I have a new Savage model 11 with only a few dozen rounds through it. It shoots factory Winchester 55 grain PSPs into an inch at 3500 FPS (chrono).
My initial handloads were 50 grain V max, new winchester brass, CCI BR2 primers, & 35.1 grains of Re 15. Brass was full length resized, trimmed to book "trim to" length, neck turned, then neck sized with a Lee collet sizer. COAL was book length - in this particular rifle, well off the lands.
Book velocity for this load is around 3600 FPS. My chrono (12 ft from muzzle) was reading around 2600 FPS, and the barrel had very heavy powder fouling when I cleaned it after the range session.
I think the chrono reading was true - I fired 2 factory rounds through the rifle before the reloads and got the expected 3500 FPS, and I shot another rifle over the chrono during the same session & got expected velocities. I got a reasonable group (around an inch). There were no pressure signs.
Does anyone have any idea what could have caused my very low velocity readings and very dirty barrel? - I wonder if neck turning & neck sizing new brass might have something to do with it?

How old and in what condition is the powder?

The "very dirty barrel" was caused by the low chamber pressure your load produced and would be expected. Either the primers were very bad, the powder was bad (or wrong), or you didn't load 35.1 grains of it. What about your powder scale, has it been checked? Did you misread it when you loaded the test rounds? Any of them left to open up and check the charge?

BTW, neck turning brass for a factory chamber is usually not a good idea, it just increases the radial neck clearance and reduces bullet support upon firing. Neck turn if you have a tight necked chamber or thick necked brass.

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Powder is maybe 2 years old, & it has been kept tightly closed in the original container. There is no odor, and I used the same lot to load some .308 that shot very well and produced good velocities and groups at the same range session. Reloader 15 is listed in the Hornady manual for the 50 gr Vmax (about middle of the road in burn rate) and 35.1 grains is maybe 5% below max.
Primers were the first ones used from a box of CCI BR2 that I bought 4 or 5 months ago. They seated with about the right amount of pressure (I use a Lee Autoprime) and seated to the right depth. I use Winchester large rifle primers for my .308, but I figured that with the smaller case of the 22-250, the benchrest primers might provide a bit more consistency.
I'm kind of anal about my reloads - the powder was weighed on a beam scale & and double checked on an electronic scale before it was poured into the brass. I loaded a few rounds with a 35.4 grain charge (next step up) that I didn't shoot because of the strange velocity readings - I'll open one & check the charge to be sure it is right, but I shot all the 35.1 grain loads (only made 3 - i'm still experimenting) because I kept thinking it was chrono error.
I thought I would try neck turning to see if it would create more even neck tension/consistency.
My seating die is a Forster. Flash holes were deburred & primer pockets were uniformed.

I don't think your reloading can be faulted nor does it sound like your powder's freshness is a problem, but isn't RL-15 kinda slow for a 22-250??? Especially with 50gr bullets.

Actually, Alliant's website shows a max load of 38.1 grains with the 50-grain V-MAX for 3916 fps, the highest velocity Alliant lists for that bullet weight. The OP's load is ~8% below max, not enough to account for the very low velocity and pressure experienced. I'd have to say that he loaded a lot less than 35.1 grains based on what he's told us.

Your load is several grains under what I am using in my Savage. I have to agree with the others it sounds like this particular load was on the light side. What neck thickness did you turn to? I have a Savage Predator DBM in 22-250. This is the only load I have ever shot in it.

55gr SBK over 37.1gr RL15, CCI BR2's, OAL to Ogive 2.095 which is a .005 jump, avg. 3725fps from the 22" barrel.

The last 3 shot group before the gun went into the safe was .156"

Cover dog - I did not mic the necks, I used a K&M neck turning tool and adjusted the cutter in small increments until I was removing brass from 1/2 to 2/3 of the surface of the neck - just enough to clean up any major variance in thickness.
Sounds like the consensus is an undercharge, & I guess that seems like the logical cause to me too. I usually do a quick visual check before seating to see that the powder is up near the shoulder, but I do not remember whether I did or not on this specific batch. Since this was one of my first loads in this caliber, I guess it is possible that I used the wrong funnel & spilled some powder while filling the cases, but I would have had to lose a lot to decrease velocity that much, & there was no obvious powder spill on or around my loading block. I have not dissassembled the batch I loaded at 35.4 grains to check their powder weight, but I will tomorrow. I suppose a double mis-weigh is a possibility, but I really have a hard time believing that I did that.
The digital scale is an almost fail safe double check, & I never fail to use it. If the 35.4 grain batch weighs out OK, I will shoot them over the chrony & see if I have a bad batch of primers (seems like the only other possibility)
Thanks to all for the thoughts and suggestions.

Just weigh a primed case and bullet and then weigh one of your finished rounds. The difference will be the weight of your powder.

Thanks Wasbeeman - I should have thought of that. The 35.4 grain loads are spot on. I'll shoot them & see what happens. Thanks for all the suggestions.

First of all, I don't have nor do I load for a .22-250 so any comments may not apply. When I have a reloading problem, I usually check out the "Accurate load" for the bullet in question in my Sierra book. I realize that you're using a Hornady V-max. Rldr-15 is the "Accurate powder" listed in Sierra and your powder charge is close to their "Accurate load" as well.
As an off the wall observation, is it possible that you are getting chrono errors? You say your 12 ft. from the muzzle. I'm wondering if you could be too close and having problems with muzzle blast. Also, as an idea, why not measure the length to the ogive of some factory rounds - you say you had some 55 gr. factory ammo- then pull the bullet & weigh the charge? Then, you'd at least have a powder weight starting point for a 55 gr. bullet. You could then load up some 55 gr. loads using a powder appropriate for that weight bullet. This might give you another indication of any problems.
Just a couple of ideas but I'm probably all wet.
Bear in Fairbanks

Chrono error was my first thought - that's why I shot all 3 rounds after seeeing the low velocities, but I got good velocity readings in the same range session from my .308 and from a 7mm Rem mag - velocities were consistent with the loads I was shooting, and the bench, the rest & the chrono were in the same place during the same session. i also got good velocity readings with the factory Winchesters in the 22-250 prior to shooting the handloads, so I'm pretty sure my setup & chrono were OK.
Length (COAL & length to ogive) of my loads are not significantly different from factory & SAMI length - I have not started messing witnh seating depth yet - searching for a good bullet & powder/charge to start.
Seems like it almost has to be a low powder charge - maybe I mentally transposed the numbers & used 31.5 rather than 35.1 - I will shoot the rounds I checked for proper powder weight & see what happens.
Thanks all.

Sounds like it's time to try a different load in the gun, and eliminate one more variable.