I shoot a .50 grain Sierra Blitzking 4000 fps. and group at .300 all day everyday.That chambering and Hart Rifle Barrels are a serious force to deal with. I wish I had all that time I spent piddling with the 22-250 AI back.The only thing I can see real gains in over a .220 Swift in .22 caliber is a 6mm-22(Texas Trophy Hunter).However I have no personal experience with the TTH. Just my two cents...

Look at the 22 cheetah, it is faster than the swift, or, go for the swift improved. Fireforming the 22-250 AI isn't difficult you just fire regular 22-250 stuff through it, and case prep is no different than any other caliber. If you are out hunting and lose your handloads, you can walk into po-dunk junction and find 22-250 ammo most anywhere. Because of the 40 degree shoulder of the AI, the majority of the burn occurs in the case so barrel life is a little prolonged, you can expect 7-14 percent velocity increase in the Ackley configuration whether it be the 22-250 or the Swift. Improving the Swift has the added benefit of much better brass life.

Malm

Toomany Tools: I have 3 close Varminting buddies that utilize the Ackley Improved 22-250 cartridge.
All 3 Rifles are very accurate and attain noteworthy velocities. I have shot along side them and have been impressed! The brass life is better with the AI than regular 22-250 brass even with the increased velocity! At present I am building a 257 Roberts Ackley Improved and have owned a 6mm Ackley Improved for many years. Lots of good things about Ackley Improved cartridges and only the one drawback and that is the fireforming effort. I live with it.
As long as you are starting from scratch with your rig I would go with the Ackley Improved 22-250. Be sure and use enough barrel length to compliment that round and create the full velocity potential. I would go with 26" barrel length minimum.
Good luck with your new Varminter!
Hold into the wind
VarmintGuy