I tried out some new loads in my 7 x57 Mauser today , something very funny happened when testing out the new charges.

44 gr s335 (Sonchem)
Average velociy for 5 shots were 2889 Fps.
Loaded with 140 gr BT.
VC : 65.65
44.3 gr s335
Average velocity for 5 shots were 2915 Fps.
Loaded with 140 gr BT
VC : 65.80
44.9 gr s335
Average velocity for 5 shots were 2719 Fps.
Loaded with 140 gr Bt
VC : 60.55
With all the factors being exactly equal, loaded with same bacth of cases, primers , powders, bullets etc.As you can see with the last charge almost 1 gr higher than first charge but with almost 100 fps less.
I was using a Chrony Beta model set 5 meters from the muzzle.
Ambient temp : 23
Relative Humidity: 38 %
I hope that is all you need to help explain.
Regards
Rudie Potgieter

Rudie, I once chronographed a load in my .30/40 Ruger No. 3 that gave an average of over 2800 FPS for five shots with 200 grain bullets, a result I just could not believe. So I reloaded the same cases with the exact same load, and refired the string for velocity about three hours later that same day. The velocity averaged 150 FPS SLOWER the second time! So I concluded that something had caused a false result the first time. Your result MIGHT BE something like that. However, there is also a "crossover point" with any given powder type and bullet weight where an increase in powder charge will give less, if any, increase in velocity compared to the degree of increase that had been occuring up to that point. This "point of diminishing returns" seems to be more noticeable when using "ball" propellants, and may be what you've encountered. I assume your s335 is a "ball" powder?? In some instances, when the "crossover point" is reached, any increase in powder charge can cause a DROP in muzzle velocity, instead of just a reduction in the increase in speed you had been getting up until then. In all probability, this is what you have encountered. Once, when working with the 7X57mm case, I was attempting to develop a high speed load with the 120 grain bullet and Winchester 760 ball powder. No matter how much powder I used, I could not break 3100 FPS! But, with the same powder, I could safely get over 3000 FPS with a 140-grain bullet! I suspect I was experiencing a similar phenomenon to the results you got.

You have exceeded the point of max. effiency for your ammo-rifle combo. What does a charge of 44.6gr do ?
You should be looking for a linier relationship between powder charge increase and volacity increase, as you reach the top end (for the combo) the curve will flatten out or even decrease in volacity (as you are seeing)
IMHO your 44.9gr load is to hot, back it down.

I suspsect that your chronograph is giving you false readings. A ten-shot string or more will give you a data base from which to measure a mean velocity. You'll want to figure high/low velocity, extreme spread, and mean velocity. You can compute standard deviation -- where 2/3 of your loads fall within this velocity spread.

You may be doing all this already.

I've seen loads that vary in velocity by 150 fps or more. Case capacity, seating depth, primer ignition, barrel temp. can all affect the velocity of a round.

You need a string of ten shots, or more.

At the top end of velocity for a load, you reach "bore capacity" . . . And your gains in velocity flatten as you increase your charge weight.

Once you reach bore capacity, you start blowing hot powder down the bore which is hard on barrels.

I don't understand your references to "VC" and "s" . . .

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Sorry Guys
I think there must have been a misunderstanding, as you would see at the very first load I typed s335 (Sonchem) , This is a South - African Powder. i think you might have it mistaken for a American Powder Tailgunner. The MAX listed in the Sonchem Manual states 44.7 as a MAX for 150 gr , As I am loading 140 grainers I cant see it being too hot there is absolutely no pressure signs yet , in fact the higher load shows less pressure than the first one, Incase you guys were wondering I weigh every charge that goes into that brass. Genghis to answer your Question the VC I am talking about is the Velocity per Charge of grain, and the s is just a prefix model for the powder I am using . I would also like to answer to Eldeguello question abut the powder , this is of the extruded type
Thanks for the input so far Guys.

Regards
Rudie

I'm no expert, but I've experienced the samething a couple different times.

That being, a decrease in velocity at some point, while working up loads with the same powder.

Just figured I had exceeded the most efficient amount of powder for that combo (powder, bullet, primer, rifle, etc.) and backed off a little bit on the charge.

If you need more speed, you'll probably need to try another powder.

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Brian

Rudie,

No offense but my guess is that you either misweighed charge number three or mismarked the loads. I had a similar experience one time so I saved one of the suspect cartridges, took it home and pulled the bullet and found that the powder charge was not what I had marked.

Although they are not perfect, I make it a habit never to second guess my chonograph.

I hope you try the exact same loads again. I would be very interested to know the results of a second test.

I will add that around 2900 fps is about the most one can expect to get out of a 7X57 with 140 grain bullets so it does sound like you are working around max.

Rudie
I wasn't looking at any data for any powder, just the numbers you are reporting, and I said nothing about pressure. What I said about a linier relationship you are calling VC.
Look at your VC numbers
65.6, 65.8, 60.5 the first 2 are linier the 3rd had dropped way off the slope.
As someone else posted, reshoot the series, but this time include a set with a 44.6gr S-335 load.

Tailunner.

I am so sorry if we misunderstood each other , as far as I can remember there is a hodgdon powder with 335 model number , sorry I tought you confused it with one of your powders around there .

Regards

Rudie

No problem, Rudie