C.O.L. V I.V.

The Old Salt

On a day with 85 deg F temps I fired several 10 rd groups and recorded the IVs. A month later I rfe-fired the same loads, same powder loads, primer, & case. There were only 2 differences. C.O.L. and temp. Temp was down 25 deg F to 60 deg F. Original day's group had C.O.L.s that attempted to keep the Ogive 0.010" off the rifling. The second day's groups were loaded w C.O.L.s long enough that the bullets were seated by contact w the rifling. The second day's groupings had I.V. 125 to 150 fps slower.

Why? Analysis invited.

Additional data to consider
Cal. 308, 168 SMK
H4895 loads from below to above max,
Win brass, Rem 9 1/2 primers,
24" custom barrel w 1"/10" twist

MG1890

Longer OAL's probably raised peak chamber pressure slightly, but not enough to overcome the lower velocity due to the 25 degree temperature difference.

And, although the outside temperature was 25 degrees different, could or was the ammunition hotter than 85 degrees on the 1st day, like maybe sitting in the sun versus in the shade?

charliemeyer007

I worry about above max charges. Is the neck tension so light the bullets actually seat deeper. I usually shoot for about 0.003 out of the rifling, 0.010 seems like a lot for custom loaded ammo to me.

Chemical reaction are temperature sensitive. As kid I had winter loads, they were only shot out of a cold rifle.

How were the group sizes? 150 fps out of like 2700 isn't a lot.

http://en.wikipedia.org/wiki/Internal_ballistics

The Old Salt

Loads had been in the car all afternoon parked in the sun, but were in the shade prior to shooting which took 2 1/2 hrs. First day's bullets were loaded 0.005" to 0.010" short of the rifling. Second day"s bullets had a COL of 2.856 guaranteeing every bullet was finish seated by the rifling.

These bullets were hand selected out of 225 bullets. All the bullets were measured from the Ogive (0.297") to the bullet base & overall bullet length. From these measurements the Ogive to bullet nose length was determined. Even w 168g SMKs there is a variation of 0.010 in Ogive to nose length.

I selected bullets w similar Ogive to nose lengths to eliminate them as a source of variation.

Your response suggests you believe the IV difference can be associated w the powder temp difference. Seems like the delta in IV is too great for the delta in temps.

The Old Salt

Forgot the groups! Wasn't shooting for groupings, just IV, but I believe the groups were in the 1" range both days.

RobOz

What is "IV"?

The Old Salt

IV? Initial Velosity
I was testing loads for their velocities. I was also fire forming new brass to my chamber.

MG1890

Well, if there was truly only (2) variables introduced, logic says the difference in velocity was caused by the variables. I will stay with temperature as the root cause.

The Old Salt

Thx for the responses. If I ever get over this sinus thing & get back to the range, I'll do a follow up w more data. Plan to shoot groups with the same charge wt and different COLs. Temps is in someone else's hands.

Hawk Carse

You could carry ammo to the range in a cooler that would keep it close to the temperature of your shop until you were ready to shoot it.

remingtongeo

I have never had temp problems with H4895. I've tested it "cooking" and frozen.
You changed the internal ballistics in the round. The small jump in the chamber enough to do it. I'm going to guess that the standard deviation was less with the rounds loaded with the ogive on the rifling.

The Old Salt

COLs used were 2.856 & 2.846. Each round was measured to +/- 0.0015". These bullets were hand selected for consistent Ogive to nose measurements. They would rest on the rifling at a COL of 2.848. So the 2.846 rounds are just .002 of the rifling where the others were seated by the rifling. ES differences between the 2 are not major and split between which were less. Because what has been submitted here about chrony being too close, wind impact (natural or muzzle blast from neighbors)on the readings, consistant height above the eyes, I have decided to reshoot the entire set of loads under better conditions. I have also FINALLY received my Grizzley Bald Eagle Slingshot Cast Iron after being on backorder!!! Used it Saturday. WOW! Next time I should have better groups, and better chrony results. Once again, thanks to all for contributing.