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Muzzle velocity variation
Discussion regarding the reloading of ammunition and tuning of loads for accuracy
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dhc4ever
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PostPosted: Tue Jun 14, 2011 2:28 pm    Post subject: Muzzle velocity variation Reply with quote

I have a friend who is having problrms with one of the calibers he reloads for .257 roberts.
Over 10 shots through a chronograph he had a maximum diviation of 347 fps, minimum was around 60fps.
This is using brass fired in that rifle, adi powder, remington large rifle primers and a quality projectile.
Powder is measured with a lyman electronic powder measure and he's seating 40 thou off the lands.
He assures me its not the powder thrower as he uses it on 3 other calibers and they dont have a problem.
My recommendation was check his trim length, and neck thickness.
A bit of reasearch last night also brought up annealing the necks as a way to reduce variations.
So I am going to recommend;
1 He put 10 more rounds through the chrony as is
2 He trims his brass and tries again
3 He outside turns the neck of his brass to even out any thickness variations and tries again
4 He anneals the necks of his brass and tries again
So does anyone have any other Ideas as to the cause of this problem and are my recommendations worthwhile?

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Ominivision1
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PostPosted: Tue Jun 14, 2011 2:37 pm    Post subject: Re: Muzzle velocity variation Reply with quote

First thing I would look at is the batteries in the chrony, put new ones in and try again.
Second thing is does he have the screen on, is it a cloudy or sunny day?

One trick I have used on really bright sunny days is to take a black marker and paint the whole bullet. But I would say try the batteries first and go from there.

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Dawgdad
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PostPosted: Tue Jun 14, 2011 2:53 pm    Post subject: Re: Muzzle velocity variation Reply with quote

batteries were my first thought too.

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gelandangan
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PostPosted: Tue Jun 14, 2011 3:02 pm    Post subject: Re: Muzzle velocity variation Reply with quote

is the load within 95% of case capacity?
position of "empty" area in the case often causes burning time variations.

Are the powder suitable for the caliber?
using a powder too slow often causes large variations, specially if the barrel is a "loose" barrel in relation tot he projectile.

are the powder "dry" ? owing to recent drenching, some hydrophilic powders may have a top layer that is of higher humidity than the lower layers.

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slimjim
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PostPosted: Tue Jun 14, 2011 5:56 pm    Post subject: Re: Muzzle velocity variation Reply with quote

What are the specifics of the load and what were the actual velocities recorded?

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terry264
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PostPosted: Tue Jun 14, 2011 6:28 pm    Post subject: Re: Muzzle velocity variation Reply with quote

What distance from the muzzle is he placing the chronograph? He may be too close, that will do it.
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Aloysius
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PostPosted: Tue Jun 14, 2011 11:48 pm    Post subject: Re: Muzzle velocity variation Reply with quote

And additional:
- does the primer match the powder? The combination small caliber/rather big cartridge may ask for slow powder. Slow powder often asks for magnum primers.
- and second: check also bulletweigth - barrel twist.

when something in your combination is giving unused powder, nobody can predict how much powder didn't burn. Check the barrel for residues after a shot, the more 'dirt' the less efficient your powder yield.

But as already said: check you chronograph first (or using additional another chrono or use another caliber on the same chrono).
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Elvis
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PostPosted: Wed Jun 15, 2011 12:52 am    Post subject: Re: Muzzle velocity variation Reply with quote

a wise man once said " a chronograph has stuffed up more good loads than anything else I know"
that equates to about a 10 % velocity variation so something isnt right somewhere.

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SingleShotLover
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PostPosted: Wed Jun 15, 2011 5:03 am    Post subject: Re: Muzzle velocity variation Reply with quote

You have good recommendations, but I would caution you to go real easy on the neck turning. Only trim the "high" spots without getting into the neck itself. Wholesale turning can easily cause insufficient neck tension which opens a whole other can of worms.

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Vince
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PostPosted: Wed Jun 15, 2011 5:43 am    Post subject: Re: Muzzle velocity variation Reply with quote

You say "one of the calibres he reloads for .257 Roberts"...what is the actual calibre if it isn't the .257 Roberts?

What ADI powder is he loading...what charge weight of the powder is he loading...what weight and brand of projectile is he using?

I have had chronos let me down in the past, so the suggestions the other guys have made are well worth checking. Does he do his shooting at SSAA Ripley? If so, they have a chrono available for hire there, but you need your own battery. Might be an easy way to test with a different chrono.

Cheers, Vince

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chambered221
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PostPosted: Wed Jun 15, 2011 6:09 am    Post subject: Re: Muzzle velocity variation Reply with quote

A 300fps spread should have an obvious difference in recoil !!!

If there's no difference in felt recoil or how the rifle reacts I'd start with the chronograph.
Keep in mind that the sensors are triggered by a passing shadow. Consistent light at the proper angle is the key. A partially cloudy day when the sun is going in and out of the clouds can be troublesome.

Like omni I have also used a black magic marker to paint my bullets in certain situations.
I've also been known to use a .22LR in setting up and to verify everything is working properly.

If it is load related I seriously doubt the issue will be corrected with neck tuning !!!

Shooting some factory ammo and the re-loads in the same shooting session could help in determining were the issue is.

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Elvis
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PostPosted: Wed Jun 15, 2011 11:35 pm    Post subject: Re: Muzzle velocity variation Reply with quote

what shape groups is he getting??? by my rough guestamation if hes shooting at a range greater then say 150-200yrs he SHOULD be getting verticle stringing if the velocity is that varied.

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camel
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PostPosted: Thu Jun 16, 2011 2:37 am    Post subject: Re: Muzzle velocity variation Reply with quote

Check which DI powder hes using, then check how old the tim of powder is. ADI powders are very stable generally but sometimes even they can get it wrong. AR2209 seems to be the best powder for 257Rob. I had a ruger years ago that loved the stuff. All loads from 75gn 87gn 100gn and 120 gn projectiles were perfect horizontally and only the height varies.

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SingleShotLover
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PostPosted: Thu Jun 16, 2011 4:51 am    Post subject: Re: Muzzle velocity variation Reply with quote

I think a close examination of the powder is in order. If it isn't of a uniform consistency and shape all bets are off. I just don't reasonably see any of the other ideas as causing this wide of a velocity swing. 100 or so fps, maybe. 300 and more, I really doubt it. The only time I've seen 300+ fps variation from shot to shot was while using CleanShot, a black powder substitute no longer made (obviously with good cause). It's composition ranged from dust to substantial kernels which made any load consistency a lost hope. Shots ranged from loud "booms" to mere "pops" from shot to shot using identical charges by volume..

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Bushmaster
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PostPosted: Thu Jun 16, 2011 7:51 am    Post subject: Re: Muzzle velocity variation Reply with quote

Aah The "simple answer"...Try another powder and see if that improves the accuracy. If it does then the rifle just doen't like ADI powder. Until you have assertained that it isn't [or is] the powder I wouldn't change anything except the powder.

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