Velocity Jumps with Formed Cases?

[brn2hnt]

Has anyone seen velocity jumps between new brass and brass that’s been fired in the chamber and then bumped the shoulder back?

Measured velocity on the same powder charge with fresh and fired brass. Fresh was consistently 30-40 fps slower.


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[Nealm66]

It depends on how tight the chamber is. Energy is lost if brass expands

 

[Nealm66]

I wouldn’t say a loose chamber is any less accurate in my experience. Just helps to fireform before load development

 

[rayporter]

atmospheric conditions can easily account for 30 fps. even from morning to evening on the same day.

 

[MuleyFever]

atmospheric conditions can easily account for 30 fps. even from morning to evening on the same day.

What specifically causes this and how?

 

[brn2hnt]

atmospheric conditions can easily account for 30 fps. even from morning to evening on the same day.

Even with a “temp stable” powder like H1000?


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[Brent111368]

I would think that formed brass should create less velocity due to increased case capacity. And by increased I mean ever so slightly. Yes, I could see temperature causing your increase. Temperature stable powder, regardless, if you fire rounds that are starting out much warmer due to outside temps, they are going to produce slightly higher velocity, I’d think.

 

[brn2hnt]

Temps were warmer, but only by 10* or so. Nothing dramatic.


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[rayporter]

air pressure, humidity and temperature are inter related in ways i may never grasp.
warmer temps lower air resistance as well as help the burn rate.

there is more to it than just temperature. i will look for some explanations to post. i know the guy but dont know if i can find it. he has an indoor tunnel that i have shot at and is good at explaining.

in march i was shooting a match and for 8 groups i shot 30.6 gr for 3440 fps. it shot well. it was a 50 deg day with fog and light mist all day. the temperature stayed very steady at about 52 deg.
on group 9 i had a lot of vertical at 200yd. where i had been shooting a .4 i suddenly shot at ,7
the temp and humidity did not change but my velocity was 3419. the shooter next to me said he seen bad vertical too. a half hour later we shot group 10 and the velocity was 3440 and the group was a .390

neither of us seen what caused the dip in velocity [ and we all had temp and humidity gauges. ]
all of which happened over about 45 minutes. with a chronograph set up all the time you can see some odd things.

normally over the course of the day you will watch the fog burn off and temps rise. and as the bullet speeds up i might lower my powder down as much as .7 gr to chase the tune and velocity.


edit
much of this sort of technicalities will be lost in the noise when talking of hunting rifles.

 

[rayporter]

this is by Gene Beggs -he has a tunnel in texas and has done some research on htis. it is all on the ppc which holds but 30 gr of powder so you can ignore the amounts of powder he gives,
he has a better article that i am searching for.


Cooler air is thicker than warm air and creates more drag on the bullet as it accelerates down the bore. If we have the rifle in tune early in the morning when temp is 70 degrees, it will be in tune only until temperature increases. Here in Midland, temperature usually increases about 20 degrees by noon and if we do nothing to compensate, the rifle will be completely out of tune when temp reaches 90 degrees. When tuning with the powder the in-tune nodes appear at 1.2 grain intervals. This means that you cannot possibly be more than .6 grains out of tune and furthermore, unless you are already up against the max load, you can get the rifle in tune by going either up or down.

So let's forget about the air in the barrel and just agree that when the air gets warmer, it offers less resistance to the bullet as it accelerates down the bore and the bullets begin to exit early. We compensate by reducing the load slightly. How much? .6 grains for each 20 degree increase in temp. Or,, .15 grains for each 5 degrees increase; .30 grains for each 10 degrees. If you're using a Culver type measure, each number on the dial represents .6 grains of Vitavertical; each half number is of course .30 grains which is the smallest correction I bother to make. If my rifle is shooting dots at 70 degrees and 54 clicks, I'll reduce the load to 53.5 when temp reaches 80 degrees, 53.0 at 90. Clear

 

[wind gypsy]

It depends on how tight the chamber is. Energy is lost if brass expands

This is the only explanation ive see that makes sense.

I've had it happen recently by about 40 FPS on once fired vs virgin. However, the virgin brass was the first 140ish rounds through the barrel so i'm not 100% certain it's just the brass. I do still have some loaded up in virgin so I can do a head to head comparison when I get a chance.

 

[Nealm66]

Not a bad idea to use once fired brass for load development. Annealing can also help avoid some variables. Funny sometimes how well them dam cheap bullets shoot when you’re just trying to fireform some new brass. Hate that

 

[Nealm66]

I rececently found some cheap old 270 caliber bullets that were probably around 15 years old and were target bullets. No idea why I had them but figured I’d use them to form some new brass. I just threw a load together and pow pow, not really caring. They shot amazing! So I thought, well , that might be kinda fun so I checked and they don’t make them anymore. I had a similar problem years ago with a 300 win mag that just loved this certain bullet that,I can’t remember exactly what it was but it wasn’t supposed to shoot as well as it did. So be careful when selecting something to fireform