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Having problems with getting consistant velocity

 
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  #1  
Old 12-19-2001, 04:13 PM
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Join Date: Jul 2001
Location: San Antonio, Texas
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Having problems with getting consistant velocity

I've started load developement for my 270-300ultra. I broke in the barrel just as advised by Lilja, after about the tenth or twelth shot there has been no copper fouling worth mentioning. Three wet patches of BBS and a couple dry and it's clean. I've never had a barrel clean so easily.
I've been shooting five shot groups and the best so far has been right at an inch @100.

Velocities have been pretty high (135MK @3800 w/102gr and 150 Ballistic tip @3400 w/96gr), but inconsistant, as much as 75fps between highest and lowest shot. I figured that when I fix the velocity problem the groups will tighten up.
All brass used was weighed sorted to be within 270-271.9 grains. Norma 404jeff showed same inconsistant velocities as Rem 300RUM brass. All were sized down and trimmed to 2.840" then necks turned to .011" per side with a few @.0105" just to see if there was any difference. With bullets seated necks were @.299" and .298" for .300" chamber neck. Note No sizing required to reload fired brass.
Federal 215 primers were used to light off the WC872.
Sierra 135gr matchking seated to touch the lands. With the MK out to touch the lands only about half the neck is in contact with the bullet, thinking maybe bullet pull is inconsistant?

Any help would be greatly appreciated.
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Old 12-19-2001, 08:49 PM
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Re: Having problems with getting consistant velocity

txhunter, how are your pressures? Sometimes if you don't have your pressure up enough you will get inconsistent velocitys. I have noticed this in my 300 Ultra. How does the norma Brass look after being fire formed in your chamber? It sounds like you must have used the base dimensions for the ultra if you can interchange brass. I was told that there would be a large bulge in the norma brass just ahead of the base. Is this true? Does it look any different then the Ultra brass?
Thanks,
Vince
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Old 12-19-2001, 10:49 PM
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Re: Having problems with getting consistant velocity

Pressures?
I'm guestimating between 50-55Kpsi.

Bulge in 404 brass?
Yes, but it is not as big as I was lead to believe it would be. But then again just looking at the fired rum brass there is no noticable expansion ring. I measured some @.550" unfired and .552" fired. I believe the 404 base size is about .540, so .012" expansion, .006" per side is not as much as the 270win barrel that was originaly on this rifle. I'll sdouble check the numbers when my new set of dial calipers get here. The old pair I had crapped out on me yesterday.
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Old 12-19-2001, 11:34 PM
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Re: Having problems with getting consistant velocity

Hello TXHunter

Using WC872? I use that powder in all my LR hunting rifles and it works fine.

Is it hot there where your testing the loads. Wc 872 is a heat sinsative ball powder whereas it will throw flyers as the chamber heat builds. This is more common when the temperature is hot outside.

An age old saying to remember here is;
High velocity will kill accuracy in almost every caliber.
Try loading down a bit to reduce the velocity by 200 FPS to start and then work back up a half gain at a time till the groups tighten up. Stop there and you will have reached the accuray potential of that load. Try other powders also. How about bullet seating depth? Work with that also. Some guns shoot with the bullet in the lands some shoot better .050" off the lands.

Check your brass as far as wall thickness and weight. Seperate them in lots or match brass. Use the brass from one lot for your testing to get the best results.

I think the problem is excessive velocity and a bit of a powder reduction may just do the trick. Of course Remington brass leaves a lot to be desired too, if that's what your using.

You would be surprised the lower velocities for 1000 yard match shooting most of the fellows use at Williamsport. At least 100 to 200 FPS lower then what the case will support in most instances.

Good luck
DC
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Old 12-20-2001, 07:20 PM
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Re: Having problems with getting consistant velocity

Hey Darryl

Now you've been doing this alot longer than I have so I will take your word for it, that velocity kills accuracy. But, and this is a big but to me. How and why?

There are two possible answers that I can think of.
1. As you reach the velocity potential(ie max pressure) for the case capacity with a particular bullet, the internal pressures are less predictable and inconsistant. More velocity variation and groups open up. Which in my case I don't think I'm that close yet.

2. There is something about pushing bullets that fast that causes them to be less stable.

My most educated guess which seems to be supported by your last statement(At least 100 to 200 FPS lower then what the case will support in most instances.) would be number 1.

Now before I worry about the accuracy to much I'm looking at the large variation in speed. 75fps spread is a lot! At least in my book. Does this problem occur more often in such large capacity cases? With other cases (270win, 30-06, 243win, and 222rem) I was able to deep the variation down around 25fps or less.

The first thing I've done was to seat the bullets so that there is full contact between the neck and bullet. Will see if that has any effect on velocity variation. If that fixes the problem then I'll start tweeking the load. Quickload says 3950fps will be max but I'm thinking 4000fps might be possible with out blowing primers. I'll probably push it on up just to see how fast it will go but I'm hoping to stay right around 3750-3800fps.

Temperature has been unusualy cool for Texas, it's actualy been staying below 80 degrees. Temp during firing session was about 67-72F. The barrel was cleaned and allowed to cool between each five shots.

[ 12-20-2001: Message edited by: txhunter ]
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Old 12-20-2001, 08:08 PM
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Re: Having problems with getting consistant velocity

Hello Tx

The problem with the bigger cases is usually there are too many varibles the bigger you go especially when they are necked down very far. Can make for a very overbore situation.

More powder, bigger cases (with room for error in manufacturing), more chamber heat, and relativly small bullets with a lower BC being driven down the barrel at extreme velocity all add up to eratic bullet behavior downrange from shot to shot. Some bullets will simply not give good accuracy at extreme speeds. If you have ever seen the back end (Boat tail)base of a bullet that has been fired, it sometimes is pushed into an uneven or distortion which will cause rapid decay of shot to shot accuracy.

Excessive speed has always been the cause of inaccuracy in rifles especially in 1000 yard match guns. We get the best accuracy when we slow them down a bit. There seems to be a certain speed that most all bullet weights will perform well at. For instance, I can and did run my 300 gr 338/416 Rigby imp to 3310 FPS. It shot extremely flat at that speed. My 100 yard groups were 1 1/2" and should have been one cloverleaf hole. After losing my cases after 2 loadings, I decided to lower the speed because the high BC of the bullet, really didn't need that much speed to perform well at extended ranges. I slowed it down to 3150 & 3200 FPS and I extended my brass life to 7 or 8 loadings and the accuracy is better (cloverleaf) and the efectivness of the 300 gr Sierra MK bullet is still there.

It seems that most shooters want to drive the bullet as fast as they can for whatever reason. I use to think this way untill I started working with higher BC bullets that were heavier. I'm an old 6.5/300 Weatherby fan (still have two of them) with long barrels and know what some calibers are capable of when pertaining to speed and accuracy.
Excessive Speed not only kills accuracy, it destroys barrels and cases MUCH faster.

I will never forget a statement from the well known Williamsport gunsmith, Howard Wolfe, who said (after we were talking about excessive speed) "I never heard a deer or elk complain as to how many extra clicks were put on to kill it."

Remember also that some bullets will group well at 100 yards and do lousy at further ranges while others will group well at 500 and 1000 yards better then closer yardages.
It's in the stabilization range of the bullet.

Maybe you can push it further but, in my experiance, it only made it worse.

Let us know your outcome as I will soon have one to work with on the 300 Ultra IMP case but, I will be using the 30 cal 240 gr Sierra which has a much higher BC then the 270s.

Good luck to you.
Darryl cassel
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  #7  
Old 12-20-2001, 08:19 PM
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Re: Having problems with getting consistant velocity

My 300 Ultra with 220gr Mk's and RL25 or H1000 gives me E.S.'s in the 10fps to 15fps range.
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