God groups but bad ES

[azgutpile]

I have a custom built 300 RUM that I had built two years ago that It shoots sub 1/4" MOA from 100 to 700 yards (3 shot groups); however, when I use my chrono, it shows an ES of about 25 fps. Even if I throw out the most extreme velocities, it would still show an ES of 20 fps. What can be causing this?

When I shoot at 750 yards, the groups tend to be slightly more horizontal than vertical.

Here are the specs of my rifle.

Stiller Predator Action
Schneider P5 barrel, 1:11 twist -
5.5 taper, 28"+ Muzzle Break
McMillan A 3-5 Stock
5.5x22x56 Night Force
Badger Rings

Nosler Brass
86 grains of H1,000
Federal 215 M Primers
Berger 210 Hunting VLDs
loaded .020 off the lands
OAL is 3.798 (4.127 with comparator)
2,988 to 3,013 FPS

I measure every load exactly on an older RCBS Balance Beam scale
RCBS rock chucker press
RCBS full length dies and seater

Just as an FYI, I have no problem achieving a low ES when I reload for my friends 300 RUM (6- ES), or my Kimber 300WSM (9- ES); however these guns only average .75" group at 100 yards.

The other thing that is throwing me off is when I last shot at 750 yards; I shot 3 groups that were (1.5") (3.2") (2.5"). When I do the math on my Ipod (ballistic FTE), and figure a 25 fps ES, my bullets should be printing in a 6" vertical group.

I know I should be happy with what I'm getting, but I can't help but think what my 1,000 yard groups would be like if I had an ES of 5?

Where should I begin my search for th cure?

Justin

 

[scenarshooter]

2.5" groups at 750?.....mission acclomplished. Throw away the chronograph.

 

[bota]

^^I agree. But you could try a few loads with some different powder/ seating depth. Shoot groups at 750 without the chrono, and pay attention to vertical spread. What type of vertical spread do the other rifles you load for get?

This is what i do, and my current rifle prints groups with almost no vertical dispersion (1/2") @600yds....

 

[Broz]

Move your chronograph farther away from the muzzle and retest. 18' plus.

Jeff

 

[Boss Hoss]

25 fps is actually marginally acceptable for a sporter. My cut off for competition loads is 15fps. The only thing I question is your shot string. When verifying the final load I always shoot a couple of 10 shot strings to verify my ES.

 

[Wingnut]

I would have to say groups don't lie. Those are good groups for that large of an ES, I would personally question the chrono. Was it a partly cloudy day? I found that most chronographs are very sensitive to light conditions and where the sun is in the sky.

 

[cowboy]

Move your chronograph farther away from the muzzle and retest. 18' plus.
Jeff

Broz gives good advise.

I think what Broz is saying is that you have to be assured you are not getting any effect from muzzle blast on your chrony.

Heck - set your chrony up at 100 yds if your rifle is shooting like that and then you'll really know what is happening. Use your 100 yd. velocities and play with a ballistic calculator to get your muzzle velocity.

 

[Marine SS]

Your chonograph is broken

 

[Don Ward]

1/4 MOA from 100 to 700 yards? Take "yes" for an answer and go hunting. It would be tough to shoot well enough to notice any improvement if you did find something to change.

 

[azsugarbear]

The ES measurement is just a means to an end. We strive for a low ES as a way of finding consistency, which generally equals accuracy, or small groups. If you are currently shooting consistently small groups that are holding out to 700+ yards - then forget about the ES. You have found the Holy Grail!

 

[MontanaRifleman]

Do you want small ES or do you want small groups?

Just because your chrony is spitting stuff out doesn't mean it's necessarily true.

Bullet holes are reality.

 

[azgutpile]

Re: Good groups but bad ES

I guess figured that if I could improve my ES, even by a little bit, my groups would improve even more.

Next time I go out, I will try the Chrony at 18' and at 100 yards and see if they settle down. Other than that, I think I will invest in some better dies and see what that does for me.

I do have another question though.

Does a match grade chambers tend to have higher chamber pressures compared to a standard chamber? It appears that I always have to stay on the lower end of the load book data. If I go up three grains I start seeing slight plunger marks on the bass. I know it is what it is and I have to live with it; I just want to make sure I learn from all my experiences.

Thanks for all you help.

 

[groper]

Lesson #1 - chronies lie, groups dont...

Second point is - im not sure what your doing with your ballistic calculator, but my calculator is telling me that a 25fps velocity spread = 2.3in vertical POI shift @ 750yds, not 6inches...?

Lastly, have you tried a different primer? the fed 215 is the hottest primer on the market, you might try a slightly cooler one but dont go too cold in the 300RUM... id also give retumbo a go instead of H1000- you might have better luck with it - i did in my 338edge which was against the trend as most seem to do better with H1000...

Failing that, you can neck turn your brass, sort it by weight and case wall thickness and use a collet neck sizing die and vary your neck tension - also sort your bullets by bearing surface length... all these help with ES and theres not much else you can do after that...

 

[MontanaRifleman]

Re: Good groups but bad ES

I guess figured that if I could improve my ES, even by a little bit, my groups would improve even more.

Next time I go out, I will try the Chrony at 18' and at 100 yards and see if they settle down. Other than that, I think I will invest in some better dies and see what that does for me.

I do have another question though.

Does a match grade chambers tend to have higher chamber pressures compared to a standard chamber? It appears that I always have to stay on the lower end of the load book data. If I go up three grains I start seeing slight plunger marks on the bass. I know it is what it is and I have to live with it; I just want to make sure I learn from all my experiences.

Thanks for all you help.

A 300 RUM has limited barrel life. If I was you, I would be very happy with .25 MOA @ 750 yds. In fact, I would be very happy with .5 MOA @ 750 yds for hunting. If I was a BR competitor, sure, I would do whatever I could to squeeze every last bit of accuracy out that I could but I always believe my groups more than my chronys. If you are getting consistant .25 MOA groups @ 750 yds your chrony is lying to you and you have a competitive winning BR rfile and load. If you want to burn up barrel life trying to make the chrony give you better numbers, that's your call. As for me, I would take that rifle and load and start getting very familiar with it at various distances and conditions. And, maybe when you move the chrony back a few feet, it might help, but who really cares? We have a very accomplished LRH member who has made multiple 1K plus one shot kills and he doesn't use a chrony at all.

And if you sell your dies, let me know. Have you checked your runout on brass and seated bullets? as far as dies go, that's all you really need to be concerned with other than maybe neck tension. If your getting good concentricity, you can't ask for much more from your dies.

I usually find very little to disagree on with Groper, but sorting brass by weight assumes that the weight is proportional to the volume of the case, which ain't necessarily so. The best thing to do IMO, is to sort by lot.

Yes, chambers and barrels can have an affect on pressure, as well as a lot of other things, but it's not a big deal if you're getting adequate velocity and accuracy.

Keep up the good shooting.

-Mark