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Old 12-03-2011, 06:30 PM   #1
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Best Velocity for Precision?

Hi Folks,

I'm wondering if there's a general rule of thumb with regard to accuracy from certain weight (or length) bullets from a certain twist.

For example, if I load a .30 caliber 150 grain round for a 1:10 twist, I might be able to adjust the load to anywhere from 2200fps to 3000fps. But would the highest precision be found in around 2400fps to 2600fps?

How exactly does this work? I'm mostly into milsurps and I don't suppose the militaries of the world pay particular attention to precision except in certain specialized applications.

I do like to experiment, but is there a velocity range in which I should look for the best precision?

I do always start at the starting loads and work up.

Thanks!

Josh
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Old 12-03-2011, 10:01 PM   #2
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Josh,
Once you have an optimum twist rate for the bullet weight, the most accurate velocity is a matter of barrel harmonics, and what velocity has the most uniform harmonics, so that the vibrating barrel will be at the same point every time when the bullet exits the muzzle.

I will attach a page link showing all the ways a barrel moves when fired. It will astound you and make you wonder how we can hit a thing with a gun. But the animations are greatly exaggerated so that we can see what goes on. Note that all these movements are simultaneous at each frequency. The hoped for result is what is known in electronics and radio, as a standing wave. A wave form on a wire or on an oscilloscope screen that harmonizes with the length of the circuit or in this case barrel and appears to become perfectly stationary, again, so that the muzzle is not twisted out of position in a random wave pattern at the moment of each firing. So in essence, start low and work up toward max and stop when satisfied or you go past the point of best accuracy.

http://www.varmintal.com/amode.htm

Browse all of Varmint Al's web site for some really good info and entertainment.

PS: This varying harmonic is the reason the BOSS adjustable compensators work. Fine tuning the standing wave by adjusting the length of the barrel a fraction. Ham operators do the same adjusting the length of antennas for radio frequency waves.
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Old 12-03-2011, 11:16 PM   #3
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Josh,

You just opened Pandora's Box!

Velocity, up or down, is only part of the puzzle.

Generally speaking, the most accurate loads are found below the maximum pressures/velocities for a cartridge. Mind you, that's not an absolute.

Then there's twist rates. While most .30 cals use a 1:10" you can find anything from a 1:10 to a 1:14. You can get faster twists for longer projectiles. The long 240 gr Sierra Match King requires a faster twist than 1:10"(1:8"?). Stabiltiy effects accuracy. One reason a lot of folks specify 1:11 1/4" twist for 175 grain SMK's. How much it effects accuracy I can't say.

The weak link is the loose nut behind the butt plate...always.

Of course, then we can get into brass quality, weight, neck thickness, yada, yada, yada...

Primers, too.

You can make yourself crazy.

One writer just did an extensive work up on the .223. The end result is find what shoots well for you...in your rifle, smile, and be happy.

My basic thought on bullet stability is it's better to overstabilize than understabilize. Some folks think best accuracy is found just this side of bullet instability.

That's why gun writers never run out of things to write about...;-)

Wes

Varmint Al's has some really interesting stuff to peruse. Just remember we don't shoot oscilliscope printouts...we shoot bullets.



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Old 12-03-2011, 11:34 PM   #4
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Quote:
Originally Posted by Josh Smith View Post
I'm wondering if there's a general rule of thumb with regard to accuracy from certain weight (or length) bullets from a certain twist.
Josh
Mate, if there was there would probably be only one bullet and one velocity for each calibre on the market.

As was said earlier, find what works for you and your rifle and enjoy.
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Old 12-04-2011, 01:10 AM   #5
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Thanks folks.

Here's the deal: I'm actually above max according to Hornady on the 7.62x54R loads. I use 47 grains of Varget. This is Hodgdon starting load, but 0.5grns above max according to Hornady. I did call Hodgdon and they assured me the pressure was safe. That load is pushing around 2700FPS from their test barrel.

The 7.92x57J gets loaded with 46.5 grains of Varget, which, according to Hornady's test barrel works out to something around 2550FPS. Hornady's shows a bit faster. Additionally, Hornady's loads show this as mid-range while Hodgdon shows it as 1grn over starting.

Now, I'm not sure that pushing the 7.62x54R as fast as I am is accomplishing anything. I'd sort of like to start working backwards on that cartridge, but especially on the 7.92x57J load using Hornady's data, as I'm not real comfortable pushing the old Gew88 quite that fast.

I'd like to shoot for around 2500fps in both, bumping velocity up or down a few feet per second to stabilize the bullet as needed.

The longest shot I can foresee ever taking would be 200 meters or so. Certainly no more than that, and probably not that far as there are only a couple places where I could take a shot that far, don't know why I would, and would have trouble seeing much past it with my 20/40 vision which I feel is a bad habit to correct when shooting if one carries a firearm for defense.

I'll be saving powder, brass, and undue wear on the rifles, especially the old German gal. I'm still in pressure specs for Gooey's action, but not by a whole heck of a lot. No reason to stress it.

The Mosinka I don't think could care less about how much stress I put the action under, but because of the primitive gas venting system (it does have one; you can see it if you smoke the action with the bolt closed, but it's certainly not up to, say, Remington 700 standards), I'd rather not push it too high, either.

Looking back on performance, I really sort of like the old .30-40 Krag, which ran around 2500fps. Heck, the .30-30 runs less and gets used plenty!

Does anyone see any practical disadvantages to loading the old working gals down to 2500FPS give or take a few feet per second? I started low and worked up, and now would like to fall back a bit.

Any and all thoughts are welcome.

Thank you!

Josh
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Old 12-04-2011, 07:26 AM   #6
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I had a time getting a Mauser 7x57mm to shoot accurately with 139gr spitzers. Lopping the end off the barrel and recrowning for fresh rifling at the muzzle (some people have the barrel counterbored a bit if they don't want to shorten it). Then worked up to near max and it dialed right in finally. At an estimated 2750 fps.

But if you look at original loads, it calls for the much longer 173gr bullet, at 2300fps. So it is not only that lighter bullets fly faster, but that you seem to have to push them faster to increase the spin rate, in order to get the same stability that the heavier bullet gets at lower velocity. I couldn't stabilize 120 gr ballistic tips at all. I couldn't drive them fast enough without exceeding max pressure.

Therefore, if you wish to shoot a slower milder load, rather than just slowing down, lets say, a 150 gr bullet, switch to a heavier bullet. In the Mosin a 181 factory loading (make not specified) runs right around 2500fps in the short rifle and around 2600 in the long barrel, where as a 150gr test load ran 2840fps in the long barrel or the normal velocity for the .308 Winchester with 150gr bullets.

A heavier bullet increases the sectional density (an attribute of its killing power or terminal ballistics) and increases the ballistic co-efficient, which is often a huge advantage to accuracy and stability, and also velocity retention at longer ranges.

The same research suggests that for the Mauser, bullets in the 200 gr. class would give best performance and around 2500-2600fps velocities with the proper powder charges.

The old rule of thumb has always been Slower=Heavier:Heavier=Slower vs. Faster=Lighter:Lighter=Faster. You can cast a 150 grain full wadcutter for a .45-70 and push it fairly fast but never stabilize a bullet that is as wide as it is long. Hence big heavy bullets that can only go slow but slam the hell out of things, but very accurately. Or you can go to about a 40gr pill in a .22-250 at 4000fps for explosive expansion, that tends to destabilize past 300-400 yards and are useless beyond that, and only good for small varmints in that range. But we find that the most efficient and beneficial is somewhere in the middle like 150gr .30 caliber bullet at 2850 fps to be an all around load for deer at 0-300 yards or sniping enemy humans at ranges up to 800-1000 yards depending on the sniper. But he will likeley be using the slightly slower but also slightly higher BC bullet of 162 gr, purely for the marginal increase in accuracy out past the average deer hunter's range.
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Old 12-04-2011, 11:29 AM   #7
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Hi Joe,

I'm trying to keep the bullets on the lighter side because, in the case of the 7.92J and the Gooey rifle, I don't want to be any harder on that old action than I have to be.

The 7.62x54R gets 147 to 150 grain bullets too, because I bought (five hundred, maybe?) years back for like $50, which seemed like a good price at the time for pulled bullets when stuff was hard to find, powder, primers, etc.

So you're saying that I will probably find best precision up around 2800fps with the Mosinka? I have plenty of room to work either which way.

The original 7.92x57J load used a 220 grain bullet. Loading 200 grain jacketed round nose gets me very, very good precision, but I'm not 100% sure that the heavier bullets aren't stressing the action, even with a relatively sedate 2200fps (or so).

Plus, those 0.318" bullets are expensive! About 37 cents apiece for the 150 grain, the cheapest, all the way up to about 75 cents apiece for the 200 grainers.

I really need to call Lee about a custom sizer...

I'm going to call the 47 grains of Varget a middle-of-the-road load for the 147 to 150 grain bullets in the 7.62x54R. Might bump it up to 47.2 grains as I found some notes and recall that as being my accuracy load. A 47 grain load is published at a mean of 2800fps, and only one surplus load I can find pushes it into the realm of 3000fps. We'll see what happens there, I guess!

As for the 7.92x57J, I only came off the minimum of 45.5 grains (around 2500fps) because I was backing primers out. Turns out that was a bad run of R-P cases; Winchester cases haven't done that (formed them from .30-06).

I'm not really sure where I need to set my max for the J load at, but I'm likely very near it. These Gew88 rifles are known to hold up to surplus ammo which uses a higher velocity and larger bullet than is recommended, but this definitely shortens life and Turkish armorers noted catastrophic failures in WWI and II with the new JS ammo an the Gew88's action.

So, I'm going to work backwards on that one until I find optimum precision in the other direction. According to Hornady, I can go all the way down to 43.5 grains, their starting load, for a published 2400fps. Plenty of room to play there.

I need to actually buy a chronograph now as the guy I used to borrow one from passed back in Feb, but I just only found out about it recently! Life is strange like that sometimes. Might ask for one for Christmas.

I'm concerned about wearing out brass but have yet to find any signs of anything. I just keep them trimmed and run a wire to check for case head separation. I only neck size so as not to overwork the brass in generously proportioned chambers. How many loadings is brass usually good for?

But anyway, does all this sound like a sane, safe plan gents?

Thanks!

Josh
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Old 12-04-2011, 12:33 PM   #8
 
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Safe, sane and sensible. As you have noticed the Hornady and Speer manuals (and others) are very conservative and the Hodgdon site lists max loads that truly are MAX. Based on my 30-06 experience I'd have recommended a 2500fps starting point but your rifle will tell you what loads it likes, all you have to do is find them. I've only had a few rifles that like max loads, I generally get what I (and my rifle) want before we get there.
Cases will last longer with milder loads, as will your powder supply.
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Old 12-12-2011, 12:14 AM   #9
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If you get bullets too long for the twist rate of the barrel, they will destabilize and keyhole. Barrel length, stiffness, bullet weight, powder charge, and burn characteristics will all affect accuracy.
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