Got the data in there as best I could figure, and my actual MV is about 150 fps faster than what GRT says.  Rifle, below book max, no pressure signs.  Where is my input error?

Good grief!, at least post a screen shot or something to give us a clue.

GRT is very much still in development.  It's a reasonable starting tool, but don't expect it to be gospel at this early stage.  It has a long way to go.

Here's something you can try to perhaps answer your own question.  It is more difficult to describe than it is to do but here goes.

SENSITIVITY STUDY

Start out by verifying ALL the inputs are correct, not just the unique ones for your cartridge.

Then, do a sensitivity study.  That is, vary each input value a little bit, then see what that small change does to either pressure or velocity.

In this manner, one at a time, you may find which input values that create the largest changes in output values.  You may also find other to which the outputs are relatively insensitive.

For example, vary case volume ("water weight") by 1%, then 3% and maybe 10%.  Write the results down the inputs and results for each iteration.  Does that change the velocity or pressure by a lot, or not?  

Do the same for OAL (seating depth), powder temperature, powder mass, barrel diameter of grooves,... etc.

In this way you will discern what is most important and what is not.  You might find there is only one thing which can reasonably explain your 150 fps shortfall.  You can then drill down on that (or those) characteristics.

What powder?

GRT is fine.  Use it to good effect.

Well that didn't work.

I typically get about 5-10 fps variance from what GRT predicts and real world results, a margin of "error" that's more than acceptable to me. A variance like that with a 2,000 fps load is 0.002% to 0.005%. All depends on the quality of information that you give it -- including as much information about the firearm as you can.

That said, as someone said earlier, GRT is still in development. A good example of this is that most powders available to us here in North America from companies like Hodgdon have been "reverse engineered" by the community of developers. Where European powder manufacturers have been fairly open in providing technical data to GRT and its community, Hodgdon not so much.

Some powders have been tested extensively but others less so. Does that explain your variance? Perhaps.

At any rate, you do need to provide more information to us before we can figure out why you received the results you did.

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91.4 chronos over the garmin at 2787.  10 shots with 12.5 es and 4.1 sd.  This is a newer rifle.  Was using 90.5 at 2700 when the barrel decided to speed up, settled in at 2750, but the groups opened up vertically.  Thought a little more powder might help, so tried 90.8, 91.1, and 91.4.  Tightened up but at a hair over an inch at 100.  

Savage 112 target magnum 26" barreled action in a heavy chassis.  Barnes brass (Hornady?), retumbo, cci250, and the 285 eldm.  Charged with a warmed up, calibrated chargemaster supreme.  Sorry about the lack of info earlier, was trying to figure out how to post the pic, and went to bed instead.

You are mostly showing the outputs.  How about the inputs?

Just screen shot the entire full-screen display.

I don't know if I (or anyone) can actually solve your mystery, as I do not shoot that cartridge.  I do know, given good inputs, I have gotten excellent agreement between GRT's outputs and my live fire measurements.  That includes using a somewhat similar powder - Ramshot MAGNUM.

Have you played with bore diameter?

Is your case volume value measured or just an estimate?

I'll shut up now.  I have to get back at it now, lots of stuff to do here.

The default water capacity is 114.  I only measured one fired cased  for water capacity, and it was the 116.8.  Just playing around with it...  But the good measured ES makes me think they are all close.  I did play around with adjusting it, and it didn't significantly change anything.  I will work on the entire screen shot.

Have not played with bore diameter.

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You could ask Charlie over at the Discord GRT support server...
GRT Support Channel
Save the above screen under a different filename and then drag & drop it (the .grtload file) onto your message to him, I'm sure he would analyze it for you and respond.

ETA... I see you entered you fired case info, which changed the default values some... that is good, but you kept the projectile default values (OAL etc)... now you need to measure the lot of bullets you have (as with cases, it's best to take measurements of several and average - I typically do 10... although 20 would be statically better, I find it too tedious). That might make a little difference.
After you get your chrono velocities entered into a measurements tab (green + next to results tab), click the OBT menu button to compute the OBT charge adjustment... You might have to reduce the "initial Pressure" (IP) from 5221 to something under 4700 to get OBT to work...
Then don't press any more buttons but look at the graph it shows and copy the values from K and Ba to your main screen powder model... this will adjust the result data and show new velocity and pressure... if everything goes good, it should be closer to reality...

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Good stuff, thanks!  I did just that, without any additional measuring of anything yet.  I used my 91.4 gr charge velocities for the 91.5 gr measurements.  Close enough for the experiment and some learning.  88.5 chronoed at 2650 before the barrel speed up, so this might be very close.  And if the pressures are anywhere near accurate, I should drop back to 88.5 and will do that this weekend.  Extend the brass life, too, hopefully.

I will get more accurate input soon.  Thanks again.

Once you have done the best you can with physical dimensions, the powder parameters are also subject to variations.

The powder issues are not just a way to match the results, they really do happen in reality on a batch to batch tolerance issue basis.

GRT and QL take some time to master, so give yourself a chance to learn the ropes.

Once you have done some model calibration with a given example that uses a common powder, it is good to try those parameters on a completely different gun/recipe to see how well the second model will calibrate due to the powder parameters.
Once you have done several jobs with a variety of parameters, you will gain confidence at predicting unknown territory and you will also have enough experience to anticipate the weight to put on each unknown.

Keep at it, tenacity counts as much as IQ so don't give up.

I ran GRT last night to investigate what struck me as anomalously high velocities posted by another shooter.

Indeed, GRT showed much lower values than were posted by that other person for the charge weights and barrel lengths he posted.

I reran GRT to review my similar loads & chronograph measurements.  I checked GRT for multiple powder types, charge weights and multiple barrel lengths for which I have data.  The GRT results came out spot-on with my measurements - only 7 fps different, or less.  All my loads were well within safe pressure limits.

I ran GRT again, trying to see what charge weights and pressures would be required to produce the velocities posted by that other shooter.  HOLY SMOKE!  

This is one way to use GRT - check your loads to ensure they are reasonably safe.

Averaged 10 cases for volume, and 10 bullets for length.  GRT predicted 2570 for velocity at 88.4 gr.

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But this is what I got.

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 This is 3x fired brass, no ejector marks, and never a sticky bolt lift.  

These were loaded with a Chargemaster Supreme.  1 charge checked on the cheap Hornady digital scale.  Was .1 different.

Anybody have Quickload that can run the numbers?  Thanks.

Two parameters that can affect changes that most people don't play with on GRT.

Example 6.5CM, 140gr ELD-M, IMR4451, 26" barrel length every other parameter default.

Bullet Jump

40gr IMR 4451 - 2550.6 fps with a bullet jump of 0.120"
40gr IMR 4451 - 2557.3 fps with a bullet jump of 0.100"
40gr IMR 4451 - 2573.2 fps with a bullet jump of 0.050"
40gr IMR 4451 - 2582.0 fps with a bullet jump of 0.025"
40gr IMR 4451 - 2589.1 fps with a bullet jump of 0.005"

40gr IMR 4451 - 2589.1 fps with a bullet jump parameter disabled/locked

Initial Pressure (click the star for the assist option) -





2589.1fps estimate to 2628fps (38.9fps difference) just by changing to magnum primer settings in the "Initial Pressure" assist.

Bullet resistance is a major parameter in GRT that a lot of people don't mess with because of the warning when you go to unlock it. It's something you'd have to do because the GRT bullet friction default values and the projectile profiles don't take into account bullet bearing surface relative to the total projectile length. It does take into account bullet tail profile though. For example a secant ogive and tangent ogive profile have very different bullet bearing surface values but there's nothing in GRT (that I'm aware of) where you can enter nose profile and bearing surface measurements. Lower friction = higher velocity and also lower max chamber pressure.

For the OP, if you modify the bullet resistance to around -8500 psi, you can get very close to the velocity results you obtained.

88.4gr - 2680.1fps
90.5gr - 2749 fps
91.4gr - 2779.1fps

I know you said you tried 90.8 and 91.1gr but you didn't say what velocities they were. If you have that data somewhere, you can compare it with the -8500 psi in the bullet resistance parameter and if it matches close enough, then you know that your specific barrel + the 285gr ELD-M bullet requires around -8500psi bullet resistance parameter change. You would use the bullet resistance value as the starting point if you were to change to from Retumbo to a different powder. Changing barrels and/or bullets would change the resistance parameter as well as bullet jump parameter.

I have often thought that the bullet shank length has to play a significant role. One might think that there would be a 'base to ogive' input value but if there is I can't find it... I've had predictions spot on, 125 fps hi/lo and everything in between...  I take the results with 2 grains of salt and if during shooting i see any of the traditional pressure signs, I stop right there and don't shoot anything higher.  Frequently when I plug in the new chrono data and recalibrate K & Ba the pressure goes up.  Its good but you still need to use some common sense...

Usually the default for bullet jump when left lock is equivalent to like bullet jam, so increasing jump only decreases velocity but it's a good idea to measure bullet jump and input correct values that way you don't end up with erroneous initial pressure or bullet resistance values. So if your chrono values are 70fps higher than GRT estimates and assuming you're not using magnum primers, then I'd only modify the bullet resistance and use -3950 psi. Compare GRT data above and below 41.8gr to see if the data values match up.

Estimations with -3950psi and 0.5gr increment above and below 41.8gr
42.3gr - 2743.2fps
41.8gr - 2710.3fps
41.3gr - 2677.4fps