I used the last of my Federal std LR primers working up loads for my 10.75. Friday my loads chrono'd 2160 with the std primers. Today,the same load using Federal match LR primers averaged 2070fps(the 5 shots varied 7fps). 90fps difference? Does this sound right?

This sounds like a rather wide variation for two primers which are supposedly identical except for the manufacturing QC.

You don't mention other possible variables:

1. Same cases?
2. Same temerature and humidity at time of firing?
3. Same humidity at time of loading? (Could your powder have absorbed significant water between the first and second batches?)
4. Is there some possible chronograph error, ie., could the screen spacing have been slightly different on the second set up?

Sure, a different primer can cause a 4% variation in average velocity, but such a variation with two primers that are supposedly of identical briscience would seem unusual.

How was the velocity standard deviation with each of the two primer lots?

Same cases(Horneber), same weather(hot and humid) for both loading and shooting. The only difference is I shot the second string about 3 inches closer to the sensor(lower) Thanks, Bob

Bob, is your chronograph a Chrony or similar machine with a short (about one foot) screen spacing? If so, it is possible to introduce 90fps of "error" if the axis of the chronograph is turned slightly from the axis of the bullet travel. This is not really "error", but rather the chronograph reading is reflecting that it is further between the sensors if you go from one corner of the near sensor field to the opposite corner of the far sensor field, thus the chronograph will sense more time between screens and read a lower velocity; by the same token, it is shorter (and takes the bullet less time) to go straight from the center of the first sensor to the center of the second.

You can do the actual math to see what the maximum potential "error" introduced is. Let's say that your sensor width is four inches and your screen spacing is twelve inches. This makes a 4"x12" rectangle through which the bullet passes. It is 12" directly from the center of the first sensor to the center of the second sensor. But it is 12.65" from one corner to the other, or about 5.4% further. This means the maximum theoretical difference in velocity readings due to axial misalignment at 2000 fps is slightly over 100 fps.

This may not even be close to explaining your velocity differences, but many times shooters don't realize how much chronograph "error" they can introduce by slightly mispacing the screens or by having the chronograph slightly misaligned with the direction of the bullet's travel.

Stonecreek, Thanks for the education. I will make sure all is square when I give it another shot this weekend. Bob

I agree that shooting at a slight angle will cause wide readings with a Chrony, but I'll also say that I think we are going to be hearing a lot of complaints about component consistancy due to the ramped up production. I know QC is supposed to catch things but have worked in plants before and know that QC sometimes takes a back seat to we need it now. I am sure there is going to be some mixed lots as well. I don't think safety will be compromised but do think QC had to drop a bit. I am glad that I didn't and still don't need any components right now for this reason. You know that the faster you go the less accurate you are and this applies to QC inpectors as well; not to mention adding in the wider tolerances that get built in to ramped up production in the first place - What was junk last week is fine today.

Airgun1, Primers and powder I have had for several years. Horneber brass is, hopefully, of decent quality. The only recently mfg component that may be iffy is the 400gr Hornady DGX. Perhaps I should try some Swift A-Frames. Tomorrow morning, back to the range.