Manuals and Pressure Data

[Unclenick]

Bart,

I've run through the free recoil calculation before, and a 7 lb rifle firing a 155 grain .308 bullet at 2900 fps is traveling about 10 fps rearward at the moment the bullet base arrives at the muzzle. Since you saw a bigger velocity drop than that, I suspect it has something to do with rearward recoil packing the burning powder forward in the case. That could slow ignition and increase what QuickLOAD calls the Weighting Factor. Only a 10% increase in the amount of powder going forward with the bullet would take another 10 fps off the velocity.

Originally Posted by wncchester

I KNOW my chronograph is not stable enough to depend on to count within 10-20 fps around 3,000 fps on a daily basis; in fact, the thermal drift effects of a 10 degree ambient temp change is probably going to change both the clock frequency and counter stability that much or more!

…Slightly changing the bullet's passage angle OR it's height thru the sky screens will too…
…those detector things aren't made with high value optics so they aren't all that precisely focused.

I think your last point is the salient one: the detectors are the weak spot. Since there is only a small overall light level change from unfocused shadows that they are detecting, their sensitivity would have to be controlled to match for high location precision by that method and the bullet would have to pass over each screen at the exact same height. But I think most units just rely on the sensor's native sensitivity and the distance between the screens and the use of cylindrical lenses to minimize the error. So it's not a high precision measuring approach and I think is a reason changing light conditions affect some units badly.

RSI's Jim Ristow says the CED and Oehler did best compared to Doppler radar in German military testing of units intended for armorers. But that was before the German made KVM-21 was available. This 2006 review claims two PVM-21 units agreed within 1.5 ft/s on a .223 round's velocity. It's the only unit I know that asks the bullet diameter before you shoot and makes an adjustment for that shadow size to avoid false triggering. It reads horizontal IR illuminated screens and has a 16 MHz clock.

The crystal clock oscillators really shouldn't be an issue. To drift 10 fps out of 3,000 fps (0.33%) over 10°F, you're talking 0.033%/°F, or 0.06%/°C, or 600 ppm/°C drift. I've been designing circuits for fifty years, and I would be challenged to design a crystal oscillator that unstable. The least expensive, lowest precision 4 MHz crystal that Digi-Key sells has 100 ppm maximum frequency drift over -40°C-+85°C, with an absolute accuracy of 30 PPM. Worst case error due to drift over that whole temperature range and combined with a center frequency at its most extreme tolerance limit should be on the order of ±0.2 ft/s from that source.

Velocity error due to angling shots across the screens is a concern with compact chronographs (1 foot screen spacing). The error will be unilaterally slow readings, and it will be proportional to one minus the cosine of the angle off the centerline axis through the screens. For 10 fps out of 3000, that angle will be arccosine of 1-0.003333, or the arccosine of 0.9967, which is about 4.7°. It's possible for someone with 1 foot screen spacing to make that kind of alignment error. If you have 4 ft spacing, like the Oehler long pipe, it's nearly impossible to settle for such poor alignment as the screens then have so much overlap they are hard to shoot through without damaging the chronograph.

Some time ago I drew up what the different degrees of error would look like in the horizontal plane to a shooter 15 feet from the midpoint between screens. It shows 1 ft, 2 ft, and 4 ft sky screen spacing, with the screens rotated off-axis with the line of sight at 2° intervals up to 10°. I put all the angle error horizontal for simplicity, but tilt on any axis affects velocity readings the same way.

A 10 fps error at 3000 fps would be between rows 3 and 4 in the illustration, with the 1 ft spacing being the left column and 4 foot spacing being the right column. You can see how someone might settle for that degree of misalignment with 1 foot spaced sky screens, but with 2 foot spacing, it would be less likely, and with 4 foot spacing, highly improbable.

[Bart B.]

Unclenick, I "SAAMI twisted" all rounds used for velocity tests. Both with .308 Win and .30-.338 Win Mag. They were loaded with reasonable repeatability but with about 95% to 97% charge density for each, some powder position irregularities could cause that much change.

More interesting was with the 12 pound .308 shot slung up in prone as well as standing without a sling, muzzle velocities from prone were higher than standing by about 15 fps. Again, with the same loading techniques; rifle held level all the time.

[44 AMP]

To me this makes their manual less reliable then say Hodgdon that published data based on actual pressures.

You're certainly welcome to look at it that way. I look at it differently.

Reloading manuals are all equally reliable, because they are guidelines, not rules. ARRRRR!!! (pirate voice)

They tell you what they got, with what they used. NOT what you will get with what you use, other than in general terms.

Don't get hung up about teeny differences in velocities or pressures. Each gun and combination of ammo components and all the other factors is an individual.

Reloading manuals max loads are not absolutes, and cannot be, other than for those specific materials used in the testing. Generally we will get similar results with similar components and factors, but precise replication is an unrealistic expectation. Some combinations of factors will give higher velocities and pressures than the tested components, some will be lower.

[steve4102]

Originally Posted by 44 AMP

Reloading manuals are all equally reliable, because they are guidelines, not rules. ARRRRR!!! (pirate voice)

They tell you what they got, with what they used. NOT what you will get with what you use, other than in general terms.

True, but in the case of the Sierra manual, "What they got" is suspect as they did not test their loads for pressure. They used the same flawed brass reading techniques that handloaders use.

Reading brass has been proven to be more and more unreliable as new pressure testing techniques become available, yet Sierra still uses this inaccurate technique. To me that makes their data no more reliable than the average Joe in the Internet posting his handloads as safe and tested.

[Bart B.]

Few people realize that all all modern pressure measuring systems using the piezo electric or strain gauge technique are not the same. Unlike micrometers and calipers across all makes and models plus their use techniques getting numbers measuring some dimension with all will be close enough to at least the 4th decimal place, the placement and calibration of these electronic pressure measuring systems on test barrels can easily have a few to several percent spreads.

While some reloading recipies list a pressure, do they all use the same test barreled actions with the same bore, chamber and firing pin specifications? If not, there's going to be a big pressure spread in any one of their loads used in all sorts of test hardware.

Therefore, it's my opinion that virtually all printed load data stating pressures are measured with a rubber yardstick. Hodgdon's data lists barrel lengths and pressure system types (psi or cup) used, but no details bore, groove nor chamber dimensions. Nor do they mention what pressure measuring standard's used, such as SAAMI or some other system. Even if one uses the same recipie and lots of components Hodgdon did in their tests, that load will produce different pressure and velocity in your barrel of the same length.

[steve4102]

Yes, but no matter how you slice it, Pressure tested data compared to Reading Brass Tea Leaves? I'll take Pressure data over Tea Leaves any day, especially when the two differ greatly.

[Clark]

steve4102
Yes, but no matter how you slice it, Pressure tested data compared to Reading Brass Tea Leaves? I'll take Pressure data over Tea Leaves any day, especially when the two differ greatly.

When reloading for an individual strong rifle like a Rem700, Win M70, Sav 110, 98 Mauser, etc, the threshold of long brass life is the important thing, and knowing the true pressure can be a red herring.

[44 AMP]

"What they got" is suspect as they did not test their loads for pressure. They used the same flawed brass reading techniques that handloaders use.

I guess that would depend on your concept of "flawed".

Pressure tested data compared to Reading Brass Tea Leaves? I'll take Pressure data over Tea Leaves any day, especially when the two differ greatly.

That's fine, I'll keep reading my Tea Leaves the way I have been for over 40 years. I know that brass signs as not "accurate" in the sense that one gets a precise pressure value. But that does not matter very much.

What matters is you stay within the safe working range of pressure, for your gun, with the components you are using. Pressure info is nice to have, its another guideline, but it really matters little to me if a load is 38.8Kpsi or 42.5K CUP. What matters is that it is within the usual safety range of the gun.

[old roper]

Clark, I seen your post about the groups that Bart B shoot


Since May 1997 I have been trying to read everything he posts on the internet. There are lots of great shooters and lots of great posters, but only he seems to share all his secrets.

Quote:
Leafing through my copy of the May 1997 issue of Precision Shooting, I
encountered an ad for Krieger Barrels, Inc. that showed an actual-size
copy of a 20-shot group shot at 800 yards by "Bert Bobbit [sic] with
his Krieger Barrelled PALMA rifle." Now this group has a .942" mean
radius, with an extreme spread of 3.325. If it were a 5-shot group,
you'd say, "Somebody else has shot that well at 1,000 yards."


There is few things that need to be corrected. Bart B did not shoot a Palma rifle he shot a scope rifle with 8x redfield scope according to his post on another site.

It's just like his post about turning down being a Navy Seal Sniper( but he was never a Seal) or turning down being a instructor for the Navy Seal Sniper at the Marine base in NV since he was a special ops guy.

Clark you might want to read up on Bart little more he just a living legend.

[Bart B.]

Corrections. . . .Old Roper's misstating what I've put in print. . . .again.

That was a legal NRA and International Palma rifle with a Weaver T20 scope on it instead of using its normal metallic sights required in competition.

I didn't turn down my request from the SEALs to my detailer to be a sniper as well as a sniper instructor at the US Army Depot in Hawthorne, NV; my detailer at the Pentagon cancelled them; 'twas his perogative as he was responsible for keeping those with my NEC where they were needed. I didn't even know about it until my detailer called me to let me know what was happening.

[old roper]

Bart B, I guess you don't remember this post

I was on the USN Rifle Team. The SEAL Team bosses asked the USN Small Arms Marksmanship Program to give them the names of a handful of their top long range competitors and they did just that. Was first sought to go to their training facility at Hawthorne, NV as a sniper. Later sought to go there as an instructor, but my detailer learned most students went "in country" along with their instructor, so he didn't release me.

I was a chief instructor in San Diego at the time managing a group of people training ships crews in large arms marksmanship. That's the skills and knowledge needed to shoot shipboard guns and guided missiles effectively as opposed to small arms marksmanship involving rifles and pistols shooting bullets effectively.

[Clark]

Here are Bart's modest reactions when we were amazed he was in the magazine:

Bartbob 5/10/97
I've heard that my name was mispelled in John Krieger's PS Magazine ad,
but I don't take Precision Shooting anymore. Guess I'll have to call
Krieger and suggest someone check the information I sent them. Thanks to
this post's author for spelling it correctly and for the kind words, too.
Although this 20-shot group is impressive to some folks, it is very
representative of what modern Palma rifles can do. Several folks have
rifles and load ammo that reqularly performs just as well. And there's a
few that, in my opinion, shoot a little bit better. They use Stolle
Panda, Paramount/RPA, Win. M70, or square-bottom-sleeved Rem. M700/40X
actions, but usually with Krieger or Obermeyer or Medesha barrels.

I only wish I could shoot groups like that with metallic sights when the
wind blows and I'm not using a front-hand rest.

Bart Bobbitt

Bartbob 5/13/97
I used a .308 Win. with Sierra's 155-gr. Palma bullet with 45.3gr. of
IMR4895 and RWS Primers in full-length sized WCC60 match cases. Had a 20X
scope on the English Paramount action and shot prone with a bag under my
front hand to steady the rifle. It was about 6AM in dead-calm wind
conditions.
BB

[Bart B.]

Yes, roper, I remember that.

Now lots of folks can see your mistakes.

[old roper]

Bart B, remember this post and I'm still waiting to hear from you

Back in the '50's when Winchester's earlier broach rifled barrels went into the .30-06 and .300 H&H target rifles, as well as those made after 1964 in .30-06 with hammer forged barrels, like yours, bullets from 180 to 200 grain weights all performed with about equal accuracy. The best scores at the longer ranges were shot with Winchester or Western 197 and 200 grain match bullets from each cartridge as the barrel's all had 1:10 twists and those bullets were .3087" to .3088" in diameter.

I tested a friend's circa 1971 .30-06 Winchester 70 much like yours but with its receiver properly epoxy bedded with Devcon plastic steel shooting Lapua D46 185-gr. match bullets with .3092" diameters with 57 grains of IMR4350. It shot about 1 MOA at 1000 yards as tested with a Weaver T20 target scope. Tried Sierra 180, 190 and 200 grain HPMK's and none shot any better than about 1.7 MOA. Test groups were 20 shots each.

Prior to 1972, the NRA long range target had a 20" V ring inside a 36" 5 ring that was black for the aiming bullseye. Since 1971, the NRA long range target's had a 10" X ring inside a 20" ten ring inside a 30" 9 ring inside a 44" 8 ring which is the black aiming bullseye. One needs much more accurate rifles and ammo these days to shoot perfect scores. It was the much better accuracy of the .308 Winchester and .30-.338 Win Mag over the .30-06 and .300 H&H that caused that.

However, with the right bullets in a good Win. 70 target rifle in .30-06, any powder that shoots them as fast and consistant as IMR4350 does will do the trick. Be sure to full length size your fired cases; nobody ever got consistant good accuracy with either cartridge neck only sizing their fired cases.
_________________________________________________________________

I'm not sure what your point is about target size and as we ex-marine remember this Marine

Carlos N. Hathcock II

On May 20th, 1959, at 17 years of age, Carlos N. Hathcock II fulfilled his childhood dream by enlisting in the United States Marine Corps. His ability as a marksman was soon recognized by the instructors on the rifle range at Camp Pendleton where he was undergoing recruit training. Later, while based in Hawaii as a member of Company E, 2nd Battalion, 4th Marines, Carlos won the Pacific Division rifle championship. Following his assignment in Hawaii, Hathcock was transferred to Marine Air Station, Cherry Point, North Carolina, where he quickly found himself shooting competitively again. This time he set the Marine Corps record on the "A" Course with a score of 248 points out of a possible 250, a record that stands today. The highlight of his competitive shooting career occurred in 1965 when Carlos out-shot over 3000 other servicemen competing to win the coveted Wimbledon Cup at Camp Perry.

Are you saying that Carlos win @ Camp Perry 1965 for the Wimbledon Cup @ 1000yds had anything to do with target size? I'll be glad to come up to Loveland and have little talk with you and explain what Honor means which you seem to lack. PM with place to meet you I can't wait.
__________________
Semper Fi
Edit/Delete Message

[Bart B.]

Yes, target size had lots to do with scores back then. A hit 18" from center would not drop a point. After 1972, a point was dropped with a hit 10.016" from center. When scoring sizes got smaller, some folks quit shooting long range matches because perfect scores were harder to shoot. Easy to figure out for most folks.

Hathcock's score in 1965 was average among all those shot in the 1960's but was blown away in 1970 by a Navy team shooter using a borrowed rifle for the first time.

http://en.m.wikipedia.org/wiki/Wimbledon_Cup Click on Trophy Winners.

I was stationed a few years later with Capt Ray Green USMC after he shot the record that still stands in the old target. His score was shot in what many say was the best conditions to date.