243Win GRT pressure predictions vs Book Data

[Nimbus73]

I'm asking for a little help to understand. I have loaded but not shot yet a short ladder using the 105gr Match Burner and H4350 in my 243Win. I want to see if it will stabilize. I made 10 rounds, 2ea at .3gr intervals. My intentions were to shoot 5 at 100yds and 5 and 200yds to see if I have any tumble or key holing. I recently downloaded GRT, trying to learn it and I've been comparing some of my other shooting results to its predictions and for the most part I'm impressed. This is my concern. I used Barnes load data for my source and started in the upper third of their load range at 41.3gr H4350 thinking I would be well under max pressure and ending with 42.5gr being .3 grains over their max, but when I entered the data into GRT for my loads it tells me I'm way over pressure. GRT predicts 40.7gr to be about 59.7kpsi and 42.2gr at 67kpsi and 42.5 up to 68.6kpsi. How much faith should be given to the pressure predictions GRT puts out? Is the Barnes data too spicy? I've loaded 100gr Sierra SBT's up to 42.5gr seeking more velocity from my 20" barrel before I realized I'm pushing way too hard and have backed off to 40.3 gr. If GRT is even remotely close for this recipe, I need to pull these rounds and find a safer ladder. What have your charges been for this or similar 105gr projectiles in 243Win. and what are your experiences?

 

[Martin52]

I'm asking for a little help to understand. I have loaded but not shot yet a short ladder using the 105gr Match Burner and H4350 in my 243Win. I want to see if it will stabilize. I made 10 rounds, 2ea at .3gr intervals. My intentions were to shoot 5 at 100yds and 5 and 200yds to see if I have any tumble or key holing. I recently downloaded GRT, trying to learn it and I've been comparing some of my other shooting results to its predictions and for the most part I'm impressed. This is my concern. I used Barnes load data for my source and started in the upper third of their load range at 41.3gr H4350 thinking I would be well under max pressure and ending with 42.5gr being .3 grains over their max, but when I entered the data into GRT for my loads it tells me I'm way over pressure. GRT predicts 40.7gr to be about 59.7kpsi and 42.2gr at 67kpsi and 42.5 up to 68.6kpsi. How much faith should be given to the pressure predictions GRT puts out? Is the Barnes data too spicy? I've loaded 100gr Sierra SBT's up to 42.5gr seeking more velocity from my 20" barrel before I realized I'm pushing way too hard and have backed off to 40.3 gr. If GRT is even remotely close for this recipe, I need to pull these rounds and find a safer ladder. What have your charges been for this or similar 105gr projectiles in 243Win. and what are your experiences?

I'd focus on one thing initially, getting some base data to inform GRT, maybe 10 rounds all at 39.0 gr H4350. Enter the velocities for these 10 rounds into GRT and your other specific data (case volume, case length etc) and run OBT. GRT will pick up you are using a 20 inch barrel (the Barnes data assumes 24 inch). I have used GRT for a while now, it can be tricky but I find it a useful guide.

 

[243Lapua]

Your better of with a different twist with that bullet. Or a different bullet with that twist. 105 gr would stabalize better in a 1-8 twist, and I would use 4831sc

 

[Nimbus73]

I'd focus on one thing initially, getting some base data to inform GRT, maybe 10 rounds all at 39.0 gr H4350. Enter the velocities for these 10 rounds into GRT and your other specific data (case volume, case length etc) and run OBT. GRT will pick up you are using a 20 inch barrel (the Barnes data assumes 24 inch). I have used GRT for a while now, it can be tricky but I find it a useful guide.

I did that with a load I tested recently in a different cartridge that I was quite pleased with and according to the OBT I was .12 grains off of their prediction. I did change the barrel length parameter and temperature on the Match Burner load to see how the prediction changed. Obviously, the velocity changed, but with temperature change there was little pressure improvement at 40 degrees. Note my parameters on the GRT attachment. I know from my Sierra 100gr SBT load I've settled on I'm getting about 2830fps for velocity, so I expect this bullet to somewhat less. I just don't get the pressure GRT says for what Barnes says should be a safe load.

Your better of with a different twist with that bullet. Or a different bullet with that twist. 105 gr would stabilize better in a 1-8 twist, and I would use 4831sc

I don't expect great things knowing my twist rate is less than ideal. I only bought one box of these bullets just to see what they do in my gun. I think I'm going to pull these bullets and remake them with a lower charge or maybe use a slower burning powder. I have some H4831sc and a few even slower but in most of the various projectiles I've tried in this cartridge, H4350 has been my preferred powder.

 

[SeabeeKen]

Seldom is GRT going to line up with book data. Differences in rifles, powder and bullet lot numbers, brass etc make it so. Be sure to measure everything. Water capacity of FIRED case, bullet length stc, it makes a difference. According to Gordon, OBT is only 40% accurate. IME it is more like 75%.
GRT is only a prediction.
With regards to Barnes data, I have recently been helping a friend with his 44 mag revolver loads. He used Barnes but couldnt get their listed powder charge in the case. It turns out that Barnes used Remington brass and he used starline. Different capacities which a call to Barnes confirmed. I recently found a similar situation in my 308 using Barnes data. Err on the side of safety

Once your powder model is calibrated, dont forget to save it into the user tab for future use in that rifle

 

[Nimbus73]

Thanks Seabeeken. I have a bunch of once fired Remington Brass I've saved from my previous barrel I'm in the process of fire forming, but my main rotation of brass is Federal. I also have 100 rounds of virgin Starline. I'm just learning the ins and outs of GRT, but it concerned me that it thinks the loads I made are potentially way over pressure so that's why I'm asking for feedback.

 

[Martin52]

A couple of grains difference in case H2O capacity between different head stamps will impact pressure and load. As SeebeeKen says (Post #5) measure the internal capacity of a fired case before resizing (I leave the fired primer in to do this). A Remington case will be different (greater capacity due to thinner case wall) to Starline and different again to Federal when fired in the same (your) rifle. The overall loaded cartridge length is important also. Because you have said Federal is your main rotation I would focus on these for the purpose of modelling in GRT, then shoot a few to get velocities, input velocities in GRT for OBT and re-test.