I don't have a chronograph (but hope to remedy that soon), but from actually shooting my loaded ammo and considering the resulting trajectory, I'm convinced that most of my ammo is producing velocities LOWER than what I'm calculating it to be. I'm curious if others experience this and if so, why? What is it I'm not taking into consideration when I make my calculations that I almost always come out with a number higher than the ammo actually produces?

Question:
Based on actual velocity readings with a chronograph or as indicated by actual trial trajectories--the ACTUAL velocity of my loaded ammo is almost always:

Choices:
SLOWER than what I calculated it would be.
FASTER than what I calculated it would be
THE SAME (approx.) as I calculated it would be

 

calculations are just that, calculated:
What do you base you calculated velocity on?
In my opinion its just a guess. sometimes you can get pretty close but most of the time our way off.

Whenever someone shoots ammo through my chronograph I warn them ahead of time that they will most likely be disappointed if they have used the "book" to estimate the speed of their ammo. 95% of the time it turns out that way with the speeds being 50-150 fps slower.

One interesting case was shooting the same ammo through two different rifles. One had a 26" barrel and the other a 22" barrel. The 22" barrel produced the same velocity as the other one due to tighter dimensions in barrel/chamber allowing the pressure to be higher.

Since I have an Oehler chrono, I don't calculate velocity but my experience is that velocities are generally lower than what is listed in loading manuals. Usually the manual lists what the load was worked up in and in many cases it is a universal receiver so they can measure pressure and it is often shot through a 26 inch barrel. Your 22 inch sporter will give different velocity and quite often it will be lower.
I use a chrono whenever I work up a new load. knowing the actual average velocity allows you to calculate ballistics much more accurately.

I learned not to 'calculate' velocity long ago.

I have a .308 Win that gets great velocity compared to the loading data.....all the rest of my guns will run 50 or more FPS slower.

Well, the only time I "calculated" my velocity, I used the same aiming point at ranges from 100 to 600yd (5shot groups at each, 30 rounds total) and adjusted the velocity to provide a "best fit curve". IIRC it came up within 100fps of actual chronograph velocity, and half of that difference was due to using a uncorrected BC for that projectile.
I didn't (and still don't) consider it a wasted exercise however, as my drop chart was developed from the same shooting session (and nothing beats knowing your actual drops at various ranges)

I used to use a range that was quite popular when I lived in California, and I was one of only a handful of shooters that had a chonograph. So naturally I would offer it to curious shooters that wanted to send a few rounds across the screens. I would do the shooting of coarse..

But I wish I had a dollar for every time I heard this statement.. "Wow, that’s too slow! It's supposed to be going faster than that!?! That thing can't be right!"

I would be happily retired by now!

So take that for what it's worth. Your new chronograph will open up a whole new world for ya!

quote:

Originally posted by vapodog:
I have a .308 Win that gets great velocity compared to the loading data.....all the rest of my guns will run 50 or more FPS slower.

I haven't testd all my rifles yet but I have one that does better than expected as well. I have a 6x47 with a 20" barrel that shoots right along with Sierra's published speeds out of either a 24/26". I don't have access to my manual right now. When I got to run it over a chrony I was hoping for 2900+ fpw and got 3066 with a load that was supposed to do 3000 fps from Sierra.

I suppose I'm curious as to why few people get the same velocities that the load manuals list for certain charges of powder with certain projectiles. It seems that most get lower velocities than those listed.

Red C.
IMHO it's mostly due to a sloppy chamber/throat with a secondary factor being a larger than nominal spec bore/groove that is typical of production chambers VS the much tighter specs of a precision barrel being cut by someone that gives a damn.
I GAINED 270fps by going from 26" with a sloppy throat (.3125) to a 25" with a tight throat (.3083). Same receiver, same barrel (Shilen #5), same lot of ammo.
Chamber 1 was cut with a throat/chamber reamer by someone that was just in it for the money, #2 was cut with a chamber reamer only (separate throat cutter NOT used) by someone who cares about producing only the best for his customers.

Every rifle is machined differently. My Rock River AR-15 runs fast. 52 SMK, 25 gr H335 gives me 3100 fos and Sierra say it's 2900. For 77 SMK with 24 gr Varget I get 2700, Sierra say 23.9 is 2550. My load is over on Sierra's chart. M193, 55 grain ball, is loaded to 3200, I get 3400.

quote:

I suppose I'm curious as to why few people get the same velocities that the load manuals list for certain charges of powder with certain projectiles. It seems that most get lower velocities than those listed.

1. Load books are worked up with new brass (read thicker) and reloaders use fired brass.


2. When the bigs make a rifle, they start with a reamer that is larger then spec. Once the reamer wears down to minimum spec, it is thrown away, and they start with a new one. This means most factory chambers are larger then min spec.

3. see #2 above, the same thing can be said for the mandrel's in the hammer forged process.

All of these factors lead to lower pressure, and lower velocity.

I've found that measured is usually lower. I've also found that once I get a measured velocity for a given powder and then adjust the burn rate in QL the forcasts then get real close.

Shoot factory ammunition over your chronograph. Assume that is a maximum load.

Ignore the velocities in manuals and stuff. That is only for comparision, you might get it, probably not.

If you don't, the world has not come to an end.

quote:

Originally posted by Antelope Sniper:
1. Load books are worked up with new brass (read thicker) and reloaders use fired brass.

I get lower velocities with new brass. My guess is that the bullet goes a bit further before the brass flex out to the chamber with new brass, and max pressure occur when the bullet is a bit further down, max pressure is then lower because of the added space.

I havent tested this a lot, but so far..

quote:

Originally posted by Red C.:
I suppose I'm curious as to why few people get the same velocities that the load manuals list for certain charges of powder with certain projectiles. It seems that most get lower velocities than those listed.

When the load manuals work up loads for their books they almost always use barrels and chambers that are minimum specification for that cartridge. If, for example, they're working up a load for a 30-06 their test rifle will have a barrel and chamber that is as tight as can be and still be within SAAMI specs. If the allowable bore diameter for a .30 cal runs .3075-.3085 then theirs will be a .3075 bore. That's just an example, I have no idea what the tolerances for a .30 cal bore actually are. Same thing with the chamber, theirs will be the smallest it can be while actually meeting 30-06 specs. By doing this their test rifle will generate the most pressure (and velocity) with a given load combination that an "in spec" 30-06 is capable of giving. Therefore they are guaranteeing that any consumer of their data will not have higher pressure than them in their own rifle for any given load they publish. Virtually all factory rifles have looser bore dimensions and chamber dimensions than the minimum SAAMI specification, therefore any load published in their load book is almost guaranteed to give lower pressure, and thus velocity, than what the load developer got out of their min spec rifle.

Basically, the load books are covering their asses by doing things this way. They're trying to guarantee that no one overpressures a rifle with their loads. The problem is that it gives us somewhat unrealistic data to work with. That's why I work up all my loads over a chronograph, I know the book data isn't that reliable. If a published load isn't giving me the velocity it's supposed to then I'll investigate and sometimes go a grain or two above what the book says to get the velocity back.

Thank the lawyers for making all this necessary.

If you look at the firearms/barrels used for testing the loads and facilities I cann't match that with what I'm using. Most shooting is done indoors I was at Sierra 2yrs ago sure would like to have their shooting tunnel. for pressure testing they send all that out to a seperate lab.

I survived a long time before I got my Oehler 35P next year my 35P be 28yrs old only reason I got it was starting build have some Ackley calibers build.

Chronograph is just one of the tools.

Mine are always a tad faster than the book predicts… in all rifles.

I load middle of the road for velocity so it is not a big concern…

That said, I use Fed primers and only Speer Loads use Fed Primers, so that may be the diff.

I posted this on another post. I load the 30-06 for 3 different people. With the same load shot over my croney.. 52 grs of 4064 150 sierra spbt. from the highest to the lowest was 2950 to 2840. all 22 " barrels..alot depends on your rifle..

Just be careful with shooting at different distances and then "reverse engineering" a muzzle velocity. As I've mentioned elsewhere I have a 308W load that shoots 3000fps but that shows a drop at 300 yards which would indicate that it is much slower.

I suspect the only way to really know the velocity at the muzzle is to measure it and even then you couldn't calculate your drop and be sure it would even be in the ball park. My calculated drops are pretty good to about 220 - 250 yards, by 300 yards they are wrong by 6 - 7 inches.

Red:

The best thing I ever did, was invest in a chronograph. Over the past 20 years it has answered more questions, and solved more conondrums, than I could ever imagine.

As to your question, some loads/rifles are faster, some are slower, and some are close to the book. Whatever book it may be. You'll never know until you run them over some screens.

Also with the pelethora of loading manuals and data available, is it any wonder folks are easily confused? The cronograph eliminates all this confusion.

Jerry

Thanks for all your responses. They were very helpful in my understanding the difference in velocities.