The gunsmith is ready to install a Shilen stainless-steel select-match 12-inch-twist barrel on my left-hand Remington 700 action. It is a #5 blank currently 30-inches long.

He has both a 220 Swift and a 220 Swift Improved reamer. What do ya'll think ?

Had intended to keep the barrel as long as possible, but some people are saying that anything over 26-inches is a detriment.

Not building a featherweight or a carry rifle. This will be shot at prairie dogs from either a bench or prone and will be driven in a one-ton truck to within feet of the shooting place.

With the 12-inch-twist, what bullets would ya'll start with ?

What do you moly-users think ?

Have been shooting 220 Swifts for over thirty years, but this is the first custom-barrel Swift. Have used either IMR 4064 or WW 760 with conventional bullets in the 50 to 55 grain range up until now.

This rifle will share duties with an existing heavy-barrel 22/6mm Remington with a 28-inch barrel and a conventional 14-inch-twist for innocent varmints.

Hammer

i have both, both are great. i like the improved best but just for the added case life. both have 28inch pipes.
1/12 twist will be fine up to 60-62 grains. both my rifles love the 55gr Vmax, 1 has a Shilen SM the other a Douglas XX, both shoot under 1/2 moa. the improved is 125-150fps faster.
4064 is my stand by powder but i have been working with N550 lately with VERY good results.

I'd stay with the std 220 Swift I were you, two reasons, you are obviously very familiar with it and you have a faster 224 in the 22/6mm,which I doubt really would beat a Swift Imp by much.
Re: the barrel length issue.
Don't let other guys tell you what's best for you.I put a 28-1/2" barrel on my Sendero and many guys told me it was too long. I think they just didn't want my barrel longer than their 26 inchers! Even the barrel maker was shocked when I told him what I wanted.I replied that if I wanted a 26" Sendero I could go to the store and buy one.For a rifle that's shot primarily off bags or a bipod the extra length is no problem...and it's free velocity.