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Does it make a difference if I use Speer, Sierra or Nosler when loading for the same weight of bullet.
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It can. Different bullet have different shapes, bearing surface area, densities and hardnesses that can effect the resistance the bullet poses to engraving in the rifling and moving down the bore. This can make pressures different even though the bullet weights are the same.
That being said, if you cannot find specific data, and you feel comfortable experimenting a little in a VERY careful and methodical fashion, then you can get use data for other similar bullets to work up a load with the bullet of your choice.
Take the data you have and reduce it by a full 10%. If the load data indicated that a specific powder should not be reduced that much (as is the case with some of the slower burning powders, but I doubt this will be a problem with any of the medium speed rifle powders used for the .22-250), try a different powder. My personal favorite that I've used a lot in the .22-250 is Varget. It's clean, produces respectable velocity and I've always managed very good to excellent accuracy with it. I've also had good luck with H-380, though as a ball powder it is said to be somewhat sensitive to temperature.
Anyway, reduce the load by a full 10% and work up SLOWLY in increments no more than 1% of the published MAX load. For a .22-250 this will be about 0.3 grains. Shoot the work up paying close attention to any pressure signs. Sticky bolt lift or extraction are prime indicators of high pressure. Flattened or cratered primers can also suggest high pressure, but it's not 100% since other factors can cause these problems. Sticky bolt lift, brass flow into the extractor or ejector area of the case head, and the best sign is if the case just ahead of the web has expanded, but it requires a micrometer to measure this (a standard caliper isn't accurate enough). If you see any pressure signs, stop immediately, back down to the last load that showed no signs and this is your MAX load for your rifle. I have never had any problems using this method. If you want to be more cautious, make your load increments 0.2 grains.
If you are in doubt, don't try it. Remember, safety first.
Mike