Is M.V. affected that much by bullet jump?

[West Calamus]

First 10 shots out of new 300 win mag with Shilen barrel. First 5 was IMR 4831 - 67,2 gr. 200 gr. Sierra gameKing, bullet jump- .120 and M.V. 2862 fps. Second day another 5 with exact same load but with a ,090 jump and a M.V. of 2743 fps. Temp was within 5 degrees each day. Seems like a awful lot of M.V. variation with only a COL change or am I missing something else you think. Each group was only 3 to 4 fps up and down from average velocity.

 

[Dmagna]

Still virgin brass? I don't start load development unless it's on once fired in my 300 win, sometimes with the belted magnums the brass has to be fired to get the consistency. Seating the bullet out could have effected case capacity as well.

 

[West Calamus]

Still virgin brass? I don't start load development unless it's on once fired in my 300 win, sometimes with the belted magnums the brass has to be fired to get the consistency. Seating the bullet out could have effected case capacity as well.

excellent observation. the 2860 group was once fired in a different chamber and the 2740 MV was virgin brass.

 

[Dmagna]

excellent observation. the 2860 group was once fired in a different chamber and the 2740 MV was virgin brass.

There's your answer,

 

[Gwine]

Looks like you got the answer, but to your original question. I just did a seating depth test this weekend from .030" off to .100" off in .010" increments. Chrono'd all shots. SD of 9, ES of 37 with 24 shots. The average for each seating depth was within 10fps fastest to slowest.

 

[BrentM]

I see you have your answer....Virgin brass is way is undersized and takes more powder to get equal speeds of formed.

MV is affected and it looks like U shape or upside down bell curve. Using exact same powder charge and only changing depth from the lands to seating to the point of compressed charge....speed goes high to low to high. I recently did a test for a FB forum and showed 0 to 120 off and the speed changed 40 fps. 90 to 120, unless compressing the charge, would not net a lot of difference. 10-20 fps max would be my expectation.

 

[baldhunter]

Jump does affect the velocity,but chronographs are also affected by lighting too.I've watched my velocity increase as I got closer to the lands.I was shooting to the east one morning at the range.As the sun started getting higher my velocity changed around 50-100fps.It was from how the chronograph was seeing the bullet crossing over the two eyes.I now shoot at a different spot at the range where I'm shooting to the north.I get repeatable velocities now.I have a Pro Chrono.It's dang accurate in it's reading too.I also have a Magneto Speed.The velocity readings between the two are just a few fps difference.If I'm testing loads for both velocity and groups I like the ProChrono.If I just want to check velocity I use the Magneto Speed.
Here is an interesting article on bullet jump and how it affects pressure and velocity

Internal Ballistics - Hornady Manufacturing, Inc

Internal Ballistics Video Library...

www.hornady.com

 

[Calvin45]

Sounds like a verdict has been reached, a brass difference, but let me state nonetheless that differences in jump can very significantly affect chamber pressure at times, notably if one tries "jamming" the bullet into the lands, all the worse if it's a bullet with lots of bearing surface or a thick jacketed bullet. Learned this the hard way when I was newer to loading and was made to believe beat accuracy would always be had from kissing or straight up jamming the lands. Can make an otherwise perfectly safe load a Primer popper.

 

[Mc Fraser]

First 10 shots out of new 300 win mag with Shilen barrel. First 5 was IMR 4831 - 67,2 gr. 200 gr. Sierra gameKing, bullet jump- .120 and M.V. 2862 fps. Second day another 5 with exact same load but with a ,090 jump and a M.V. of 2743 fps. Temp was within 5 degrees each day. Seems like a awful lot of M.V. variation with only a COL change or am I missing something else you think. Each group was only 3 to 4 fps up and down from average velocity.

My 300 wm brass can change by as much as .015in from one shot to another. I have to bump the shoulder every time.

 

[Tulsa Reiner]

Looks like you got the answer, but to your original question. I just did a seating depth test this weekend from .030" off to .100" off in .010" increments. Chrono'd all shots. SD of 9, ES of 37 with 24 shots. The average for each seating depth was within 10fps fastest to slowest.

Gwine: what cartridge were you testing, and what were your load specifics? (I'm fascinated by your results.)

 

[Beluebow]

Things that may affect your velocities based on your post. A 119fps spread isn't gonna come from a .030 seating depth change imo.

1.New barrel...new barrel velocities tend to be all over the map until they settle down.

2.Chronograph.....some of the cheaper ones will give different reading from day to day. The better ones seem to be consistent.

3. New brass versus fired brass....if it was new brass both days...see the above.

4. Scale issues?

 

[Calvin45]

Things that may affect your velocities based on your post. A 119fps spread isn't gonna come from a .030 seating depth change imo.

1.New barrel...new barrel velocities tend to be all over the map until they settle down.

2.Chronograph.....some of the cheaper ones will give different reading from day to day. The better ones seem to be consistent.

3. New brass versus fired brass....if it was new brass both days...see the above.

4. Scale issues?

great to bring up the possibility of scale issues and new Barrel effects. That'd do it.
OP - have you ever checked your scale against another?

 

[Gwine]

Gwine: what cartridge were you testing, and what were your load specifics? (I'm fascinated by your results.)

22" 22 Creedmoor
once fired Alpha brass
80eldm
40.7g H4350
CCI large
shots 82-105 on barrel

At .100" off, the base of the bullet should just be at the top of the powder, ish. So no where near compressed. 40.9 is where I started seeing ejector marks on the brass while doing pressure ladder.
2209fps and 2246fps were the outliers. 2220fps avg.

 

[West Calamus]

Follow up test to prove if it was virgin vs. once fired brass for 120 fps swing in muzzle velocity. Exact same load as original test in post 1 with all bullets set .070 off lands. Sig brass out of same box and not weight sorted. Alternated shots between virgin, once fired,virgin etc. for a total of 8 shots (all the virgin brass that I had). I threw out one virgin brass shot that was 40 fps off remaining group. Virgin brass average 2748 fps. Once fired brass average 2867 fps for once again a 120 fps advantage to the once fired. I realize that this only applies to my load, brass and rifle and am not making any blanket statements. These were shots 17 through 25 down the pipe. I appreciate all the comments and suggestions and a shout out to Dmagna.