'm assuming fit is not so much a matter of geometry but shouldering various rifles to see what feels right? Glass is where I really need help. Is AO a big help on a range? Does magnification need to be sufficient to see holes or is a spotting scope a better idea? What would be the preferred reticle?
Another question has come to mind. Does a bull barrel make a difference?
I'll try to help you some here. My experience comes from High Power Metallic Rifle Silhouette competition. I am not an expert as I have only been shooting HPMRS for a little over 3 years now.
Yes. Pick up, hold, caress and shoulder as many different mfrs and models as you can. The aesthetics of the various makes and models will probably have an impact on you, too. FWIW, I have two HPMRS guns. One is a Tikka T3 Lite with plastic stock in .270 Win. that will shoot 2-3/4" five round groups at 500 meters. The other is a Tikka T3 Hunter in a wood stock chambered for 6.5x55 Swedish Mauser. It shoots slightly better groups at 500 meters than the .270. Both rifles wear Vortex 6.5-20x50 w/side focus Viper scopes. For the money, the Vortex scopes are hard to beat. The turret detents are crisp, solid and repeatable. They track wonderfully and can always be returned to zero. Also, once you have your rifle zeroed, setting the turrets to zero is as simple as lifting the turret against its spring, rotating it to zero and releasing it back onto the detents. There are many more manufacturers of good scopes out there besides Vortex, Leupold, Zeiss, Unertl, ad infinitum. My advice is to not go cheap on your scope and/or rings.
AO? Yes and no. Some type of focus / parallax adjustment is going to be critical if you are going to shoot prairie poodles. I personally prefer the side focus. Because the HPMRS course of fire requires 4 different ranges (200m, 300m, 385m, 500m) being able to correct the parallax is very important. The side focus scopes are much easier for me. You might prefer the adjustable objective, though. You really need to try both before you decide.
Magnification depends on what you want to do.
When I have a new rifle to work up loads for, I generally mount my Weaver 36x target scope just so I can see exactly how steady I am holding the target picture - and my 100 yard load work-up target is a 1/4" black square that I aim for the lower right corner of. (Exactly where the bullets land are not important as long as they are on the paper and do not destroy my aiming point.) Once I have settled on a load, my preferred scope is 6.5-20 for HPMRS and something along the line of a 4-16 Weaver, 4.5-15.5 Nikon or 6-24 Konus. (Here's a 4.5-16 30mm tube Konus I want to try someday:
http://www.midwayusa.com/product/98...adjustments-illuminated-mil-dot-reticle-matte) I prefer the 30mm tube over the 1" tube because they are noticeably brighter and I will splurge for that when I have the $ to spend and when they are on sale. For the most part, if I want to see the bullet holes at 200m and farther, I rely on a good spotting scope. Mid range priced Konus, Leupold, Zeiss etc. are the minimum (IMHO) for a spotting scope. I believe you should spend more on a spotting scope than you do for your best rifle scope. You will use it more and there is little that is more frustrating than not being able to see your target clearly through a spotting scope. Mine is a mid-range priced Burris and I cannot recommend it. (I intend to replace it someday, but there are more immediate financial concerns that must be addressed first.)
Reticles are a personal thing and dependent on the specific task. My HPMRS scopes are calibrated in MOA and have stadia marks in each direction. My shooting partner prefers a fine crosshair with a target dot. My advice is to look through a lot of scopes at your local Cabela's, Bass Pro or Sportsman's Warehouse and buy whatever reticle feels right to you.
There is really no substitute for a good quality rifle scope. I believe a guy should buy as much scope as he can afford plus 20%. If that means save for a little while before buying, then so be it. I have several scopes I could do without and will replace someday.
Bull barrels are a whole 'nother subject. By an d large, judging from the shooting you have described, bull barrel is better (stiffer) and shorter is 'more stiffer'. From here I will defer to the more knowledgeable than I.
I hope this is helpful.