So I just ordered a barrel for my .25-06 and am considering going AI, how much extra velocity do you guys realistically feel you're getting?
To be totally honest, everything else equal on the rifle and within the same pressures, your going to only see maybe 250fps of realistic gain depending on the bullet weights and powders used.
That said, when I built my AI version, I went with a 28" finished lenght barrel. I did this expressly to get all I felt I could from the usable case capacity and the slower powders. I however also went with a 1-9 twist Broughton 4-1 Magnum Sporter 5C barrel, which now I feel has cut some velocity off my top end loads, but I don't mind as I am getting fantastic groups with the bullet weight I am wanting to shoot. How much did I loose over the standard 1-10 is up in the air, and personally I am not sweating it.
When I started out forming cases and working towards finding my top end loads I used two sets of 25 cases. The first set was standard Winchester 25-06 cases, the second set was Lapua 30-06 cases which were necked down to the point I could just chamber them with a slight felt resistance when the bolt closed. Neither had the bullets into the lands and both sets formed perfectly the first firing. After forming they all were checked for length and faced off to the same lenght.
To determine where my max load was going to be I took 5 each of the cases and the three powders I wanted to use and begin to work up. I had researched every load I could find for the standard 25 using 120gr bullets and quite a few for the 257 WBY which used 24-26" barrel lengths. I sort of picked a middle ground for a top end velocity and went for it. Having the extra 4" of barrel and figuring the added velocity for it I easily got up into the WBY velocity range with the 120gr bullets, and then some, before I started to see any sign of flattened primer or case head marks. This said however with the straightened out case of the AI being what it is and holding the chamber better than the more tapered standard version does, this can and does mask some indications of pressure, so once I saw these signs I backed off a couple of grains to use as my max load. Even with this I am easily able to push the 120gr loads to 3350fps with the case loaded with Ramshot Magnum and the added length of the 28" barrel.
Had I gone with the 1-10, I might have gained a few more FPS, but I am already in the velocity range I was wanting to be and getting sub MOA groups out past 300yds, all with excellent case life, and none of the added trimming I has having to do with the standard version every three or so loads.
I have seen plenty of folks who get the usual 150'ish FPS over the standard version say that they like the switch, but weren't overly impressed with the gain. IMO to build one is a personal preference. I felt if I was going to go for it I might as well start off with all I could and worst case I could always have the barrel cut back if I wasn't happy, but couldn't add on if needed. I also chose the 5C bore simply due to reports of easier cleaning, less fouling, and some reports of added velocity. Can't say one way or the other, but I know it only takes a short scrub, and I am done, and there is hardly any fouling what so ever when I do. I also cannot argue one bit with the accuracy I have seen across the board with the bullets I have tried. This may have more to do with the fellow who put it all together than the components, but I haven't really shot anything through it yet that I would consider horrible, and even my FF loads will group around an inch at 200yds.
Hope this helps.