I was all set to build 17 machIV until last week when 2 people I know little of suggested 17 Hornet any one out there have oppinion??

What action will you building it on, and what type of performance are you after? A 17 hornet on a Ruger 77/22 (77/17?) would be a pretty slick little rifle. Then again, if you want to really get the 17 cal to strut its stuff, the hornet case is a tad small.

I shoot both the Mach IV and the 17 Ackley Hornet. Most of my shots are taken at 100 to 150 yds. However I have taken several crows between 175 and 200 yds with the Mach IV. A year ago, before I got my 17 AH I would have sworn that It was my favorite. However, today I'm not so sure. I had Virgin Valley Guns make me a 22" 17 Ackley Hornet Barrell to fit a contender frame I have. They did a trigger job on the frame and reduced the pull to 12oz. I load 10 grains of Lil Gun and a 22 gr. berger bullet and get approx 3700fps from the Ackley hornet. The 17-mach IV will do approx 3800 w/ a 25 gr V-max using 15.8 gr of h4198. The mach-IV is louder and kills are more dramatic but I believe I've become partial to the ackley hornet. Both cartridges will group sub inch if I do my part. Either will eviscerate crows and squirrels. I've nailed three feral cats in the last several weeks with the 17 AH. No exit and they dropped in their tracks.
As you probably know, both have to be fire formed. All in all I think the 17 AH is easier to load. Good luck on which ever you decide. I don't think you can be mistaken whichever you choose. In fact you might get one of each over a period of time. I did and don't regret it.

I'm just about to tell the gunsmith to go ahead with a mach IV, built on a Sako L461 action. I've vacillated for the last couple of weeks between the mach 4 and the 17 Hornet. The noise level of the Hornet is very appealing, but in the end, the extra legs of the mach have given it the nod. I suspect I would enjoy either.

I am putting this on switch barrel XP-100,(late model 7 ver) barrel length will be 16in

You will be very happy with your choice in 17 Mach4. I have 5x..17's including 17 Ackley Hornet, 17 Mach4 Sako L461, 17/222 Rimmed and 17 Remington both Sako's in light and heavy barrels.

The 17 Ackley with Hornady's 20 gn V-max is turning up 3680 FPS (chronographed) and shoots a flat as my 220 Swift. It is I MUST say one of the finest SMALL game rifles I own. As an experiment I was fireforming cases and was suprised to see that little pill knocked down full size steel chickens every shot at 200 metres.

17 Mach4 is very efficient cartridge with a mere 17.2 gns of powder achieving 3800fps with 25gn Hornady HP. There is a considerable difference in power with a projectile that is 25% heavier. No problems to date in taking dog sized game to 225 metres.

A word of caution in forming cases. I did not anneal my new Remington cases before forming and ended up with 2 out of 100 that did not split on the first firing. I also neck turn the cases to an even thickness before forming as the brass can get overly thick during the necking down process.

Overall an excellent choice. Rember that it is a only small game rifle and enjoy it to the fullest.