[300rum]

hello all!

i started load development for my wife 260 rem model 7 this morning.

i use remington once fired brass
cci br 2 primers
nosler 120 gr. ballistic tips
and h4350 powder
coal is 2.780"

i tried the newberry method for the first time. as far as grouping goes
i'm satified, for a first attempt.
here's the target.


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group number three has a flier, probably my error, i just don't know.

so here's the chrono i got on those
group 1 is 44.8 grains vel. was 2744,2789,2005???
group 2 is 45.2 grains vel. was 2803,1969,1766
group 3 is 45.6 grains vel. was 2772,2002,2650
group 4 is 46.0 grains vel. was 1999,2063,2715
group 5 is 46.4 grains vel. was 2054,2867,2825.

i also shot 3, 3 shot group with my 308 and factory loads, and all were around 2700 fps.
so what do you long time reloaders think?
i never experienced something like this with this chrono so far.
after the first few odd readings i relocated the setup somewhere else
to make sure the sun angle wasn't playing tricks on the chrono.( prochrono pal)
all the loads were carefully weighted on a beam scale.
scale was zeroed before and checked after loading session.
all the shot felt the same (recoil and muzzle blast).
should i use magnum primers with this powder?

is it possible that it's the load?
wouls i get groups like that at 100 yard with some bullets going 800 fps slower?

i'm a bit lost right now.
thank's for your help.

 

[unclenick]

From the loads, the higher numbers are most likely close to correct. The low ones are most likely false early triggering of the near sky screen by muzzle blast or blow-by unburned powder particles before the bullet actually arrives. The factory loads may be using a faster powder that leaves lower muzzle pressure for the blast or a fine spherical propellant whose grains can't trip the screen.

15 feet seems like the safest minimum chronograph distance for avoiding the problem. One fellow said his .338 Lapua Magnum (IIRC?) needs 18 feet before false triggering stops.

 

[TMan]

The two problems I've had with a chronograph are lighting, and battery life. Either can give you totally wacky numbers. Mechanically, if your unit folds, even a small amount of angle in the housing, can add lots of speed to your projectile. I once encountered an archery shop that set a paper shim in the hinge on their Krony. It added about 30-40fps to the bow speed, and customer satisfaction.

When my bow picks up 200fps, I know it's lying, and when I get extermely hi/lo readings from handgun or rifle loads, with no apparent sign of hi/lo pressures, I also suspect "other" reasons.

Over time, I have come to believe my Krony, when I get the same numbers (statistically) over multiple trips to the range.

So, it's possible, I suppose, but not likely.