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Very Hot Topic (More than 25 Replies) Standard deviation vs. spread on velocities (Read 21621 times)
kkid66
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Re: Standard deviation vs. spread on velocities
Reply #15 - Oct 15th, 2018 at 11:16am
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westerner wrote on Oct 14th, 2018 at 11:02pm:
I always wondered about deviations on a chronograph. What do you do after you buy a chronograph and get the smallest deviation but your rifle still shoots large groups? Say you get all the numbers right like deviation and velocity but still inaccurate, then what? 

I had a cheap chrono and would say It was better not to have one. I now use an Oehler.

                               Joe.

  
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marlinguy
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Re: Standard deviation vs. spread on velocities
Reply #16 - Oct 15th, 2018 at 11:36am
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I have an Oehler also, but I rarely use my chronograph. I work up loads and go for group sizes to obtain best accuracy. If I have issues that fight me to get good group sizes, then I drag out the chronograph to see if it tells me anything. But I don't work up loads based on chronograph readings.
  

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BP
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Re: Standard deviation vs. spread on velocities
Reply #17 - Oct 15th, 2018 at 3:10pm
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westerner wrote on Oct 15th, 2018 at 2:20pm:
I have a Oehler also. I never use it. I find it much easier to use a target. If I'm shooting 250s, velocity and deviation numbers make no difference. 


                     Joe. 




I got my doubts that a head-shot squirrel worries much about velocity and deviation numbers either.    Wink
  

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calledflyer
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Re: Standard deviation vs. spread on velocities
Reply #18 - Oct 15th, 2018 at 4:03pm
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Yeah, but a squirrel like that probably hasn't got any brains, either. Roll Eyes
  
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BP
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Re: Standard deviation vs. spread on velocities
Reply #19 - Oct 15th, 2018 at 4:08pm
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calledflyer wrote on Oct 15th, 2018 at 4:03pm:
Yeah, but a squirrel like that probably hasn't got any brains, either. Roll Eyes

Sorta like a paper target?    Grin

  

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svartkruttgris#369
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Re: Standard deviation vs. spread on velocities
Reply #20 - Oct 15th, 2018 at 4:49pm
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Having used a cheap chrono for decades and now a MagnetoSpeed unit, most value they provide to me, as a hunter, is velocity of loads I am interested in. Paper targets and POI on dead critters are SOP for real world accuracy of 1st shot.

Only loads I have much interest in are those that fill case under seated bullet and still have pressures safe for each rifle and velocities suited to bullet and critter. That alone seems to keep velocity spreads low.
  
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Re: Standard deviation vs. spread on velocities
Reply #21 - Oct 15th, 2018 at 7:24pm
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I don't bother with the chronograph for very close shooting ( 200 yards ), but I wouldn't dream of bringing ammunition to a long range match ( or even only 600 yards ) without doing some chronograph testing.   A load can look excellent at 200-300 and have enough velocity variation that a poor finish is guaranteed at long range.

Chris.
  
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Domino81
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Re: Standard deviation vs. spread on velocities
Reply #22 - Oct 26th, 2018 at 8:16am
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I have exchanged the nipple to oryginal one with smaller ignition channel.. I also did a test with underpowder newspaper wads. 

New nipple caused the max spread to sabilize at mid 40's while with newspaper wads i was able to get down to mid 30's. Neither of these did solve the problem of group sizing. 

While cleaning brass I have extracted couple of unburned remainigs of wads. What makes me wonder is that there are visible differences in ignition. The wholes burned out by ignition in these wads are different in size. I guess, that might have something to do with differences in alignment between igition channel and flash whole from one case to another. It heavily depends on how consistently barrel meets the receiver vs. the uniformity of 38-55 brass that I used to fabricate 35-30 cases. I was able to put 3 bullets into 1,5 inch group when velocities in one group of shots were within 5-8fps spread range. 

I'll try to lessen ignition channel's diameter, so that miss-alignment will have less influence in the ignition. 

Generally 32-40 and 38-55 are supposibly more vulnerable to hotter rifle caps causing spreads in velocities. BP Cartridge Reloading Primer book mentions even 80-90 FPS spreads between shots.. The remedy for that was pilstol primer instead of Rifle one, so deffinately making ignition weaker should have positive infuence on spreads. 

Here is couple of overpowder wads remains. 
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I have also extracted couple of my bullets from range and it seems that this isn't really the bore riding design I wanted to order from accuratemolds using dimensions from slugged bore. The nose instead of .353 drops .351 with 8 BHN alloy. On the orher hand bands are perfectly fitting to groove dimensions. 

I will try harder alloy (12-13 BHN?) and maybe FFFg Swiss powder - but I do not know whether that will be enough to make a bullet a 3 thousands bigger on the nose part to perfectly fit the bore. On the other hand from what I have found out the smaller the grains the bigger the spread. 

Couple more pics: 
Bullets - cleary no sign of nose contact to lands. 
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« Last Edit: Oct 26th, 2018 at 8:35am by Domino81 »  
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Re: Standard deviation vs. spread on velocities
Reply #23 - Oct 26th, 2018 at 1:57pm
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Dominik,
It looks like your making some progress.

I agree that the bullet doesn't fit well. If ACCURATE made the mold and it doesn't cast to what you ordered, I would call them and see what they have to say. I would want the bore riding section to be at least .354, cast with nothing harder than 20/1 (approx 10 BHN). I would also want the driving bands to be .001 over groove diameter.

If you requested a .353 bore ride and got .351, I would hope that they would make it good.

If you ever have another mold made, I would use a tapered bore ride, starting at .353 and ending at .355. That will help center the cartridge in the chamber. If the lands are wide, I'd suggest .353 - .354.

The other thing about your situation, is that we can't say what your accuracy potential is. I would think that 2", five shot groups@ 100 yds, would be good. DeadEyeBly, might be better able to address that.

Frank
  

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Re: Standard deviation vs. spread on velocities
Reply #24 - Oct 26th, 2018 at 2:01pm
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When you ordered your mold, what alloy did you specify you were using?
  
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Domino81
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Re: Standard deviation vs. spread on velocities
Reply #25 - Oct 26th, 2018 at 3:34pm
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When I was ordering, I specified 1:30 alloy. 0+/-

@frnkeore - I'll e-mail them for sure, see where it gets me. It's my 4th mould from accurate and not the last for sure. 

tapered bore ride sounds like something I haven't tried so far, and is definitely interesting direction. I wanted actually to submit a new design based on the one i found on page, but Tom suggested that I just put notes in the order and he will reflect that in design.. Do I need special software to design bullet?

It's my first breechloader and it feels like discovering completely new land Smiley 

When it comes to accuracy, these were supposed to be very accurate percussion .35 Sporting Rifles. 2 inches at 100 yards and 4 inches at 200 should be achievable, even though the barrel has only 1 in 28 inch twist that limits the weight of the bullet to c.a. 200 grains and a length of .75'. I have another mould that is throwing 156 grains one grease groove bullets, however got initial results that were more promising gron the heavier ones. My gunsmith have tested the barrel and receiver in a bench rest installation and confirmed that he was able to achieve 1 1/2 inch 5-shot group at 100 yards, however these were breech seated, powder poured into the chamber and than the and of chamber plugged with a shortened case. The problem with the smaller bullets is that because of their shape, they stop when bands reach the rifling. When I put the case full of powder inside, the bullet stops 1/5 inch ahead of the case, that I presume is a bit too far for breech seating in a 150 years old barrel. The longer ones stop 1/8 inch ahead of case that seems to be safer. 

When it comes to cases I find another problem - Starline brass that I used and adopted for 35-30 are having too thin necks comparing to original cases. The original bullet mentioned in Maynard's catalogue was .370. When I fire-formed the brass, outer diameter of neck fits the one from the catalogue, but i get 7-8 thousands of an inch space in the case using bullets that are .369 Sad. I format the necks, so that they hold the bullet in centered on bands, but I know that will not secure accuracy to my expectations. 

  
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Re: Standard deviation vs. spread on velocities
Reply #26 - Oct 26th, 2018 at 4:26pm
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Dominik, I don't have any words of wisdom that will solve your accuracy issues for sure. You say your using reformed 38/55 cases. Are you just using them with the primer pockets left empty or with a deactivated primer with a small hole drilled in them or with plugged and redrilled flash holes. Most percussion shooters claim that large flash holes deteriorate accuracy. Most Maynard shooters in the N-SSA are quite happy if they can get 1 1/2" groups at 50 yards. 

If you can get a nipple for the small RWS pistol primers you could try them. A 7mmx1 threaded nipple should be able to adapt to the Maynard nipple thread.
  
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Domino81
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Re: Standard deviation vs. spread on velocities
Reply #27 - Oct 29th, 2018 at 6:42am
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Primer pockets are filed in with beryllium ball bearing, so that when the ignition hits from the channel it should hit and ommit the ball to initiate ignition. When the pressure rises it is supposed to seal the gas inside the case. It is working very well in 45-70 and 45-90 Sharps percussion replicas with reliable and consistent velocities. 

The problem i discovered is the size of ignition channel that needs to be smaller than the flash hole. It is larger unfortunately. If there is any concentric inconsistency in primer pockets, than there will be differences in the quality of ignition. We will work with my gunsmith to make the ignition channel smaller to be less vulnerable for primer pockets concentricity variances and will see what happens. 


We will also try the small caps nipple that would give me more options with overpowder wads. 

Regarding the bullets, I have been casting this weekend. My alloy that was supposed to be 13 BHN (Lead + Tin + Antymony) occured to be 22 BHN which would point to linotype. The noses are now .353 with .371 in bands (that I could size to .370), so technically that should be correspoding to .354 in lands and .368 in grooves. I do not know however If Is hould be shooting these kind of alloys in 150 years old barrel.. 

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Re: Standard deviation vs. spread on velocities
Reply #28 - Oct 29th, 2018 at 4:36pm
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Dominik,
This might be a good bullet to try.

Frank
  

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Re: Standard deviation vs. spread on velocities
Reply #29 - Oct 30th, 2018 at 11:31am
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comment: SD has statistical validity, extreme spread doesn't. I doubt the rifle or the load cares. Wink

It might be fruitful to wander over to the north-south skirmish assoc site and pick their brains. They are helpful people and experienced in dealing with exactly your problem. 

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