Speed decreases with more powder

[Revolver]

I've been working on a load for my S&W 500 using a 375 grain spire point and Accurate 1680 powder. FPS kept rising until my last batch, it went down. The last two loads were compressed, or pretty close to it. Is this normal?

38.5gr 1236
40.0gr 1337
40.9gr 1360
41.5gr 1351

Also today, testing some 308 loads with benchmark powder the speed quit rising from 38.0 to 39.0 grains...

36.1 2233
37.0 2264
38.0 2381
39.0 2382

My method is to chrono 5 shots, delete one wild shot if exists, then average the remaining 4 shots. I don't know if that's a good method, but it seemed reasonable to me.

[geargnasher]

Assuming your data was sufficient to be statistically meaningful, yes.

Gear

[Hickory]

The theory of diminished return is in play.

[Revolver]

info added above

[Revolver]

Thanks for the info guys. I'm learning as I go!

[geargnasher]

You need an absolute minimum of seven valid data points per load to establish trend. It's been a long time since I took statistics, but I think 37 is one of the magic numbers for group trend. Rather than average, figure the mean.

The effect you observed in your .500 is a result of load density behavior that some powders exhibit. I've seen it with 2400 and Reloder 7 in straight-walled cases under heavy boolits, right at the point of compression the velocity either levels out or declines. Some powder/cartridge/boolit combinations will dangerously spike pressures at the point where compression begins to take place, others not so much, so be very careful drawing any "certainties" from your observations.

The effect in your .308 is one of the methods some folks use for load workups, sort of akin to the "ladder method", where you begin with a starting load and increase the charge in consistent, small increments until the point of impact levels stops rising for two or three loads, indicating a velocity plateau. The "sweet spot" for accuracy and consistency throughout a broad temperature range may be within that window. I have a pretty strong opinion that it is more a function of barrel harmonics than powder behavior in bottlenecked rifle cartridges.

Gear

[delta1]

I see velocity increase in some of my black powder loads that were under to much compression. When I backed off in powder but kept the compression die the same, velocity increased and the es and sd are better also.

[243winxb]

Velocity decreases when maximum has been reached at times. Dont now if that applies here. I didnt check your data to max.

[cbrick]

I use this as a warning sign in mid capacity cartridges such as the 7BR and many other such cases with mid to slow powders while increasing charges by 1/2 grain, tells me . . . ok dummy, that's far enough back down. Dunno if it applies to larger volume cases such as the 308. In the mid capacity cases it's where accuracy, consistency of the SD and velocity head south.

I use nothing less than 10 shot chrono tests and that only gives me a reasonable idea of consistency. I throw out none of the shots fired in a test, that one shot could well be trying to tell you something about the consistency of the load. Actually it's probably telling you a great deal about consistency.

Rick

[snuffy]

I've had that happen in more than one bottle-necked rifle cartridge.

To me, that signals that I've reached to pressure limit for that powder, case, bullet, and primer combo. If you add more powder, the pressure climbs, but the velocity does NOT!

Measurement of the pressure ring just above the extractor groove will tell you the internal pressure has gone way above the safe pressure point for that powder. Usually the primer pocket also expands. Increases of just .0002 to .0003 is too much. It tells you have permanent expansion of the case head.

Shade tree reloaders use that as a poor mans pressure gauge. It has been said to measure a factory load using similar bullet weights. Because factory loaded ammo is mostly loaded to max pressure,(so the published velocity is reached.) When your handloads get to the factory measurement, you're pretty close to max pressure.

[1Shirt]

Agree with Hickory, have also seen it more than once.
1Shirt!

[runfiverun]

and a velocity decrease DOES NOT mean a pressure decrease or plateau.

[cbrick]

Shade tree reloaders use that as a poor mans pressure gauge. It has been said to measure a factory load using similar bullet weights. Because factory loaded ammo is mostly loaded to max pressure,(so the published velocity is reached.) When your handloads get to the factory measurement, you're pretty close to max pressure.

Ken Waters wrote for years in Handloader that this was his method of determining max pressure. Didn't like this method myself, not all cases will expand the same with the same pressure in them. You could have one case that says all is well and the very next case is saying uh-oh.

Rick

[mpmarty]

Compressed charges of some powders don't ignite as well as non compressed charges thus giving less velocity.

[runfiverun]

ahh,, while others want compression to burn more efficiently.

airc ken also recommended pulling down a few rounds from the factory ammo and using the cases for your load to compare the CHE it was supposed to be an apples to apples comparison of the brass.

near velocity and near CHE measurements should equal near pressures.

[theperfessor]

The more powder you have the more mass of gas along with the bullet that has to go out the muzzle.

[stubshaft]

When you see a diminishing velocity increase per grain of powder you've reached the limit of that powder/barrel combination.

[Echo]

Y Rather than average, figure the mean.

Gear

Oh, Criminy, Gear - the Average IS the Mean! Good grief...

[44man]

Revolver, see why this is the best place? [smilie=s: You get straight answers.
What you see can happen with ANY cartridge. More pressure does not mean more velocity.
But do not toss wild readings, it means something is wrong. Yeah, sometimes the chrono will not pick up the boolit, just gas so that is different.
One shot with a high spike in velocity or lower is an indication of ignition problems. Could be the primer or the powder you are using. Powder compression will cause ignition problems.

[Revolver]

Thanks for all of this info, I have tried to integrate some of it into my collective (brain).

Utilimately it seems that I've made a booboo. The 308 info was for a cast boolit and now I see read that I should keep them under 2k fps. :oops:

36.1 2233
37.0 2264
38.0 2381
39.0 2382

[cbrick]

Utilimately it seems that I've made a booboo. The 308 info was for a cast boolit and now I see read that I should keep them under 2k fps. :oops:

36.1 2233
37.0 2264
38.0 2381
39.0 2382

Nothing wrong with cast over 2,000 fps "IF" it's working. By working I mean is it accurate? Is it consistent? Is it leading? You never said what bullet your using, if it's a light bullet velocity will of course be higher. In my 308 I shoot 160 to 180 grains right around 1900 fps. Not because it's under 2000 but because as I worked up the load that's where these bullets grouped well and with repeated 10 shot chrono tests the chrono told me it is a very consistent load. Also for the shooting I mostly do with this rifle it is at a power level that is adequate, mostly 200 meter handgun silhouette targets.

Consistency of the load is the prime reason and prime value of using a chrono. If all you want is a vague idea of velocity fine, shoot 5 shots but that's all you will have . . . A vague idea.

The more shots fired the more confidence you can have that additional shots fired with that load made with the same components in that firearm will fall within the parameters of the tested rounds. When 5 shots are fired and the one shot that is screaming at you that this load IS NOT consistent is omitted you can have full confidence that many of the additional rounds fired will be well outside the parameters of the test rounds.

Hope this helps.

Rick

[popper]

Echo - average and mean are NOT the same thing. Sum/# vs 1/2 above, 1/2 below.

[geargnasher]

Echo - average and mean are NOT the same thing. Sum/# vs 1/2 above, 1/2 below.

You gots it.

Gear

[C.F.Plinker]

I thought 1/2 above and 1/2 below was the median.

[HangFireW8]

I thought 1/2 above and 1/2 below was the median.

You thought right.

Mean = Average.

Median = midpoint.

C'mon folks, just type "definition:average" into Google.

HF

[felix]

This one might be mo'betta'............

http://www.purplemath.com/modules/meanmode.htm

... felix

[303Guy]

Perhaps the root mean square would be more meaningful? Oh shucks. I don't even remember what the root mean square is!:holysheep Just kidding.:mrgreen:

[bobthenailer]

I ran into this same thing when working up loads for 454 Casull revolvers , i peaked velocity at 38 grs of 296 with the Hornday 240 gr mag XTP , I tried 39 & 40 grs and got less velocity than with 38 gr charge.