Since you are going next year - you are not under time pressure to make decisions. When I go on such a hunt, I take a combination of rifles for short and long shots all the way from .17 Hornet to 6BR - but 90% of my shooting will be with either a .223 or .20 Practical. In the .223, I shoot 50 V-Max which is a very slight disadvantage to the 53 V-Max, but I had bought many 500-count boxes of the "Z-Max" packages that Hornady sold years back really cheap (like $40.00 for 500 on sale). If I couldn't keep up with my buddies shooting the 55 Blitz kings and such, I wouldn't shoot them. if I deplete my stock, I'd opt for the 53 V-Max or 55 Blitz king. When shooting a few thousand rounds - the price difference between bullets can really add up - so I'd try a few of the less-expensive bullets to see what your gun can do. For example, the price difference between using 3,000 Nosler "blems" from shooters Pro Shop and the price of 53 V-Max or Blitz Kings could pay for all or most of the cost of your upper. If most shooting were to be inside 450 yards (it usually is), the 50 grain Varmint Grenade blows things up like no other. It is ballistically inferior but very accurate and really blows them up if you want to see pieces flying 20'. I alternate with the .20 Practical shooting both 40 V-Max or 39 Blitz king (the absolute best bullet for the .20 Practical in my opinion). I leave the 32's at home when wind is expected - not to say you can't still hit targets out there.
You get much bias on this site as to what rifle (bolt or A/R). I shoot both a lot on a trip and if I could only have ONE prairie dog or squirrel rifle, it would be an A/R designed for this type of shooting. My choice would be either a .20 Practical or .223 chambering (or .22 Nosler if you didn't mind the higher price of brass as it will shoot 40 grain all the way up to 75 for longer shots or bigger game). I'd get a quality barrel, preferred 26" length, 1-9 twist on the .223 as will shoot lead-free, 1-11 tw. on the Practical, 1-8 tw. on the .22 Nosler, heavy contours on all. The only real downside in function of the .22 Nosler is the barrel heats up noticeably faster than either the .223 or .20 P. I'd also add a brake, a trigger that lets off at about 2 lbs (as good as you are going to safely get with an A/R), a heavy buttstock, a decent brass catcher and as good of a quality scope as your budget allows for (I like one which will allow for up to at least 30X magnification, though resolution is key. I'd put a brass catcher on it (I like the Caldwell model that allows you to clip it on and off with a rail that mounts to your scope base), a barrel hand guard that allows for good ventilation and finally, attach a sled to your hand guard that allows your rifle to be shot off of a quality rest. I use a toggle rest and there are some out there now that aren't too expensive. An A/R in this configuration will allow you to see your hits, make a quick visual adjustment for your follow-up shot. One can squeeze off several shots at a scattering bunch of PD's without taking your eye off the targets and the bolt gun is at a HUGE disadvantage there. You can squeeze more accuracy out of a good bolt gun, but getting fast second shots and not having to take your eye off the scope or adjusting your hold is worth more in real-world field situations. Even a good off-the-shelf upper with a 24" barrel such as those sold by Fulton armory will do 1/2" or better when tuned. I use my bolt gun when there are sparse targets or when taking extremely long shots with larger caliber rifles. There just isn't any real advantage OVERALL using a bolt gun. And I love both when I say this.
Your "pencil barrel" will not serve you well - not only because of the extra heat, but because of recoil. Even the little .223 will rock you off your sight picture without a brake. It is no fun not seeing where your misses landed - or seeing your target hit clearly. You mentioned 22/250. You will be limited to about 8 to ten shots and waiting to cool, versus a shot every ten seconds almost continuously with the .223 and .20.
If you build, I'd look at a quality barrel like Krieger, Shilen, Hart, etc. They all make great barrels. I'd tend to not trust a "no name" barrel. If the company making an upper you are looking at offers these barrels as part of their build - all the better. Fulton Armory did last time I ordered something for friends. good luck in your decision and have a fun hunt!
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