cronographing ammo

baltz526 · 08-30-2006, 04:54 AM

i have a simple cronograph, it works good enough for my needs. couple days ago i did a few tests, the main one i wanted info on was my new shotgun load. i shot a few over the crono and was suprised to find they where 100fps faster than the book stated, the only variable in the data is the crip pressure and how it compresses the load powder/wad/shot. this load is out of a recent shotgun reloading book. is a 100fps gain in velocity a normal occurance. following the load data, the first shell i made turned out perfect, no messing with crimp. these are 2 3/4" 4dram equivulent 1 3/8 oz loads, book listed 1350fps, my cronograph said 1450fps

billc_sbio · 08-30-2006, 07:33 AM

baltz,

I'm not too sure about what the differences are, but I know there's something "different" about chronographing "clouds" of shot vs. single projectiles.

(It might be due to the longer length of a shot string and the way the Chronograph Sky Screens trigger, i.e. Switch 1 on/off=Start, Switch 2 On=Stop. This would see the BACK of the shot string to START the time and the FRONT of the shot string to STOP/READ, resulting in a shorter time duration and thus a higher speed reading OR it could be due to the fact that the shot string is getting LONGER between the screens that the pros use the coil setup?)

When the "factories"/ballistic labs do their tests they use a "coil" setup (probably a pair of large coils, spaced apart, that are shot through) rather than sky screens for measuring shot charges.

I'm not certain what/how there's a difference, but there is.

This might be why your numbers are different than other, published, numbers.

Just a thought...

Regards,
Bill C.-sbio

Chesapeake · 08-30-2006, 07:46 AM

I dont know that I can explain your findings but I can tell you mine and that might help.

I dont reload shotgun but I did chrono several factory loads to see how accurate thier speeds posted on the boxes were.

I have a Pro Chrony (cheep $100 fold up deal)

If I kept the chrony within 10 feet or so of the barrel speeds were good and were within 50 fps of advertised. If the chrony was too close or too far results were all over the place.

I also think that variables like choke, back boreing, barrel length, forcing cone, gas operated auto, ect.. all make small differences that can add up to big differences. My pump with 30" tube will shoot a given load 50-100+ fps faster than my auto with a 28" backbored tube.

baltz526 · 08-30-2006, 08:03 AM

i had the crongraph set at ten feet, i do not think the wad has seperated from the shot yet? 28" barrel 870 express, so i would think, the gun would be near factory test dimensions. the pattern was blown out in the mod choke on paper at 30yds, more than i expected. but the #5's sure penitrated good. next bag of shot will be #6's. did one 70yd shot at cinder rock about chucker sized. pattern was about 36" and 1' low on target. so sniping them rock sitting jokers with a full choke will be an option. dig out the range finder, plot trajectory and boom

Hunt'nFish · 08-30-2006, 08:14 AM

Yea Baltz, I agree w/ Bill. Back bored barrel? How far away from the chono were you? if 10-15ft then I'd back up a bit and re-shoot. My Oheler has 3 screens that it uses....effectively 2 seperate chono's (screen1 to s2) and (s1 to s3).
Some times I've seen some goofy things when checking shot shell loads, proof screen data differant and such.

However, I also note that I saw velocities that where no where close to the book. As you pointed out the crimp plays a very big role in pressure build up. How do the heads look? Can you see the outline of the boltface on it? If so your WAY OVER pressure and I'd back off on the powder or let up on the crimp. I'm like you I like tight crimps. You might also consider going to a slightly slower powder and keeping the tight crimp.
Hunt'nFish

Hunt'nFish · 08-30-2006, 08:29 AM

Quote:

My pump with 30" tube will shoot a given load 50-100+ fps faster than my auto with a 28" backbored tube.

Huh, that's interesting. I thought back-boring was supposed to INCREASE velocity. This auto, was it gas action or recoil action?? I wonder how much gas guns loose. Might be that a 3" pump gun is hitting like a 3.5" gas gun.
Hunt'nFish

StickFish · 08-30-2006, 08:37 AM

Hunt'nFish & Blatz

Either of you think the altitude at Lapine would account for 100 fps?

billc_sbio · 08-30-2006, 09:25 AM

Quote:

My pump with 30" tube will shoot a given load 50-100+ fps faster than my auto with a 28" backbored tube.

Huh, that's interesting. I thought back-boring was supposed to INCREASE velocity. This auto, was it gas action or recoil action?? I wonder how much gas guns loose. Might be that a 3" pump gun is hitting like a 3.5" gas gun.
Hunt'nFish

Geez...there's 2 ways to look at Backboring.

One is less barrel resistance...but

I know it's supposed to reduce felt recoil, i.e. "softer" loads...

But why?

Let's see...it's really the push on the gun vs. the push on the shot charge/velocity...

So...NON-Backbored guns have a better seal, less VOLUME in the "cylinder", and thus higher (starting) working pressures.

Does this sound correct?

So it's those early (starting) pressures that determine how fast and what the Max pressure (and likely max velocity) is going to be. (And tight crimps help that starting pressure when using slow burning powders)

Are we SURE that Backbored is really Faster? Maybe less recoil, but faster?

Hmm...let's talk on this...think it through.

Hunt'nFish · 08-30-2006, 11:06 AM

Stickfish,
Alititude has nothing to do with muzzle velocity. Temperature of the powder at time of firing, is all that matters environmentally. (Baltz, you didn't have them laying on the dash of the truck did you?!!!

)

Altitud only comes into play for exterior ballistics, not interior. I consider muzzle velocity to be a measure of interior ballistics.

BTW, how's the TUNA! fishing been?? Me?

I need a fix.

Bill, yea, I have questioned the impact of back-boring as well. But I am not super knowledgable on the subject. I'm sure a little reading online would yield the answer though.
Hunt'nFish

StickFish · 08-30-2006, 11:14 AM

I always considered it to be exterior balistics - but I'm a greenhorn who lost his last marble. I tried to duplicate some off book loads I got from Nosler some years back and couldn't down here in the thick air - his explanation was altitude

Tuna - Have not been since the fishing got good

either out of town on business or some other thing, oh and then there is the lousy forecasts for the ocean the last seven weekends - getting that Steen's tag kind put a damper on week day fishing - I know quit crying in my beer it will get warm.

Chesapeake · 08-30-2006, 01:30 PM

Quote:

My pump with 30" tube will shoot a given load 50-100+ fps faster than my auto with a 28" backbored tube.

Huh, that's interesting. I thought back-boring was supposed to INCREASE velocity. This auto, was it gas action or recoil action?? I wonder how much gas guns loose. Might be that a 3" pump gun is hitting like a 3.5" gas gun.
Hunt'nFish

Geez...there's 2 ways to look at Backboring.

One is less barrel resistance...but

I know it's supposed to reduce felt recoil, i.e. "softer" loads...

But why?

Let's see...it's really the push on the gun vs. the push on the shot charge/velocity...

So...NON-Backbored guns have a better seal, less VOLUME in the "cylinder", and thus higher (starting) working pressures.

Does this sound correct?

So it's those early (starting) pressures that determine how fast and what the Max pressure (and likely max velocity) is going to be. (And tight crimps help that starting pressure when using slow burning powders)

Are we SURE that Backbored is really Faster? Maybe less recoil, but faster?

Hmm...let's talk on this...think it through.

My gun is gas operated so this could account for loss in velocity.

My thoughts on backbore:

1.I always thought it was to improve pattern not velocity.

2. By enlarging the internal diameter of the barrel you enlarge the area the gas must expand and fill, there fore lowering pressure, and velocity. (probably very minimal)

3. The larger internal diameter would cause the wad to have to expand more to keep a good seal. I would think more gas would escape past the shot wad thus lowereng velocity.(again probably minimal)

4. I cant imagine the reduction in friction between the wad and barrel would be reduced enough to cause a measurable velocity gain.

Again my understanding of back boring is that it is to improve patterning due to the thought that the extra room allows the shot to situate itself before it hits the choke.

Hunt'nFish · 08-30-2006, 02:04 PM

Quote:

Again my understanding of back boring is that it is to improve patterning due to the thought that the extra room allows the shot to situate itself before it hits the choke.

Perhaps.
I agree with you thoughts on points 1-4.

baltz526 · 08-30-2006, 02:11 PM

the cronograph was reading correct, usually when i get eratic readings, it shows right away. i tested the shotgun and two rifle loads, all the rifle tests where consistant. the powder i'm using is blue dot. i checked the cases for pressure signs, comparing them with once fired factory loads, they looked exactly the same

baltz526 · 08-30-2006, 02:16 PM

my understanding of back boring, is twofold, reduce felt recoil and reducing abrasion of large steel shot all the way down the bore. creating a situation where an improved cylinder choke will give a pattern of a full choke from a nonbackbored barrel. creating a safer choke system for steel shot

billc_sbio · 08-30-2006, 05:34 PM

Quote:

my understanding of back boring, is twofold, reduce felt recoil and reducing abrasion of large steel shot all the way down the bore.

AND

creating a situation where an improved cylinder choke will give a pattern of a full choke from a nonbackbored barrel. creating a safer choke system for steel shot

Agree on Recoil (it is "actual" recoil)

Hmm...not sure on reduced abrasion as we're only talking a few thou here... That's more a function of the protection/quality of the shotcup. A slightly wider bore just lets the shot slump down and still push against the sides of the shotcup.

Definitely AGREE on the function of making an exisiting choke/choke tube TIGHTER. Since the effect of the choke is a function of the ratio of the bore diameter to the choke constriction.

I'm not sure this is particularly benificial as now it makes your "open" tubes tighter (and your "tight" tubes too tight to be of any use

). What do you now do for "Open" tubes?

baltz526 · 08-30-2006, 06:32 PM

reduced abrasion on the first generation of steel shot loads. now they have the wad construction figured out, but the original wads would have holes in them after firing. when loaded with the larger pellets. i still chase the wads down on new shells to check the results of going through the choke

baltz526 · 08-30-2006, 07:43 PM

the other two loads i tested, 100gr remington coreloc 243 was 2900fps average. and the weatherby 300win mag, with rl22 77gr under a 165gr accubond as a starting load. 3040fps average. the accuracy was marginal 1.35" stringing horizontaly. i have the next test loaded at 78gr

adobe wall · 08-31-2006, 04:35 AM

most likely culprit, in my opinion, would be that the chrono was not perfectly level. I glued a small level inside of mine to ensure I set it correctly each time.

If you did level it, then I wouldn't sweat it at all. Firearms are so different that any factory data is more or less just a reference, as far as I'm concernced. Who knows how the factory test barrel was set up.

If your loads aren't showing pressure signs, press on.

jmho, aw

baltz526 · 08-31-2006, 05:06 AM

leveling is something i have never thought about, i just eyeball it, get it close and lined up with my targets.

baltz526 · 08-31-2006, 05:16 AM

i test the factory ammo in guns i plan to shoot it in. some i have tested over the years, like the 300 savage ammo has been right on published data. 150gr remington coreloc in a 24" barreled mod 99 averages right at 2650fps, the test info is in the shop, if anyone is interested i could dig it out.

adobe wall · 08-31-2006, 05:26 PM

failure to level the chrono results in the horizontal distance between the screens being shorter. Shorter actual distance between screens will result in readings higher than actual velocity.

Not using a level every time also is the cause for inconsistent velocity readings some folks have and can't figure out.

Most shotgun reload charges are thrown via a volume measure and can vary quite a bit. Barrel length, bore diameter, and all the other variables mentioned by other posters add up to the fact that I'd be more surprised if the shotgun chrono'd exactly to published factory data.

If a subsonic (less than 1050 fps +/-) load is being used, you can also get false velocity readings by the fact that the pressure wave from the round is travelling faster than the projectile. Putting a sheet of newspaper in a frame between the muzzle and chrono will correct that. There are actually quite a few ways to get false readings out of a chronograph.

hth aw

Hunt'nFish · 09-01-2006, 08:49 AM

Quote:

If a subsonic (less than 1050 fps +/-) load is being used, you can also get false velocity readings by the fact that the pressure wave from the round is travelling faster than the projectile. Putting a sheet of newspaper in a frame between the muzzle and chrono will correct that. There are actually quite a few ways to get false readings out of a chronograph.

That's an interesting thought.