Probably due to the rifles. Many early 77's were picky, due to barrel quality, throat length or both. I have also found Ruger Ultralight's to be pretty picky as well.
My 257 Roberts is a MK-2 ultralight. I bought it to be a woods and climbing stand rifle. Easy to tote with a stand on my back and the trimmer dimensions make it slightly less prone to banging into stuff. I haven't really worked it over good, but I am pretty sure it's never going to be what most people would consider accurate. With charges of 40-42 grains of I4350 and 115 NBT or NPT it's an inch and 3/4 gun. The NPT tends to anchor deer, sailing clean through even with shoulder bone involved. Since the gun is accurate enough for its intended use, I haven't really strayed too far in my load development. I finally got around to buying the chrono you suggested and was going to try it and a 100 grain TTSX load today, but mother nature has other plans.
