Problem with deviation on my handloads

[dulzura]

I'm not new to hand loading, just to precision loading. My current problem is this; I'm loading 7mm rem mag using three bullet types: Berger 168 VLD's jammed (tried different jumps, but they definitely are way more accurate with 1-2 thous jammed), Nosler 150gr ballistic silver tips (the most accurate load for my TC Venture printing under .5" @ 100 yrds) and TXS 120's. Here's the rub: I just purchased a Chrony Beta Master and tested the loads. My SD is way out. 138 to 174 fps per string deviation with the Bergers using 61.5 grains IMR 4831 at an average fps of 2850, and exactly the same (174) for the Barnes (2971 ave. fps). I'm weighing the charge with an RCBS digital powder scale/measure and have compared the resulting weights with a mechanical scale which confirmed the weights. I'm using Federal match primers and Remington cases and sort the cases in groups per the case weight. I can't figure out why there's such an extreme spread. The only wildcard might be the case neck tension. I'm using a full length Hornady sizing die which is not needed with the bolt gun. I've ordered a neck size only die which is soon to arrive. The case neck tension with the full die was weak at best, allowing me to move the bullet with a firm grip of my fingers with effort. I can understand the velocity changing from a cold barrel to a hot barrel, but have no idea why I'm getting such a spread. What do you think?

 

[Bullet bumper]

Can't say for sure what the cause is but there is a few things you could try.
You could buy a CED M2 chronograph as they are more accurate.
You could swap to H4831 powder and adjust your powder load accordingly as it is less temp sensitive. Velocity change from cold to hot may indicate the need.
Buy a Redding body die to go with the neck size only die as it will allow resizing and shoulder bumping without touching the neck area.
You could also neck turn the cases to just do a skim to clean them up about 75% of the diameter and make sure you turn all the way to mark the shoulder slightly . This gives more even neck tension and makes the neck sizing operation straighter and easier and bullet seating straighter .
If you buy a Lee collet neck sizer it will allow sizing of neck turned cases as is .
However you will need advice on how to set the die up.
Bushing dies work good with neck turned cases and allow easy adjustment of sized length. What I do is only size 2 /3 of the neck length , leaving an expanded shoulder below that . This never gets sized when using a body die as the body die does not touch any of the neck. This acts as a centering shoulder right through the whole life of the case after the first fire forming.
This effectivly regains the lost diameter of the neck turning in a factory chamber.
With a bellted magnum you may at some stage need this die also as the case can swell above the belt and be hard to size properly . Some need it some don't . Don't buy it untill you have the problem. http://www.larrywillis.com/diebig.jpg

 

[Bart B]

What I do is only size 2 /3 of the neck length , leaving an expanded shoulder below that . This never gets sized when using a body die as the body die does not touch any of the neck. This acts as a centering shoulder right through the whole life of the case after the first fire forming.

If you're not setting a fired belted case shoulder back at all (or no more than a couple thousandths) when it's sized, the front of the cartridge centers perfectly in the chamber whether there's a "centering shoulder" or not. The firing pin driving the case shoulder hard into the matching chamber shoulder centers the round as well as the bullet perfectly. Any off-center the case neck has relative to the shoulder results in the case neck being off center in the chamber neck; the bullet's aligned with the case neck so it's off center, too. Proper full length sizing of bottle neck cases ensures the case neck's centered on the case shoulder.

 

[Bart B]

My SD is way out. 138 to 174 fps per string deviation with the Bergers using 61.5 grains IMR 4831 at an average fps of 2850, and exactly the same (174) for the Barnes (2971 ave. fps). . . . . .What do you think?

With a 150 fps SD, muzzle velocity spread's about 450 fps. Is the SD (standard deviation) indicated as much as you mention? Total spread's about 3 times the standard deviation.

I think your chronograph's a rubber ruler; it's probably not measuring consistantly. Case shape and its resizing issues have nothing to do with muzzle velocity. Neck tension does, but as long as it's got no more than a 20% spread in the release force needed to get the bullet out, that's fine.

 

[dulzura]

I messed up. I've stated SD but it's really the extreme spread with an SD of 51 to 57 (+/-) depending on the load.

 

[Bart B]

Thanks for clarification on velocity.

There's a few things that'll cause that much spread.

One is a weak firing pin spring. Primer flame's not consistant and that doesn't burn the powder as consistantly.

Then there's the issue of the firing pin not protruding enough from the bolt face to properly smack the primer. It should be at least .055" and .060" is better. Insufficient protrusion can cause the same problems as a weak spring.

Another is the way one holds their rifle resting on something atop a bench. Us humans don't hold rifles against our shoulders with exactly the same amount of force. I've seen as much as 90 fps difference in average muzzle velocity with the same .308 Win. rifle and ammo by just holding the rifle very tight to virtually no pressure at all.

Neck tension by the case mouth on the bullet isn't consistant. This often happens when the case mouth's not enlarged enough when sizing fired cases. Peak pressures will be both higher and have a wider spread; that's enough to cause large spreads in muzzle velocity.

 

[rscott5028]

What Bart said. Plus a few random thoughts...

Chrony may be too close and thus affected by muzzle blast. Move it out as far as the cable will allow.

Shade the chrony from variable lighting such as clouds moving overhead.

Shade the cartridges from direct sun/heat.

Consistent brass. Same lot. Not mixing headstamps. Norma is about the best for 7RM. But, they can all be good with prep/sorting/culling.

Good brass prep including consistent neck wall thickness. Turn necks if necessary.

Neck ID should be .001-.002" less than caliber. More is just overworking brass. Less than .001" is a problem.

Uniform primer pockets and seat primers consistently. Debur flash holes.

Norma brass, H4831sc, gm215m, and 168g Bergers work well for me. You've got too many bullets and variables. Change only one thing at a time.

1-2 thousandths jam could be a problem as some could be touching and some not. Go at least .010" jump or jam. Just not right at the threshold since your measurement may not even be that precise.

Ladder test to find nodes with better grouping and ES/SD.

Use the chrony while verifying loads at long range paying close attention to vertical spread.

-- richard

 

[Bullet bumper]

If you're not setting a fired belted case shoulder back at all (or no more than a couple thousandths) when it's sized, the front of the cartridge centers perfectly in the chamber whether there's a "centering shoulder" or not. The firing pin driving the case shoulder hard into the matching chamber shoulder centers the round as well as the bullet perfectly. Any off-center the case neck has relative to the shoulder results in the case neck being off center in the chamber neck; the bullet's aligned with the case neck so it's off center, too. Proper full length sizing of bottle neck cases ensures the case neck's centered on the case shoulder.

Not totally true! It all depends on the dimentions of the sizing die , it may be tight at the shoulder diameter even though you are not bumping the shoulder angle much . This can cause some slap at the shoulder diameter , there is some shoulder clearance also. Belts do not alighn cases all that well cocentricly speaking. They mostly provide a headspace stop .
The process I outlined is not just about centering a case that is only one of the possible advantages.

 

[Bart B]

Not totally true! It all depends on the dimentions of the sizing die , it may be tight at the shoulder diameter even though you are not bumping the shoulder angle much . This can cause some slap at the shoulder diameter , there is some shoulder clearance also. Belts do not alighn cases all that well cocentricly speaking. They mostly provide a headspace stop .
The process I outlined is not just about centering a case that is only one of the possible advantages.

A full length sizing die that's tight (small in diameter?) at the shoulder might make the case diameter at that point 10 thousandths smaller that the same point in the chamber. Sometimes new cases are that much smaller than the chamber is at that point but they shoot accurate enough to win matches and set records. I've shot 1/2 MOA 15-shot test groups at 1000 yards with both brand new belted cases as well as proper full length sized ones in a SAAMI spec'd chamber. A recent 1000 yard benchrest record was set with brand new .300 Weatherby Magnum cases.

Most folks getting best accuracy with belted cases don't set their shoulder back on a fired case more than a couple thousandths. This lets them "headspace" on the shoulder and that makes 'em center perfectly up front in the chamber regardless of how much smaller in diameter they are from the chamber. There's nothing to prevent them from centering very well and consistant.

No chambered cases align themselves dead center in the chamber's back end; there's forces acting on the case that prevents that. The extractor spring pushes the case towards the chamber wall such that it contacts the chamber opposite the claw when the bolt's closed on it. That's a very repeatable way to position the back end of the case. Even if the extractor and bolt face didn't touch the case anywhere, gravity would pull its back end down so it rested on the lowest part of the chamber. Note that a .001 inch off chamber center the back of the case is, that causes about .0005 inch offset of the bullet tip from dead center in the leade opposite of where the back of the case is; the round pivots on its shoulder against the chamber shoulder. But its very repeatable from round to round. I don't think there's any way the back end of a chamberd round could "float" dead center in the chamber.

 

[dulzura]

Thanks to all the contributors of this post. You've given me a list of things to try, check, and monitor. All good stuff. Thanks again.