Headspace difference affecting group sizes

[bigedp51]

When you pull the trigger the firing pin hits the primer and pushes the case forward until the shoulder of the cartridge case contacts the shoulder of the chamber and the cartridge goes bang.

The case with .004 shoulder bump pushed the bullet .003 further into the lands.

In 1969, Tommy Roe had a hit single called "Jam Up and Jelly Tight" :

If the difference was truly caused by the shoulder being set back more on those particular cases then I'm with Ed. Try some loads with the bullets seated farther out not deeper in the case.

Actually I would go with the voice of far more experience below and go the "other way" and seat the bullets deeper and give them a little more wiggle room.

Go back to the brass that's bumped .001" and seat bullet .003" deeper and shoot a group, then .006", .009", etc. do a test that covers about .021" (7 groups) and your groups will shrink.

Erik Cortina's "Long range load development at 100 yards" posting is a wealth of information.

When reading anything by Erik Cortina it should be like a E.F. Hutton TV commercial.

When E.F. Hutton talks, people listen ...
http://www.youtube.com/watch?v=2MXqb1a3Apg

 

[bigedp51]

When initially working the load up I found the center of the node at 100 using the ocw method then I verified that with a ladder test at 300. I then took that load and started with a jam and worked out to. .039 off in .003 increments. It shot best jammed to .006 off with jammed having 7 fps e.s. for 10 shots.

And when you bumped the shoulder back .004 your bullets were jammed .003 further than before. I'm not an expert but when seating depth of a few thousandths effects your groups that close to jam the margin for error in shoulder bump has a greater effect.

I have run dry fire tests on fired cases and the impact of the firing pin can make your cases .001 shorter, just by dry firing the fired case and primer.

 

[artbosco]

Here's the sequence, for working up a load, by a Benchrest Hall of Fame inductee.

Working Up a Load …. by Lee Euber

1. Polish bullet with steel wool. Initial seating depth achieved when bullet is marked by all lands.

POWDER

2. Load 3 cases in a low grain load. Increase each 3 case load by .5 of a grain, and then shoot. Increase loads until primers show pressure.

3. Identify shots that grouped well, not so good, and then good again.

4. Pick load that shot the best. Load 3 cases with .1 grain less, and 3 with .1 grain more, for another comparison.

5. After picking charge for use, load 30 cases with the same powder charge.

SEATING DEPTH

6. Load the 30 cases in groups of 3, increasing the seating depth from touching the lands to jumping about .040 of an inch, in increments of .005 of an inch.

7. Shoot all 30 rounds without cleaning or clean after each 10.

8. Use ONE fouling shot after each cleaning.

9. Select load that shot best. Load 5 cases with that powder charge and seating depth.

10. Load 10 cases, 5 with seating depth .005 less than benchmark figure and 5 with .005 more.

11. Can compare more than once or change seating depth by .001 or .002.

12. Once you’ve picked the best seating depth, load 15 cases at that depth.

POWDER

13. Check first test by varying charge by .1 grain, for each 5 shot group.

14. Shoot in a medium wind condition, letting the gun do the work.

SEATING DEPTH

15. After picking the best load jumping the lands, compare it with the load that touches. Compare these loads again and again, on different days, in varying conditions, deciding which one is best.

16. If it doesn’t shoot to your satisfaction, restart with a different powder.

NECK TENSION

17. If you decide on the load that puts the bullet in the lands, be sure neck tension is tight enough to hold the bullet uniformly.

 

[Erik Cortina]

When you pull the trigger the firing pin hits the primer and pushes the case forward until the shoulder of the cartridge case contacts the shoulder of the chamber and the cartridge goes bang.

The case with .004 shoulder bump pushed the bullet .003 further into the lands.

In 1969, Tommy Roe had a hit single called "Jam Up and Jelly Tight" :

If the difference was truly caused by the shoulder being set back more on those particular cases then I'm with Ed. Try some loads with the bullets seated farther out not deeper in the case.

Actually I would go with the voice of far more experience below and go the "other way" and seat the bullets deeper and give them a little more wiggle room.

Go back to the brass that's bumped .001" and seat bullet .003" deeper and shoot a group, then .006", .009", etc. do a test that covers about .021" (7 groups) and your groups will shrink.

Erik Cortina's "Long range load development at 100 yards" posting is a wealth of information.

When reading anything by Erik Cortina it should be like a E.F. Hutton TV commercial.

When E.F. Hutton talks, people listen ...
http://www.youtube.com/watch?v=2MXqb1a3Apg

LOL, that's funny! ;D

 

[LRPV]

All I was saying was that with the shoulder bumped back farther the bullet would be pushed farther into the lands and those grouped better.

I now return you to the E.F. Hutton infomercial. Lol